Sumitendra Mazumdar

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Journals/Publications

• Chesler, R., & Khan, S. (2020). Wave Function Based Analysis of Dynamics versus Yield of Free Triplets in Intramolecular Singlet Fission. J. Phys. Chem. A, 124(49), 10091– 10099.
• Khan, S. (2020). Free Triplets Versus Bound Triplet-Triplet Biexciton in Intramolecular Singlet Fission Materials: Structure-Property Correlations. JOURNAL OF PHYSICAL CHEMISTRY C, 124, 1171-1177. doi:10.1021/acs.jpcc.9b09831
• Mazumdar, S. (2020). Sr$_2$RuO$_4$, like doped cuprates and barium bismuthate, is a negative charge-transfer gap even parity superconductor with $\frac{3}{4}$-filled oxygen band. Physical Review X.
• Clay, R. (2019). Theory of triangular lattice quasi-one-dimensional charge-transfer solids. Physical Review B, 100, 115158. doi:DOI10.1103/PhysRevB.100.115158
• Mazumdar, S., & Clay, R. T. (2019). From charge- and spin-ordering to superconductivity in the organic charge-transfer solids R. T. Clay, S. Mazumdar. Physics Reports, 788, 1-89.
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  Solicited review article, 104 pages, 63 figures
• Pradhan, S., & Ramasesha, S. (2019). Correlated Electronic Properties of a Graphene Nanoflake: Coronene. MOLECULES, 24, 730 [1-4]. doi:10.3390/molecules24040730
• Khan, S. (2018). Optical probes of the quantum-entangled triplet-triplet state in a heteroacene dimer. Physical Review B, 98, 165202. doi:https://doi.org/10.1103/PhysRevB.98.165202
• Mazumdar, S. (2018). Valence transition model of the pseudogap, charge order, and superconductivity in electron-doped and hole-doped copper oxidesed. Physical Review B, 98, 205153 (1-28). doi:https://doi.org/10.1103/PhysRevB.98.205153
• Mazumdar, S. (2017). Bond patterns and charge order amplitude in 1/4-filled charge-transfer solids. Physical Review B, 95, 125114. doi:10.1103/PhysRevB.95.125114
• Mazumdar, S., & Khan, S. (2017). Diagrammatic Exciton Basis Theory of the Photophysics of Pentacene Dimers. Journal of Physical Chemistry Letters, 8, 4468-4478. doi:DOI: 10.1021/acs.jpclett.7b01829
• Mazumdar, S., & Khan, S. (2017). Theory of Transient Excited State Absorptions in Pentacene and Derivatives: Triplet-Triplet Biexciton versus Free Triplets. JOURNAL OF PHYSICAL CHEMISTRY LETTERS, 8, 5943-5948. doi:DOI: 10.1021/acs.jpclett.7b02748
• Mazumdar, S., Basel, T. P., Huynh, U. N., Ehrenfreund, E., Li, G., Yang, Y., & Vardeny, Z. V. (2017). Transient Magnetophotoinduced Absorption Studies of Photoexcitations in pi-Conjugated Donor-Acceptor Copolymers. PHYSICAL REVIEW LETTERS, 119, 017401. doi:DOI: 10.1103/PhysRevLett.119.017401
• Mazumdar, S. (2016). Correlated electronic properties of some graphene nanoribbons: A DMRG study. Physical Review B, 94(3), 035139. doi:10.1103/PhysRevB.94.035139
• Mazumdar, S. (2016). Coulomb enhancement of superconducting pair-pair correlations in a 3/4-filled model for kappa-(BEDT-TTF)(2)X. Physical Review B, 93(20), 205111. doi:10.1103/PhysRevB.93.205111
• Mazumdar, S. (2016). Coulomb-enhanced superconducting pair correlations and paired-electron liquid in the frustrated quarter-filled band. Physical Review B, 93(16), 165110. doi:DOI: 10.1103/PhysRevB.93.165110
• Aryanpour, K., Dutta, T., Huynh, U. N., Vardeny, Z. V., & Mazumdar, S. (2015). Theory of Primary Photoexcitations in Donor-Acceptor Copolymers. PHYSICAL REVIEW LETTERS, 115(26).
• Aryanpour, K., Shukla, A., & Mazumdar, S. (2015). Theory of Singlet Fission in Polyenes, Acene Crystals, and Covalently Linked Acene Dimers. JOURNAL OF PHYSICAL CHEMISTRY C, 119(13), 6966-6979.
• Mazumdar, S., Aryanpour, K., Dutta, T., Huynh, U., & vardeny, Z. (2015). Theory of Primary Photoexcitations in Donor-Acceptor Copolymers. PHYSICAL REVIEW LETTERS, 115, 267401. doi:DOI: 10.1103/PhysRevLett.115.267401
• Aryanpour, K., Roberts, A., Sandhu, A., Rathore, R., Shukla, A., & Mazumdar, S. (2014). Subgap two-photon states in polycyclic aromatic hydrocarbons: Evidence for strong electron correlations. Journal of Physical Chemistry C, 118(6), 3331-3339.
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  Abstract: Strong electron correlation effects in the photophysics of quasi-one-dimensional π-conjugated organic systems such as polyenes, polyacetylenes, polydiacetylenes, etc., have been extensively studied. Far less is known on correlation effects in two-dimensional π-conjugated systems. Here we present theoretical and experimental evidence for moderate repulsive electron-electron interactions in a number of finite polycyclic aromatic hydrocarbon molecules with D6h symmetry. We show that the excited state orderings in these molecules are reversed relative to that expected within one-electron and mean-field theories. Our results reflect similarities as well as differences in the role and magnitude of electron correlation effects in these two-dimensional molecules compared to those in polyenes. © 2014 American Chemical Society.
• Aryanpour, K., Shukla, A., & Mazumdar, S. (2014). Electron correlations and two-photon states in polycyclic aromatic hydrocarbon molecules: A peculiar role of geometry. Journal of Chemical Physics, 140(10), 104301.
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  Abstract: We present numerical studies of one- and two-photon excited states ordering in a number of polycyclic aromatic hydrocarbon molecules: coronene, hexa-peri-hexabenzocoronene, and circumcoronene, all possessing D6h point group symmetry versus ovalene with D2h symmetry, within the Pariser-Parr-Pople model of interacting π-electrons. The calculated energies of the two-photon states as well as their relative two-photon absorption cross-sections within the interacting model are qualitatively different from single-particle descriptions. More remarkably, a peculiar role of molecular geometry is found. The consequence of electron correlations is far stronger for ovalene, where the lowest spin-singlet two-photon state is a quantum superposition of pairs of lowest spin triplet states, as in the linear polyenes. The same is not true for D6h group hydrocarbons. Our work indicates significant covalent character, in valence bond language, of the ground state, the lowest spin triplet state and a few of the lowest two-photon states in D2h ovalene but not in those with D6h symmetry. © 2014 AIP Publishing LLC.
• Dutta, T., & Mazumdar, S. (2014). Theory of metal-intercalated phenacenes: Why molecular valence 3 is special. Physical Review B, 89(24), 245129.
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  We develop a correlated-electron minimal model for the normal state of charged phenanthrene ions in the solidstate, within the reduced space of the two lowest antibonding molecular orbitals of phenanthrene. Our modelis general and can be easily extended to study the normal states of other polycyclic aromatic hydrocarbon superconductors. The main difference between our approach and previous correlated-electron theories of phenacenes isthat ourcalculations are exact within thereduced basis space, albeit forfinite clusters.Theenhancedexchange of electron populations between these molecular orbitals, driven by Coulomb interactions over and above the bandwidth effects, gives a theoretical description of the phenanthrene trianions that is very different from previous predictions.Exact many-bodyfiniteclustercalculationsshowthatwhilethesystemswithmolecularcharges of\ − 1and −2 are one- and two-band Mott-Hubbard semiconductors, respectively, molecular charge −3 gives two nearly 3/4-filled bands, rather than a completely filled lower band and a 1/2\ -filled upper band. The carrierdensity per active molecular orbital is thus nearly the same in the normal state of the superconducting aromaticsand organic charge-transfer solids, and may be the key to understanding unconventional superconductivity inthese molecular superconductors.
• Mazumdar, S. (2014). Comment on "Tuning the Magnetic Dimensionality by Charge Ordering in the Molecular TMTTF Salts". Physical Review Letters, 113(2), 029701. doi:DOI: 10.1103/PhysRevLett.113.029701
• Mazumdar, S. (2014). The Chemical Physics of Unconventional Superconductivity. Int. J. Quant. Chem., 114(16), 1053-1059.
• Mazumdar, S., & Dutta, T. (2014). Theory of metal-intercalated phenacenes: Why molecular valence 3 is special. PHYSICAL REVIEW B, 89(24), 245129.
• Mazumdar, S., & Dutta, T. (2014). Theory of metal-intercalated phenacenes: Why molecular valence 3 is special. PHYSICAL REVIEW B, 89(24), 245129. doi:10.1103/PhysRevB.89.245129
• Aryanpour, K., Muñoz, J., & Mazumdar, S. (2013). Does singlet fission enhance the performance of organic solar cells?. Journal of Physical Chemistry C, 117(10), 4971-4979.
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  Abstract: Singlet fission, in which the optical spin-singlet exciton dissociates into two low energy triplet excitons, has been proposed as a viable approach to enhance the quantum efficiency of organic solar cells. We show that even when singlet fission is occurring in the donor molecule, the electronic structure at the donor-acceptor interface must satisfy specific requirements for the solar cell performance to be enhanced by this process. We focus on the pentacene-C60 solar cell, and on the basis of our calculations and available experimental data, we conclude that there is not enough evidence that these requirements are met by the donor-acceptor interface here. We propose experiments that can determine whether the minimal requirement for enhanced performance driven by singlet fission is met in this and other solar cells. © 2013 American Chemical Society.
• Gomes, N., Clay, R. T., & Mazumdar, S. (2013). Absence of superconductivity and valence bond order in the Hubbard-Heisenberg model for organic charge-transfer solids. Journal of Physics Condensed Matter, 25(38).
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  PMID: 23995074;Abstract: A frustrated, effective 1/2-filled band Hubbard-Heisenberg model has been proposed for describing the strongly dimerized charge-transfer solid families κ-(ET)2X and Z[Pd(dmit)2]2. In addition to showing unconventional superconductivity, these materials also exhibit antiferromagnetism, candidate spin-liquid phases, and, in the case of Z = EtMe3P, a spin-gapped phase that has sometimes been referred to as a valence bond solid. We show that neither superconductivity nor the valence bond order phase occurs within the Hubbard-Heisenberg model. We suggest that a description based on 1 4 -filling, that is reached when the carrier concentration per molecule instead of per dimer is considered, thus may be appropriate. © 2013 IOP Publishing Ltd.
• Aryanpour, K., Mazumdar, S., & Zhao, H. (2012). Triplet excitations in carbon nanostructures. Physical Review B - Condensed Matter and Materials Physics, 85(8).
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  Abstract: We show that the energy differences between the lowest optical singlet exciton and the lowest triplet exciton in semiconducting single-walled carbon nanotubes with diameter ∼1 nm and graphene nanoribbons with widths ∼2 nm are an order of magnitude smaller than in the π-conjugated polymer poly(para-phenylenevinylene). Our calculated energy gaps between the singlet and triplet excitons are in excellent agreement with the measured values in three different nanotubes with diameters close to 1 nm. The spatial extent of the triplet exciton is nearly the same as that of the singlet exciton in wide nanotubes and nanoribbons, in contrast to that in π-conjugated polymers in which the triplet exciton exhibits strong spatial confinement. Weakly confined behavior of the triplet state begins in nanoribbons with widths as narrow as 2.5 times the graphene unit lattice vector. We discuss possible consequences of the small singlet-triplet energy difference in the carbon nanostructures on device applications. © 2012 American Physical Society.
• Clay, R. T., Dayal, S., Li, H., & Mazumdar, S. (2012). Beyond the quantum spin liquid concept in frustrated two dimensional organic superconductors. Physica Status Solidi (B) Basic Research, 249(5), 991-994.
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  Abstract: The occurrence of antiferromagnetism (AFM) in κ-(ET) 2X can be understood within an effective 1/2-filled band with dimers of ET molecules containing one hole each. We argue that while this effective model can describe the presence of AFM, a complete description for these materials requires the correct carrier density of one-half per molecule. For dimerized and strongly frustrated 1/4-filled lattices we show that a singlet-paired statecoexisting with charge ordering occurs that we have termed the paired electron crystal (PEC). Here we investigate the 1/4-filled model on a dimerized lattice, showing regions where AFM, PEC, and the Wigner-crystal occur. We point out the need to go beyond quantum spin liquid concepts for highly frustrated materials such as κ-(ET) 2Cu 2(CN) 3 and β′-EtMe 3Sb[Pd(dmit) 2] 2 which we believe are PECs at low temperatures. © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
• Clay, R. T., Song, J., Dayal, S., & Mazumdar, S. (2012). Ground state and finite temperature behavior of 1/4-filled band zigzag ladders. Journal of the Physical Society of Japan, 81(7).
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  Abstract: We consider the simplest example of lattice frustration in the 1/4-filled band, a one-dimensional chain with nextnearest neighbor interactions. For this zigzag ladder with electron-electron as well as electron-phonon interactions we present numerical results for ground state as well as thermodynamic properties. In this system the ground state bond distortion pattern is independent of electron-electron interaction strength. The spin gap from the ground state of the zigzag ladder increases with the degree of frustration. Unlike in one-dimension, where the spin-gap and charge ordering transitions can be distinct, we show that in the ladder they occur simultaneously. We discuss spin gap and charge ordering transitions in 1/4-filled materials with one, two, or three dimensional crystal structures. We show empirically that regardless of dimensionality the occurrence of simultaneous or distinct charge and magnetic transitions can be correlated with the ground state bond distortion pattern. © 2012 The Physical Society of Japan.
• Dayal, S., Clay, R. T., & Mazumdar, S. (2012). Absence of long-range superconducting correlations in the frustrated half-filled-band Hubbard model. Physical Review B - Condensed Matter and Materials Physics, 85(16).
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  Abstract: We present many-body calculations of superconducting pair-pair correlations in the ground state of the half-filled-band Hubbard model on large anisotropic triangular lattices. Our calculations cover nearly the complete range of anisotropies between the square and isotropic triangular lattice limits. We find that the superconducting pair-pair correlations decrease monotonically with increasing on site Hubbard interaction U for interpair distances greater than nearest neighbor. For the large lattices of interest here the distance dependence of the correlations approaches that for noninteracting electrons. Both these results are consistent with the absence of superconductivity in this model in the thermodynamic limit. We conclude that the effective 12-filled band Hubbard model, suggested by many authors to be appropriate for the κ-(BEDT-TTF)-based organic charge-transfer solids, does not explain the superconducting transition in these materials. © 2012 American Physical Society.
• Mazumdar, S., & Clay, R. T. (2012). Is there a common theme behind the correlated-electron superconductivity in organic charge-transfer solids, cobaltates, spinels, and fullerides?. Physica Status Solidi (B) Basic Research, 249(5), 995-998.
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  Abstract: We posit that there exist deep and fundamental relationships between the above seemingly very different materials. The carrier concentration-dependences of the electronic behavior in the conducting organic charge-transfer solids and layered cobaltates are very similar. These dependences can be explained within a single theoretical model, the extended Hubbard Hamiltonian with significant nearest neighbor Coulomb repulsion. Interestingly, superconductivity in the cobaltates seems to be restricted to bandfilling exactly or close to one-quarter, as in the organics. We show that dynamic Jahn-Teller effects and the resultant orbital ordering can lead to 1/4-filled band descriptions for both superconducting spinels and fullerides, which show evidence for both strong electron-electron and electron-phonon interactions. The orbital orderings in antiferromagnetic lattice-expanded bcc M 3C 60 and the superconductor are different in our model. Strong correlations, quarter-filled band and lattice frustration are the common characteristics shared by these unusual superconductors. © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
• Mazumdar, S., Clay, R. T., & Hongtao, L. i. (2012). Similarities in electronic properties of organic charge-transfer solids and layered cobaltates. Physica B: Condensed Matter, 407(11), 1722-1724.
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  Abstract: The apparently counterintuitive carrier concentration-dependent electronic properties of layered cobaltates have attracted wide interest. Here we point out that very similar carrier-concentration dependence has previously been noted in strongly correlated quasi-one dimensional (quasi-1D) organic charge-transfer solids. The normal states of both families can be understood, over the entire range of carrier concentration of interest, within the extended Hubbard Hamiltonian with significant intersite Coulomb interaction. As with the charge-transfer solids, superconductivity in the cobaltates appears to be limited to bandfilling of one-quarter. We point out further that there exist other families of correlated superconductors, such as spinels, where too strong correlations, geometric lattice frustration and bandfilling of one-quarter seem to be the essential features of the unconventional superconductors. © 2012 Elsevier B.V. All rights reserved.
• Aryanpour, K., Sheng, C. -., Olejnik, E., Pandit, B., Psiachos, D., Mazumdar, S., & Vardeny, Z. V. (2011). Evidence for excimer photoexcitations in an ordered π-conjugated polymer film. Physical Review B - Condensed Matter and Materials Physics, 83(15).
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  Abstract: We report pressure-dependent transient picosecond and continuous-wave photomodulation studies of disordered and ordered films of 2-methoxy-5-(2- ethylhexyloxy) poly(para-phenylenevinylene). Photoinduced absorption (PA) bands in the disordered film exhibit very weak pressure dependence and are assigned to intrachain excitons and polarons. In contrast, the ordered film exhibits two additional transient PA bands in the midinfrared that blueshift dramatically with pressure. Based on high-order configuration interaction calculations, we ascribe the PA bands in the ordered film to excimers. Our work brings insight to the exciton binding energy in ordered films versus disordered films and solutions. The reduced exciton binding energy in ordered films is due to energy states appearing below the continuum band threshold of the single strand. © 2011 American Physical Society.
• Dayal, S., Clay, R. T., Li, H., & Mazumdar, S. (2011). Paired electron crystal: Order from frustration in the quarter-filled band. Physical Review B - Condensed Matter and Materials Physics, 83(24).
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  Abstract: We present a study of the effects of simultaneous charge and spin frustration on the two-dimensional strongly correlated quarter-filled band on an anisotropic triangular lattice. Our conclusions are based on exact diagonalization studies that include electron-electron interactions as well as adiabatic electron-phonon coupling terms treated self-consistently. The broken-symmetry states that dominate in the weakly frustrated region near the rectangular lattice limit are the well known antiferromagnetic state with in-phase lattice dimerization along one direction and the Wigner crystal state with the checkerboard charge order. For moderate to strong frustration, however, the dominant phase is a novel spin-singlet paired-electron crystal (PEC), consisting of pairs of charge-rich sites separated by pairs of charge-poor sites. The PEC, with coexisting charge order and spin gap in two dimensions, is the quarter-filled band equivalent of the valence bond solid (VBS) that can appear in the frustrated half-filled band within antiferromagnetic spin Hamiltonians. We discuss the phase diagram as a function of onsite and intersite Coulomb interactions as well as electron-phonon coupling strength. We speculate that the spin-bonded pairs of the PEC can become mobile for even stronger frustration, giving rise to a paired-electron liquid. We discuss the implications of the PEC concept for understanding several classes of quarter-filled band materials that display unconventional superconductivity, focusing, in particular, on organic charge transfer solids. Our work points out the need to go beyond quantum spin liquid concepts for highly frustrated organic charge-transfer solids such as κ-(BEDT-TTF)2Cu 2(CN)3 and EtMe3Sb[Pd(dmit)2] 2, which we believe show frustration-induced charge disproportionation at low temperatures. We discuss possible application to layered cobaltates and quarter-filled band spinels. © 2011 American Physical Society.
• Li, H., Clay, R. T., & Mazumdar, S. (2011). Theory of carrier concentration-dependent electronic behavior in layered cobaltates. Physical Review Letters, 106(21).
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  PMID: 21699322;Abstract: A natural explanation for the carrier concentration-dependent electronic behavior in the layered cobaltates emerges within correlated-electron Hamiltonians with finite on-site and significant nearest neighbor hole-hole Coulomb repulsions. The nearest neighbor repulsion decreases hole double occupancy below hole density 13, but increases the same at higher hole densities. Our conclusion is valid for both single-band and three-band extended Hubbard Hamiltonians, and sheds light on concentration dependent eg′ hole occupancy within the latter. © 2011 American Physical Society.
• Aryanpour, K., Psiachos, D., & Mazumdar, S. (2010). Theory of interfacial charge-transfer complex photophysics in π -conjugated polymer-fullerene blends. Physical Review B - Condensed Matter and Materials Physics, 81(8).
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  Abstract: We present a theory of the electronic structure and photophysics of 1:1 blends of derivatives of polyparaphenylenevinylene and fullerenes. Within the same Coulomb-correlated Hamiltonian applied previously to interacting chains of single-component π -conjugated polymers, we find an exciplex state that occurs below the polymer's optical exciton. Weak absorption from the ground state occurs to the exciplex. We explain transient photoinduced absorptions in the blend, observed for both above-gap and below-gap photoexcitations, within our theory. Photoinduced absorptions for above-gap photoexcitation are from the optical exciton as well as the exciplex while for below-gap photoexcitation induced absorptions are from the exciplex alone. In neither case are free polarons generated in the time scale of the experiment. Importantly, the photophysics of films of single-component π -conjugated polymers and blends can both be understood by extending Mulliken's theory of ground-state charge transfer to the case of excited-state charge transfer. © 2010 The American Physical Society.
• Clay, R. T., Li, H., & Mazumdar, S. (2010). Bipolaron density-wave driven by antiferromagnetic correlations and frustration in organic superconductors. Physica B: Condensed Matter, 405(11 SUPPL.), S253-S255.
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  Abstract: We describe the paired electron crystal (PEC) which occurs in the interacting frustrated two-dimensional 1/4- filled band. The PEC is a charge-ordered state with nearest-neighbor spin singlets separated by pairs of vacant sites, and can be thought of as a bipolaron density wave. The PEC has been experimentally observed in the insulating state proximate to superconductivity in the organic charge-transfer solids. Increased frustration drives a PEC-to-superconductor transition in these systems. © 2010 Elsevier B.V. All rights reserved.
• Clay, R. T., Li, H., Sarkar, S., Mazumdar, S., & Saha-Dasgupta, T. (2010). Cooperative orbital ordering and Peierls instability in the checkerboard lattice with doubly degenerate orbitals. Physical Review B - Condensed Matter and Materials Physics, 82(3).
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  Abstract: It has been suggested that the metal-insulator transitions in a number of spinel materials with partially filled t2g d orbitals can be explained as orbitally driven Peierls instabilities. Motivated by these suggestions, we examine theoretically the possibility of formation of such orbitally driven states within a simplified theoretical model, a two-dimensional checkerboard lattice with two-directional metal orbitals per atomic site. We include orbital ordering and interatom electron-phonon interactions self-consistently within a semiclassical approximation, and onsite intraorbital and interorbital electron-electron interactions at the Hartree-Fock level. We find a stable, orbitally induced Peierls bond-dimerized state for carrier concentration of one electron per atom. The Peierls bond distortion pattern continues to be period two bond dimerization even when the charge density in the orbitals forming the one-dimensional band is significantly smaller than 1. In contrast, for carrier density of half an electron per atom the Peierls instability is absent within one-electron theory as well as mean-field theory of electron-electron interactions, even for nearly complete orbital ordering. We discuss the implications of our results in relation to complex charge, bond, and orbital ordering found in spinels. © 2010 The American Physical Society.
• Li, H., Clay, R. T., & Mazumdar, S. (2010). The paired-electron crystal in the two-dimensional frustrated quarter-filled band. Journal of Physics Condensed Matter, 22(27).
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  PMID: 21399248;Abstract: The competition between antiferromagnetic and spin-singlet ground states within quantum spin models and the 1/2 -filled band Hubbard model has received intense scrutiny. Here we demonstrate a frustration-induced transition from Néel antiferromagnetism to a spin-singlet state in the interacting 1/4 -filled band on an anisotropic triangular lattice. While the antiferromagnetic state has equal charge densities, 0.5, on all sites, the spin-singlet state is a paired-electron crystal, with pairs of charge-rich sites separated by pairs of charge-poor sites. The paired-electron crystal provides a natural description of the spin-gapped state proximate to superconductivity in many organic charge transfer solids. Pressure-induced superconductivity in these correlated-electron systems is likely a result of a transition from the 1/4 -filled band valence bond solid to a valence bond liquid. © 2010 IOP Publishing Ltd.
• Clay, R. T., Mazumdar, S., & Li, H. (2009). Local singlets, frustration, and unconventional superconductivity in the organic charge-transfer solids. Physica B: Condensed Matter, 404(3-4), 487-489.
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  Abstract: We suggest that superconductivity (SC) in the organic charge-transfer solids (CTS) is reached from a bond-charge density wave (BCDW). We discuss an effective model for the BCDW to SC transition, an attractive U extended Hubbard Hamiltonian with repulsive nearest neighbor interaction V. We discuss experimental consequences of the theory for different classes of CTS superconductors as well as related inorganic materials. © 2008 Elsevier B.V. All rights reserved.
• Mazumdar, S., Clay, R. T., & Li, H. (2009). From valence bond solid to unconventional superconductivity in the organic charge-transfer solids. Synthetic Metals, 159(21-22), 2419-2421.
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  Abstract: We show that superconductivity is absent within the frac(1, 2)-filled band triangular lattice repulsive Hubbard model that has been proposed for organic charge-transfer solids. We posit that organic superconductivity is rather reached from a bond-charge density wave that either constitutes the insulating state proximate to superconductivity, or is extremely close in energy to the antiferromagnetic state, and replaces the latter under pressure. The bond-charge density wave can be described within an effective attractive U extended Hubbard Hamiltonian with repulsive nearest neighbor interaction V. A first-order transition from the insulating to the superconducting state occurs within the model with increasing frustration. © 2009 Elsevier B.V.
• Psiachos, D., & Mazumdar, S. (2009). Correlated-electron description of the photophysics of thin films of π -conjugated polymers. Physical Review B - Condensed Matter and Materials Physics, 79(15).
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  Abstract: We extend the Mulliken theory of ground-state charge transfer in a donor-acceptor complex to excited-state charge transfer between pairs of identical π -conjugated oligomers, one of which is in the optically excited state and the other is in the ground state, leading to the formation of a charge-transfer exciton. Within our theory, optical absorptions from the charge-transfer exciton should include a low-energy intermolecular charge-transfer excitation, as well as distinct intramolecular excitations from both the neutral delocalized exciton component and the Coulombically bound polaron-pair component of the charge-transfer exciton. We report high-order configuration-interaction calculations for pairs of oligomers of polyparaphenylenevinylene (PPV) that go beyond our previous single configuration-interaction calculation and find all five excited-state absorptions predicted using heuristic arguments based on the Mulliken concept. Our calculated excited-state absorption spectrum exhibits strong qualitative agreement with the complete wavelength-dependent ultrafast photoinduced absorption in films of PPV derivatives, suggesting that a significant fraction of the photoinduced absorption here is from the charge-transfer exciton. We make detailed comparisons to experiments and a testable experimental prediction. © 2009 The American Physical Society.
• Wang, Z., Psiachos, D., Badilla, R. F., & Mazumdar, S. (2009). Electron-electron interaction effects on the photophysics of metallic single-walled carbon nanotubes. Journal of Physics Condensed Matter, 21(9).
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  PMID: 21817382;Abstract: Single-walled carbon nanotubes are strongly correlated systems with large Coulomb repulsion between two electrons occupying the same pz orbital. Within a molecular Hamiltonian appropriate for correlated π-electron systems, we show that optical excitations polarized parallel to the nanotube axes in the so-called metallic single-walled carbon nanotubes are excitons. Our calculated absolute exciton energies in twelve different metallic single-walled carbon nanotubes, with diameters in the range 0.8-1.4 nm, are in nearly quantitative agreement with experimental results. We have also calculated the absorption spectrum for the (21, 21) single-walled carbon nanotube in the E 22 region. Our calculated spectrum gives an excellent fit to the experimental absorption spectrum. In all cases our calculated exciton binding energies are only slightly smaller than those of semiconducting nanotubes with comparable diameters, in contradiction to results obtained within the ab initio approach, which predicts much smaller binding energies. We ascribe this difference to the difficulty of determining the behavior of systems with strong on-site Coulomb interactions within theories based on the density functional approach. As in the semiconducting nanotubes we predict in the metallic nanotubes a two-photon exciton above the lowest longitudinally polarized exciton that can be detected by ultrafast pump-probe spectroscopy. We also predict a subgap absorption polarized perpendicular to the nanotube axes below the lowest longitudinal exciton, blueshifted from the exact midgap by electron-electron interactions. © 2009 IOP Publishing Ltd.
• Clay, R. T., Li, H., & Mazumdar, S. (2008). Absence of superconductivity in the half-filled band hubbard model on the anisotropic triangular lattice. Physical Review Letters, 101(16).
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  Abstract: We report exact calculations of magnetic and superconducting pair-pair correlations for the half-filled band Hubbard model on an anisotropic triangular lattice. Our results for the magnetic phases are similar to those obtained with other techniques. The superconducting pair-pair correlations at distances beyond nearest neighbor decrease monotonically with increasing Hubbard interaction U for all anisotropy, indicating the absence of frustration-driven superconductivity within the model. © 2008 The American Physical Society.
• Mazumdar, S., & Clay, R. T. (2008). Quantum critical transition from charge-ordered to superconducting state in the negative- U extended Hubbard model on a triangular lattice. Physical Review B - Condensed Matter and Materials Physics, 77(18).
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  Abstract: We demonstrate a robust frustration-driven charge order to superconductivity transition in the half-filled negative- U extended Hubbard model. Superconductivity extends over a broad region of the parameter space. We argue that the model provides the correct insight to understanding unconventional superconductivity in the organic charge-transfer solids and other quarter-filled systems. © 2008 The American Physical Society.
• Musfeldt, J. L., Brown, S., Mazumdar, S., Clay, R. T., Mas-Torrent, M., Rovira, C., Dias, J. C., Henriques, R. T., & Almeida, M. (2008). Infrared investigation of the charge ordering pattern in the organic spin ladder candidate (DTTTF)₂Cu(mnt)₂. Solid State Sciences, 10(12), 1740-1744.
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  Abstract: We measured the variable temperature infrared response of the spin ladder candidate (DTTTF)2Cu(mnt)2 in order to distinguish between two competing ladder models, rectangular versus zigzag, proposed for this family of materials. The distortion along the stack direction below 235 K is consistent with a doubling along b through the metal-insulator transition. While this would agree with either of the ladder models, the concomitant transverse distortion rules out the rectangular ladder model and supports the zigzag scenario. Intramolecular distortions within the DTTTF building block molecule also give rise to on-site charge asymmetry.
• Wang, Z., Mazumdar, S., & Shukla, A. (2008). Photophysics of charge-transfer excitons in thin films of π -conjugated polymers. Physical Review B - Condensed Matter and Materials Physics, 78(23).
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  Abstract: We develop a theory of the electronic structure and photophysics of interacting chains of π -conjugated polymers to understand the differences between solutions and films. While photoexcitation generates only the intrachain exciton in solutions, the optical exciton as well as weakly allowed charge-transfer excitons are generated in films. We extend existing theories of the lowest polaron pair and charge-transfer excitons to obtain descriptions of the excited states of these interchain species, and we show that a significant fraction of ultrafast photoinduced absorptions in films originates from the lowest charge-transfer exciton. Our proposed mechanism explains the simultaneous observation of polaronlike-induced absorption features peculiar to films in ultrafast spectroscopy and the absence of mobile charge carriers as deduced from other experiments. We also show that there is a 1:1 correspondence between the essential states that describe the photophysics of single chains and of interacting chains that constitute thin films. © 2008 The American Physical Society.
• Cao, J., Musfeldt, J. L., Mazumdar, S., Chemova, N. A., & Whittingham, M. S. (2007). Pinned low-energy electronic excitation in metal-exchanged vanadium oxide nanoscrolls. Nano Letters, 7(8), 2351-2355.
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  PMID: 17628114;Abstract: We measured the optical properties of mixed valent vanadium oxide nanoscrolls and their metal-exchanged derivatives in order to investigate the charge dynamics in these compounds. In contrast to the prediction of a metallic state for the metal-exchanged derivatives within a rigid band model, we find that the injected charges in Mn2+-exchanged vanadium oxide nanoscrolls are pinned. A low-energy electronic excitation associated with the pinned carriers appears in the far infrared and persists at low temperature, suggesting that the nanoscrolls are weak metals in their bulk form, dominated by inhomogeneous charge disproportionation and Madelung energy effects. © 2007 American Chemical Society.
• Clay, R. T., Hardikar, R. P., & Mazumdar, S. (2007). Temperature-driven transition from the Wigner crystal to the bond-charge-density wave in the quasi-one-dimensional quarter-filled band. Physical Review B - Condensed Matter and Materials Physics, 76(20).
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  Abstract: It is known that within the interacting electron model Hamiltonian for the one-dimensional 1 4 -filled band, the singlet ground state is a Wigner crystal only if the nearest-neighbor electron-electron repulsion is larger than a critical value. We show that this critical nearest-neighbor Coulomb interaction is different for each spin subspace, with the critical value decreasing with increasing spin. As a consequence, with the lowering of temperature, there can occur a transition from a Wigner crystal charge-ordered state to a spin-Peierls state that is a bond-charge-density wave with charge occupancies different from the Wigner crystal. This transition is possible because spin excitations from the spin-Peierls state in the 1 4 -filled band are necessarily accompanied by changes in site charge densities. We apply our theory to the 1 4 -filled band quasi-one-dimensional organic charge-transfer solids, in general, and to 2:1 tetramethyltetrathiafulvalene (TMTTF) and tetramethyltetraselenafulvalene cationic salts, in particular. We believe that many recent experiments strongly indicate the Wigner crystal to bond-charge-density Wave transition in several members of the TMTTF family. We explain the occurrence of two different antiferromagnetic phases but a single spin-Peierls state in the generic phase diagram for the 2:1 cationic solids. The antiferromagnetic phases can have either the Wigner crystal or the bond-charge-spin-density wave charge occupancies. The spin-Peierls state is always a bond-charge-density wave. © 2007 The American Physical Society.
• Stafford, C. A., Cardamone, D. M., & Mazumdar, S. (2007). The quantum interference effect transistor. Nanotechnology, 18(42).
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  PMID: 21730447;Abstract: We give a detailed discussion of the quantum interference effect transistor (QuIET), a proposed device which exploits the interference between electron paths through aromatic molecules to modulate the current flow. In the off state, perfect destructive interference stemming from the molecular symmetry blocks the current, while in the on state, the current is allowed to flow by locally introducing either decoherence or elastic scattering. Details of a model calculation demonstrating the efficacy of the QuIET are presented, and various fabrication scenarios are proposed, including the possibility of using conducting polymers to connect the QuIET with multiple leads. © IOP Publishing Ltd.
• Wang, Z., Zhao, H., & Mazumdar, S. (2007). π -electron theory of transverse optical excitons in semiconducting single-walled carbon nanotubes. Physical Review B - Condensed Matter and Materials Physics, 76(11).
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  Abstract: We present a quantitative theory of optical absorption polarized transverse to the tube axes in semiconducting single-walled carbon nanotubes. Within one-electron theory, transverse optical absorption occurs at an energy that is exactly in the middle of the two lowest longitudinal absorption energies. For nonzero Coulomb interactions between the π electrons, transverse optical absorption is to an exciton state that is strongly blueshifted relative to the longitudinal excitons. Very similar behavior is observed in the π -conjugated polymer polyparaphenylenevinylene, where the optical absorption polarized predominantly perpendicular to the polymer chain axis is blueshifted relative to the absorptions polarized predominantly along the chain axis. The binding energy of the transverse exciton in the nanotubes is considerably smaller than those of the longitudinal excitons. Electron-electron interactions also reduce the relative oscillator strength of the transverse optical absorption. Our theoretical results are in excellent agreement with recent experimental measurements in four chiral nanotubes. © 2007 The American Physical Society.
• Zhao, H., & Mazumdar, S. (2007). Elucidation of the electronic structure of semiconducting single-walled carbon nanotubes by electroabsorption spectroscopy. Physical Review Letters, 98(16).
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  Abstract: We report benchmark calculations of electroabsorption in semiconducting single-walled carbon nanotubes that provide motivation to perform electroabsorption measurement on these systems. We show that electroabsorption can detect the continuum bands in different energy manifolds. Direct determination of the binding energies of excitons in higher manifolds thereby becomes possible. We also find that electroabsorption can provide evidence for Fano-type coupling between the second exciton and the lowest continuum band states. © 2007 The American Physical Society.
• Cardamone, D. M., Stafford, C. A., & Mazumdar, S. (2006). Controlling quantum transport through a single molecule. Nano Letters, 6(11), 2422-2426.
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  PMID: 17090067;Abstract: We investigate multiterminal quantum transport through single monocyclic aromatic annulene molecules, and their derivatives, using the nonequilibrium Green function approach within the self-consistent Hartree-Fock approximation. We propose a new device concept, the quantum interference effect transistor, that exploits perfect destructive interference stemming from molecular symmetry and controls current flow by introducing decoherence and/or elastic scattering that break the symmetry. This approach overcomes the fundamental problems of power dissipation and environmental sensitivity that beset nanoscale device proposals. © 2006 American Chemical Society.
• Clay, R. T., & Mazumdar, S. (2006). Charge ordering and spin gap transitions in quarter-filled ladders. Journal of Low Temperature Physics, 142(3-4), 365-370.
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  Abstract: Several new families of charge transfer solids with ladder-like structure show unusually high spin gap transition temperatures. We argue that these materials are best described by a 1/4-filled ladder with zigzag interstack bonds. We find that in the 1/4-filled zigzag ladder, a cooperative bond-charge ordering leads to the formation of local singlets and a spin gap. The spin gap for the zigzag ladder is an order of magnitude larger than that found in equivalent one dimensional systems. © Springer Science+Business Media, Inc. 2006.
• Wang, Z., Zhao, H., & Mazumdar, S. (2006). Quantitative calculations of the excitonic energy spectra of semiconducting single-walled carbon nanotubes within a π -electron model. Physical Review B - Condensed Matter and Materials Physics, 74(19).
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  Abstract: Using Coulomb correlation parameters appropriate for π -conjugated polymers (PCPs), and a nearest neighbor hopping integral that is arrived at by fitting the energy spectra of three zigzag semiconducting single-walled carbon nanotubes (S-SWCNTs), we are able to determine quantitatively the exciton energies and exciton binding energies of 29 S-SWCNTs within a semiempirical π -electron Hamiltonian that has been widely used for PCPs. Our work establishes the existence of a deep and fundamental relationship between PCPs and S-SWCNTs. © 2006 The American Physical Society.
• Yan, Y., Mazumdar, S., & Ramasesha, S. (2006). Dimerization-induced enhancement of the spin gap in the quarter-filled two-leg rectangular ladder. Physical Review B - Condensed Matter and Materials Physics, 73(22).
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  Abstract: We report density-matrix renormalization group calculations of spin gaps in the quarter-filled correlated two-leg rectangular ladder with bond-dimerization along the legs of the ladder. In the small rung-coupling region, dimerization along the leg bonds can lead to large enhancement of the spin gap. Electron-electron interactions further enhance the spin gap, which is nonzero for all values of the rung electron hopping and for arbitrarily small bond-dimerization. Very large spin gaps, as are found experimentally in quarter-filled band organic charge-transfer solids with coupled pairs of quasi-one-dimensional stacks, however, occur within the model only for large dimerization and rung electron hopping that are nearly equal to the hopping along the legs. Coexistence of charge order and spin gap is also possible within the model for not too large intersite Coulomb interactions. © 2006 The American Physical Society.
• Zhao, H., Mazumdar, S., Sheng, C. -., Tong, M., & Vardeny, Z. V. (2006). Photophysics of excitons in quasi-one-dimensional organic semiconductors: Single-walled carbon nanotubes and π -conjugated polymers. Physical Review B - Condensed Matter and Materials Physics, 73(7).
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  Abstract: The nature of the primary photoexcitations in semiconducting single-walled carbon nanotubes (S-SWCNTs) is of strong current interest. We have studied the emission spectra of S-SWCNTs and two different π -conjugated polymers in solutions and films, and have also performed ultrafast pump-probe spectroscopy on these systems with unprecedented spectral range from 0.1 to 2.6 eV. The emission spectra relative to the absorption bands are very similar in S-SWCNTs and polymers, with redshifted photoluminescence in films showing exciton migration. We also found that the transient excited state spectra of both polymers and SWCNTs contain two prominent photoinduced absorption (PA) bands (PA1 and PA2) that are due to photogenerated excitons; in the polymers these PA bands are correlated with a stimulated emission band, which is absent in the S-SWCNTs. In order to understand the similarities in the PA spectra we have performed theoretical calculations of excited state absorptions in π -conjugated polymers as well as S-SWCNTs within the same correlated electron Hamiltonian. We find strong similarities in the excitonic energy spectra of these two classes of quasi-one-dimensional materials, although there exist also subtle differences such as the occurrence of dark excitons below the optical excitons in the S-SWCNTs. In the polymers PA1 is an excited state absorption from the optical exciton to a two-photon exciton that occurs below the continuum band threshold. In the S-SWCNTs PA1 occurs from both the optical exciton and the dark exciton, to final states which are close in energy and again below the continuum band threshold. PA1 therefore gives the lower limit of the binding energy of the lowest optical exciton in both π -conjugated polymers and S-SWCNTs. The binding energy of lowest exciton that belongs to the widest S-SWCNTs with diameters ≥1 nm in films is 0.3-0.4 eV, as determined by both experimental and theoretical methods. © 2006 The American Physical Society.
• Clay, R. T., & Mazumdar, S. (2005). Cooperative density wave and giant spin gap in the quarter-filled zigzag electron ladder. Physical Review Letters, 94(20).
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  Abstract: Strong cooperative interactions occur between four different broken symmetries involving charge ordering and bond distortions in the quarter-filled correlated zigzag electron ladder. The ground state is singlet, with spin gap several times larger than in the spin-Peierls state of the one-dimensional quarter-filled chain with the same parameters. We propose the quarter-filled zigzag electron ladder model for several different organic charge transfer solids with coupled pairs of quasi-one-dimensional stacks, in which the spin-gap transition temperatures are unusually high. © 2005 The American Physical Society.
• Clay, R. T., & Mazumdar, S. (2005). Magnetism in BEDT-TTF materials. Synthetic Metals, 153(1-3), 445-448.
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  Abstract: Strong commensurate antiferromagnetism proximate to superconductivity is found in some members of the κ-(ET) family, while a spin gap (SG) is found in the θ-(ET). Both κ- and θ-(ET) materials have frustrated triangular lattice structures. We show from calculations of spin-spin correlations within the effective half-filled band triangular lattice proposed for the κ-ET, as well as for the real lattice, that long range AFM order is not obtained as a consequence of this frustration. We argue that some other mechanism reduces the magnetic frustration in these systems. We show that the low temperature magnetic states in these materials can only be understood if the effects of the cooperative charge and bond ordering transitions occurring at higher temperatures in these systems are taken into account. In the κ-ET, this co-operative transition leads to unequal hole populations on the ET dimers that form the triangular lattice. © 2005 Elsevier B.V. All rights reserved.
• Das, M., Ramasesha, S., & Mazumdar, S. (2005). Role of electron-electron interactions on spin effects in electron-hole recombination in organic light emitting diodes. Synthetic Metals, 155(2), 270-273.
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  Abstract: We investigate the possibility that high-energy singlet and triplet excited states with large electron-hole separations are generated in electron-hole recombination process, over and above the lowest singlet and triplet excitons. Using a time-dependent calculation of the interchain/intermolecular charge-transfer within model Hamiltonians that explicitly include electron-electron interactions between the π-electrons, we show that there occurs a bifurcation of the electron-hole recombination path in each of the two spin channels that leads to the generation of both the lowest energy exciton and a specific-high-energy charge-transfer state, with the matrix elements favoring the lowest energy exciton and the energy difference factor favoring the higher energy state, resulting in an enhancement of the singlet:triplet yield ratio above the value of 0.25 predicted from statistical considerations. © 2005 Elsevier B.V. All rights reserved.
• Mazumdar, S., & Clay, R. T. (2005). Charge ordering and local-singlet formation in quarter-filled band charge-transfer solids and oxides of early transition metals. Journal De Physique. IV : JP, 131, 63-.
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  Abstract: In many 1/4-filled band nonsuperconducting charge-transfer solids (CTS), such as the θ-(ET)2X and the "ladder" compounds (DT-TTF)2Au(mnt)2 a charge-ordering (CO) transition is followed by a transition to a spin-gap (SG) state. This phenomenon is of interest because (a) SG in dimensionality greater than 1 is relatively rare, and (b) similar SG transitions, preceded or accompanied by a CO transition, are also seen in 1/4-filled band oxides of early transition metals such as Ti 4O7 and α-NaV2O5. The mechanisms of the SG formations are not understood and are often controversial. We present a global perspective of the CO transitions in 1/4-filled band systems. We report calculations incorporating both electron-electron and electron-phonon interactions in 1/4-filled band (a) 1D chains (b) 2D rectangular and triangular lattices, and (c) zigzag and rectangular ladders. We show that CO in many 1/4-filled band materials involves the formation of "local dimers" or "intersite bipolarons" that are bound into a singlet state, thereby giving a SG. Our work presents a new mechanism for singlet formation in a wide variety of exotic semiconductors below the metal-semiconductor transition. We discuss the above CTS and oxides in the context of the theory, and speculate on the possible role of local dimers in the superconductivity observed in structurally related CTS and Ti- and V-oxides. © EDP Sciences.
• Yan, Y., & Mazumdar, S. (2005). Density matrix renormalization group study of conjugated polymers with transverse π-conjugation. Physical Review B - Condensed Matter and Materials Physics, 72(21).
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  Abstract: We report accurate numerical studies of excited state orderings in long hypothetical π-conjugated oligomers in which the hydrogen atoms of transpolyacetylene are replaced with conjugated side groups, within modified Hubbard models. There exists a range of the bare Coulomb repulsion for which the excited state ordering is conducive to photoluminescence in the substituted systems, even as this ordering is opposite in the unsubstituted polyenes of the same lengths. Our work provides motivation to study real π-conjugated polymers with transverse conjugation and small optical gaps. © 2005 The American Physical Society.
• Zhao, H., & Mazumdar, S. (2005). Excitons in semiconducting single-walled carbon nanotubes. Synthetic Metals, 155(2), 250-253.
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  Abstract: We report correlated-electron calculations of optically excited states in 10 semiconducting single-walled carbon nanotubes with a wide range of diameters. Optical excitation occurs to excitons whose binding energies decrease with the increasing nanotube diameter, and are smaller than the binding energy of an isolated strand of poly-(paraphenylene vinylene). The ratio of the energy of the second optical exciton polarized along the nanotube axis to that of the lowest exciton is smaller than the value predicted within single-particle theory. The experimentally observed weak photoluminescence is an intrinsic feature of semiconducting nanotubes, and is consequence of dipole-forbidden excitons occurring below the optical exciton. Excited states absorption calculations show photoinduced absorption energies are lower than or comparable to the binding energy of the lowest exciton. © 2005 Elsevier B.V. All rights reserved.
• Shukla, A., Ghosh, H., & Mazumdar, S. (2004). Ultrafast excited state absorption and charge separation in phenylene-based conjugated polymers. Synthetic Metals, 141(1-2), 59-65.
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  Abstract: Ultrafast pump-probe spectroscopy of phenylene-based π-conjugated polymers give low and high energy photoinduced absorptions (PAs) to final states with very different relaxation behavior. While the low energy two-photon state that is reached in PA relaxes to the optical exciton in ultrafast times, the higher energy two-photon state violates the Vavilov-Kasha rule and instead of relaxing to the optical exciton undergoes interchain dissociation to a polaron pair state. We have calculated the excited state absorptions of long oligomers of poly-paraphenylene and poly-paraphenylenevinylene within a rigid band correlated electron Hamiltonian, using the complete π-electron basis and the powerful multi-reference singles and double configuration interaction technique. Our calculated PA spectra well reproduce the experimental PA. We further show that there exists a fundamental difference in the natures of the wavefunctions of the final states that are reached in low and high energy PA, and the difference originates from the existence of multiple kinds of bands in phenylene-based systems within one-electron theory. We speculate that the violation of the Vavilov-Kasha rule and charge separation into polaron pairs upon excitation to the high energy two-photon state is a consequence of its specific electronic structure, and discuss an earlier work on wavelength dependent photoconductivity of poly-paraphenylenevinylene to support this conjecture. © 2003 Elsevier B.V. All rights reserved.
• Zhao, H., & Mazumdar, S. (2004). Electron-electron interaction effects on the optical excitations of semiconducting single-walled carbon nanotubes. Physical Review Letters, 93(15), 157402-1-157402-4.
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  PMID: 15524940;Abstract: Correlated-electron calculations of optically excited states in ten semiconducting single-walled carbon nanotubes with a wide range of diameters were discussed. It was shown that the optical excitation occurs to excitons whose binding energies decrease with increasing nanotube diameter and are smaller than the binding energy of an isolated strand of poly-(paraphenylene vinylene). It was found that the ratio of the second optical exciton polarized along the nanotube axis to that of the lowest exciton is smaller than the value predicted within single-particle theory. The results show that the experimentally observed weak photoluminescence is an intrinsic feature of semiconducting nanotubes.
• Clay, R. T., Campbell, D. K., & Mazumdar, S. (2003). Charge order in quasi-one-dimensional organic charge-transfer solids. Synthetic Metals, 135-136, 681-682.
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  Abstract: We examine two proposed models of the observed charge ordering (CO) in the quasi-1D, 1/4-filled organic charge transfer solids. The models are characterized by the site charge densities ..1010.. and ..1100.. respectively. Using a Peierls-extended Hubbard model to include both electron-electron and electron-phonon interactions, we show that 1) the ..1010..CO can occur only when the nearest-neighbor Coulomb repulsion is sufficiently strong; 2) a spin-Peierls state can coexist with the ..1010..CO; and 3) experimental evidence clearly supports ..1100.. CO in the (TMTSP) and the 1:2 anionic CTS, while in the (TMTTF) salts the nature of the order is not yet clear. © 2003 Published by Elsevier Science B.V.
• Clay, R. T., Mazumdar, S., & Campbell, D. K. (2003). Pattern of charge ordering in quasi-one-dimensional organic charge-transfer solids. Physical Review B - Condensed Matter and Materials Physics, 67(11), 1151211-1151219.
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  Abstract: We examine two recently proposed models of charge ordering (CO) in the nominally 1/4-filled, quasi-onedimensional (1D) organic charge-transfer solids (CTS). The two models are characterized by site charge density "cartoons" ... 1010 ... and ... 1100 ..., respectively. We use the Peierls-extended Hubbard model to incorporate both electron-electron (e-e) and electron-phonon (e-ph) interactions. We first compare the results, for the purely electronic Hamiltonian, of exact many-body calculations with those of Hartree-Fock (HF) mean-field theory. We find that HF gives qualitatively and quantitatively incorrect values for the critical nearest-neighbor Coulomb repulsion (Vc) necessary for ... 1010 ... order to become the ground state. Second, we establish that spin-Peierls order can occur in either the ... 1100 ... and ... 1010 ... states and calculate the phase diagram including both on-site and intrasite e-ph interactions. Third, we discuss the expected temperature dependence of the CO and metal-insulator transitions for both ... 1010 ... and ... 1100 ... CO states. Finally, we show that experimental observations clearly indicate the ... 1100 ... CO in the 1:2 anionic CTS and the (TMTSF)2X materials, while the results for (TMTTF)2X with narrower one-electron bandwidths are more ambiguous, likely because the nearest-neighbor Coulomb interaction in these materials is near Vc.
• Dallakyan, S., & Mazumdar, S. (2003). Sub-Poissonian shot noise in molecular wires. Applied Physics Letters, 82(15), 2488-2490.
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  Abstract: The transport behavior of polyene molecules sandwiched between two metallic contacts was discussed using the nonequilibrium Green's function formalism. The current and noise power as a function of applied voltage were calculated and they were found to decrease with increasing size of the polyene molecules. The analysis showed a crossover from Poissonian to sub-Possonian behavior in the shot noise as a function of applied voltage.
• Dallakyan, S., Chandross, M., & Mazumdar, S. (2003). Infrared light emission from π-conjugated polymers: A diagrammatic exciton basis valence bond theory. Physical Review B - Condensed Matter and Materials Physics, 68(7), 752041-7520410.
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  Abstract: There is currently a great need for solid-state lasers that emit in the infrared, as this is the operating wavelength regime for applications in telecommunications. Existing π-conjugated polymers all emit in the visible or ultraviolet, and whether or not π-conjugated polymers that emit in the infrared can be designed is an interesting challenge. On the one hand, the excited-state ordering in trans-polyacetylene, the π-conjugated polymer with a relatively small optical gap, is not conducive to light emission because of electron-electron interaction effects. On the other hand, excited-state ordering opposite to that in trans-polyacetylene is usually obtained by chemical modification that increases the effective bond alternation, which in turn increases the optical gap. We develop a theory of electron correlation effects in a model π-conjugated polymer that is obtained by replacing the hydrogen atoms of trans-polyacetylene with transverse conjugated groups and show that the effective on-site correlation in this system is smaller than the bare correlation in the unsubstituted system. An optical gap in the infrared as well as excited-state ordering conducive to light emission is thereby predicted with similar structural features.
• Mazumdar, S., Clay, R. T., & Campbell, D. K. (2003). The ubiquitous 1100 charge ordering in organic charge-transfer solids. Synthetic Metals, 137(1-3), 1317-1319.
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  Abstract: Charge and spin-orderings in the 1/4-filled organic CT solids are of strong interest, especially in view of their possible relations to organic superconductivity. We show that the charge order (CO) in both ID and 2D CT solids is of the ...1100... type, in contradiction to mean field prediction of ...1010... CO. We present detailed computations for metal-insulator and magnetic insulator-insulator transitions in the θ-ET materials. Complete agreement with experiments in several θ systems is found. Similar comparisons between theory and experiments in TCNQ, TMTTF, TMTSF, and ET materials prove the ubiquity of this phenomenon.
• Mazumdar, S., Dallakyan, S., & Chandross, M. (2003). Designing π-conjugated polymers with light emission in the infrared. Synthetic Metals, 139(3), 769-772.
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  Abstract: There is currently a great need for solid state lasers that emit in the infrared. Whether or not conjugated polymers that emit in the IR can be synthesized is an interesting theoretical challenge. We show that emission in the IR can be achieved in designer polymers in which the effective Coulomb correlation is smaller than that in existing systems. We also show that the structural requirement for having small effective Coulomb correlations is that there exist transverse π-conjugation over a few bonds in addition to longitudinal conjugation with large conjugation lengths. © 2003 Elsevier Science B.V. All rights reserved.
• Ramasesha, S., Mazumdar, S., Tandon, K., & Das, M. (2003). Electron correlation effects in electron-hole recombination and triplet-triplet scattering in organic light emitting diodes. Synthetic Metals, 139(3), 917-920.
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  Abstract: Using a time-dependent model many-body formulation, we follow the electron-hole recombination between a pair of oppositely charged polyene chains and triplet-triplet (T-T) scattering in π-conjugated systems. Electron correlations reduce the overall yields and also lead to large differences between the singlet and triplet yields. External electric field has a strong influence on the recombination products. The fraction of singlet yield, η, increases with the chain length. Heteroatoms also change this ratio significantly, as has been observed experimentally. The outcome of the triplet-triplet scattering is also significantly affected by electron correlations. © 2003 Elsevier Science B.V. All rights reserved.
• Shukla, A., Ghosh, H., & Mazumdar, S. (2003). Theory of excited-state absorption in phenylene-based π-conjugated polymers. Physical Review B - Condensed Matter and Materials Physics, 67(24), 2452031-2452039.
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  Abstract: Within a rigid-band correlated electron model for oligomers of poly-(paraphenylene) (PPP) and poly(paraphenylenevinylene) (PPV), we show that there exist two fundamentally different classes of two-photon Ag states in these systems to which photoinduced absorption (PA) can occur. At relatively lower energies there occur Ag states which are superpositions of one electron-one hole (1e-1h) and two electron-two hole (2e-2h) excitations, that are both comprised of the highest delocalized valence-band and the lowest delocalized conduction-band states only. The dominant PA is to one specific member of this class of states (the mAg). In addition to the above class of Ag states, PA can also occur to a higher energy kAg state whose 2e-2h component is different and has significant contributions from excitations involving both delocalized and localized bands. Our calculated scaled energies of the mAg and the kAg agree reasonably well to the experimentally observed low- and high-energy PA's in PPV. The calculated relative intensities of the two PA's are also in qualitative agreement with experiment. In the case of ladder-type PPP and its oligomers, we predict from our theoretical work an intense PA at an energy considerably lower than the region where PA's have been observed currently. Based on earlier work that showed that efficient charge-carrier generation occurs upon excitation to odd-parity states that involve both delocalized and localized bands, we speculate that it is the characteristic electronic nature of the kAg that leads to charge generation subsequent to excitation to this state, as found experimentally.
• Tandon, K., Ramasesha, S., & Mazumdar, S. (2003). Electron correlation effects in electron-hole recombination in organic light-emitting diodes. Physical Review B - Condensed Matter and Materials Physics, 67(4), 451091-4510919.
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  Abstract: We develop a general theory of electron-hole recombination in organic light-emitting diodes that leads to formation of emissive singlet excitons and nonemissive triplet excitons. We briefly review other existing theories and show how our approach is substantively different from these theories. Using an exact timedependent approach to the interchain/intermolecular charge transfer within a long-range interacting model we find that (i) the relative yield of the singlet exciton in polymers is considerably larger than the 25% predicted from statistical considerations, (ii) the singlet exciton yield increases with chain length in oligomers, and (iii) in small molecules containing nitrogen heteroatoms, the relative yield of the singlet exciton is considerably smaller and may be even close to 25%. The above results are independent of whether or not the bond-charge repulsion, X⊥, is included in the interchain part of the Hamiltonian for the two-chain system. The larger (smaller) yield of the singlet (triplet) exciton in carbon-based long-chain polymers is a consequence of both its ionic (covalent) nature and smaller (larger) binding energy. In nitrogen containing monomers, wave functions are closer to the noninteracting limit, and this decreases (increases) the relative yield of the singlet (triplet) exciton. Our results are in qualitative agreement with electroluminescence experiments involving both molecular and polymeric light emitters. The time-dependent approach developed here for describing intermolecular charge-transfer processes is completely general and may be applied to many other such processes.
• Ashida, M., Taguchi, Y., Tokura, Y., Clay, R. T., Mazumdar, S., Svirko, Y., & Kuwata-Gonokami, M. (2002). Dimensionality dependence of optical nonlinearity and relaxation dynamics in cuprates. Europhysics Letters, 58(3), 455-461.
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  Abstract: Femtosecond pump-probe measurements find pronounced dimensionality dependence of the optical nonlinearity in cuprates. Although the coherent two-photon absorption (TPA) and linear absorption bands nearly overlap in both quasi-one- and two-dimensional (1D and 2D) cuprates, the TPA coefficient is one order of magnitude smaller in 2D than in 1D. Furthermore, picosecond recovery of optical transparency is observed in 1D cuprates, while the recovery in 2D involves relaxation channels with a time scales of tens of picoseconds. The experimental results are interpreted within the two-band extended Hubbard model.
• Clay, R. T., Mazumdar, S., & Campbell, D. K. (2002). Charge Ordering in θ-(BEDT-TTF)₂X Materials. Journal of the Physical Society of Japan, 71(8), 1816-1819.
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  Abstract: We theoretically investigate charge ordered states on the anisotropic triangular lattice characteristic of the θ-(BEDT-TTF)2X materials. Using exact diagonalization studies, we establish that the charge order (CO) pattern corresponds to a "horizontal" stripe structure, with ... 1100... CO along the two directions with larger electron hopping (p-directions), and ... 1010... CO along the third direction (c-direction). The CO is accompanied by co-operative bond dimerizations along all three directions in the highest spin state. In the lowest spin state bonds along the p-directions are tetramerized. Our theory explains the occurrence of a charge-induced high temperature transition as well as a spin gap transition at lower temperature.
• Clay, R. T., Mazumdar, S., & Zhang, G. P. (2002). Comment on "origin of giant optical nonlinearity in charge-transfer-mott insulators: A new paradigm for nonlinear optics" [1] (multiple letters). Physical Review Letters, 89(3), 397011-397021.
• Ashida, M., Ogasawara, T., Motoyama, N., Eisaki, H., Uchida, S., Taguchi, Y., Tokura, Y., Kuwata-Gonokami, M., Ghosh, H., Shukla, A., & Mazumdar, S. (2001). Interband two-photon transition in Mott insulator as a new mechanism for ultrafast optical nonlinearity. International Journal of Modern Physics B, 15(28-30), 3628-3632.
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  Abstract: The dimensionality dependence of optical nonlinearity in cuprates was investigated by sub-picosecond pump-probe transmission measurements in the near-infrared region. It was found that cuprates with one-dimensional Cu-O networks show nonlinearity one order of magnitude larger than that of conventional band semiconductors and picosecond relaxation of the excited state. In contrast, a two-dimensional cuprate shows one order of magnitude smaller nonlinearity and slower decay of the excited state, as well as picosecond relaxation. The possibility for application of the present material to all-optical switching devices is also discussed.
• Ashida, M., Ogasawara, T., Tokura, Y., Uchida, S., Mazumdar, S., & Kuwata-Gonokami, M. (2001). One-dimensional cuprate as a nonlinear optical material for ultrafast all-optical switching. Applied Physics Letters, 78(19), 2831-2833.
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  Abstract: We examine the feasibility of ultrafast all-optical switching utilizing a one-dimensional cuprate Sr2CuO3 operable within the optical fiber communication window at room temperature by using the pump-probe spectroscopy and Z-scan measurements. The strength of the interband two-photon absorption in Sr2CuO3 is much larger than that of conventional semiconductors, and is comparable to the largest reported values in π-conjugated polymers. The intensity-dependent refractive index, however, is considerably larger than that of polymeric materials. We further show that the recovery of optical transparency after the photoinjection of carriers lies within picosecond time scale. Large nonlinearity, ultrafast response, and high damage threshold make the one-dimensional cuprate a potential material for multiterabits/second rate all-optical switch. © 2007 American Institute of Physics.
• Ashida, M., Ogasawara, T., Uchida, S., Tokura, Y., Kuwata-Gonokami, M., & Mazumdar, S. (2001). Low-dimensional perovskite copper oxide as a nonlinear optical material for all-optical switching. Conference on Lasers and Electro-Optics Europe - Technical Digest, 215-.
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  Abstract: The use of low dimensional perovskite copper oxide as a nonlinear optical material for all-optial switching (AOS) was demonstrated. Sub-picosecond photo-induced absorption change was measured for samples with different copper and oxygen networks at room temperature. The photo-induced absorption changes were found to consist of a prompt component and an exponentially decaying tail. The low-dimensional cuprates showed strong two-photon absorption (TPA) and picosecond ground state recovery at room temperature.
• Clay, R. T., Mazumdar, S., & Campbell, D. K. (2001). Re-integerization of fractional charges in the correlated quarter-filled band. Physical Review Letters, 86(18), 4084-4087.
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  PMID: 11328101;Abstract: A study was conducted to demonstrate that charge fractionalization does not occur in the low temperature 2kF spin-Peierls (SP) phase of the interacting 1D 1/4-filled band. It was shown that the charge ordering associated with the SP tetramerization causes a "re-integerization" of the fractionally charged defects that occur within the 4kF dimerized phase.
• Ghosh, H., Mazumdar, S., & Shukla, A. (2001). Electron correlation and photo physics of phenyl substituted polyacetylenes. International Journal of Modern Physics B, 15(19-20), 2793-2798.
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  Abstract: We investigate theoretically photoluminescence (PL) properties of mono and di-phenyl substituted trans-polyacetylene (t-PA), namely, poly-phenylacetylene (PPA) and polydiphenylacetylene (PDPA), respectively. PL is a consequence of the occurrence of the two-photon state (2Ag) above the optically excited state (1Bu). Usually, electron correlation leads to confinement of 1Bu as well as the ground state. We show, in contrast, that in phenyl-substituted polyacetylenes electron-electron interactions cause enhanced delocalization of quasiparticles in the optically excited state from the backbone polyene chain into the phenyl groups. This coulomb enhanced delocalization in the transverse direction leads to confinement in the longitudinal direction and causes crossover between the 1Bu and 2Ag. We further show that in the absence of electron repulsion the low energy absorption is x-polarized whereas the high energy absorption is predominantly y-polarized. In contrast, in presence of many body electron interaction both of them (low and high energy absorptions) are x-polarized. Thus photophysics of PPA/PDPA etc. are consequences of true many body effects.
• Mazumdar, S., Clay, R. T., & Campbell, D. K. (2001). The nature of the insulating state in organic superconductors. Synthetic Metals, 120(1-3), 679-682.
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  Abstract: We review our recent proposal for a unified, microscopic, many-body theory for the 2:1 organic charge transfer solids (CTS), which range from nearly 1D to strongly 2D in electronic character. Our theory predicts an insulating bond-charge density wave (BCDW) ground state in the limit of exact 1/4-filling for all anisotropies, consistent with experimental observations of coexisting density waves in the real materials. For weakly 2D materials, we find that a spin-density wave (SDW) coexists with the BCDW; the resulting BCSDW is again consistent with experiments. In contrast, both in the 1D limit and in the strongly 2D regime the systems can be in the nonmagnetic singlet state. We suggest that slightly away from exact 1/4-filling, commensurability defects can form pairs leading to superconductivity.
• Shukla, A., Ghosh, H., & Mazumdar, S. (2001). Photophysics of phenyl-substituted polyacetylenes, theory. Synthetic Metals, 116(1-3), 87-90.
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  Abstract: We present a theoretical study of the electronic structures of poly-phenylacetylene (PPA) and poly-diphenylacetylene (PDPA). Strong photoluminescence (PL) in PDPA's is a consequence of the occurrence of the 2Ag two-photon state above the optical 1Bu, in spite of the polyene backbone. This reversed energy ordering (as compared to linear polyenes and trans-polyacetylene, t-PA) is a signature of reduced effective electron correlations in PPA and PDPA. The optical gaps of the substituted materials are smaller than polyenes of the same chain length.
• Wohlgenannt, M., Tandon, K., Mazumdar, S., Ramasesha, S., & Vardeny, Z. V. (2001). Erratum: Formation cross-sections of singlet and triplet excitons in π-conjugated polymers (Nature (2001) 409 (494-497)). Nature, 411(6837), 617-.
• Wohlgenannt, M., Tandon, K., Mazumdar, S., Ramasesha, S., & Vardeny, Z. V. (2001). Formation cross-sections of singlet and triplet excitons in π-conjugated polymers. Nature, 409(6819), 494-497.
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  PMID: 11206541;Abstract: Electroluminescence in organic light-emitting diodes arises from a charge-transfer reaction between the injected positive and negative charges by which they combine to form singlet excitons that subsequently decay radiatively. The quantum yield of this process (the number of photons generated per electron or hole injected) is often thought to have a statistical upper limit of 25 per cent. This is based on the assumption that the formation cross-section of singlet excitons, σS, is approximately the same as that of any one of the three equivalent non-radiative triplet exciton states, σT; that is, σS/σT ≈ 1. However, recent experimental and theoretical work suggests that σS/σT may be greater than 1. Here we report direct measurements of σS/σT for a large number of π-conjugated polymers and oligomers. We have found that there exists a strong systematic, but not monotonic, dependence of σS/σT on the optical gap of the organic materials. We present a detailed physical picture of the charge-transfer reaction for correlated π-electrons, and quantify this process using exact valence bond calculations. The calculated σS/σT reproduces the experimentally observed trend. The calculations also show that the strong dependence of σS/σT on the optical gap is a signature of the discrete excitonic energy spectrum, in which higher energy excitonic levels participate in the charge recombination process.
• Ashida, M., Ogasawara, T., Motoyama, N., Eisaki, H., Uchida, S., Taguchi, Y., Tokura, Y., Kuwata-Gonokami, M., Ghosh, H., Shukla, A., & Mazumdar, S. (2000). Giant and ultrafast optical nonlinearity in low dimensional perovskite copper oxides. Conference on Quantum Electronics and Laser Science (QELS) - Technical Digest Series, 183-184.
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  Abstract: Discussed as peculiar optical response observed in quasi-one-dimensional (1D) cuprates, Sr2CuO3 and Sr-CuO2. Femtosecond pump-probe transmission spectra in the 1D cuprates are measured at 10-300 K. To change the pump and probe energies independently, two optical parametric oscillators, which are pumped with a regenerative amplifier and generate light pulses with approx. 200 fs with in the region of 0.3-20 μm.
• Ghosh, H., Shukla, A., & Mazumdar, S. (2000). Electron-correlation-induced transverse delocalization and longitudinal confinement in excited states of phenyl-substituted polyacetylenes. Physical Review B - Condensed Matter and Materials Physics, 62(19), 12763-12774.
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  Abstract: Electron-electron interactions in general lead to both ground-state and excited-state confinement. We show, however, that in phenyl-substituted polyacetylenes electron-electron interactions cause enhanced delocalization of quasiparticles in the optically excited state from the backbone polyene chain into the phenyl groups, which in turn leads to enhanced confinement in the chain direction. This cooperative delocalization confinement lowers the energy of the one-photon state and raises the relative energy of the lowest two-photon state. The two-photon state is slightly below the optical state in monophenyl-substituted polyacetylenes, but above the optical state in diphenyl-substituted polyacetylenes, thereby explaining the strong photoluminescence of the latter class of materials. We present a detailed mechanism of the crossover in the energies of the one- and two-photon states in these systems. In addition, we calculate the optical-absorption spectra over a wide wavelength region, and make specific predictions for the polarizations of low- and high-energy transitions that can be tested on oriented samples. Within existing theories of light emission from π-conjugated polymers, strong photoluminescence should be restricted to materials whose optical gaps are larger than that of transpolyacetylene. The present work shows that, conceptually at least, it is possible to have light emission from systems with smaller optical gaps.
• Mazumdar, S. (2000). Prospects for the polymer nanoengineer. Science, 288(5466), 630-631.
• Mazumdar, S., Clay, R. T., & Campbell, D. K. (2000). Bond-order and charge-density waves in the isotropic interacting two-dimensional quarter-filled band and the insulating state proximate to organic superconductivity. Physical Review B - Condensed Matter and Materials Physics, 62(20), 13400-13425.
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  Abstract: We report three surprising results regarding the nature of the spatial broken symmetries in the two-dimensional (2D), quarter-filled band with strong electron-electron interactions that provides a microscopic model of the 2:1 cationic organic charge-transfer solids (CTS's). First, in direct contradiction to the predictions of one-electron theory, we find a coexisting "bond-order and charge-density wave" (BCDW) insulating ground state in the 2D rectangular lattice for all anisotropies, including the isotropic limit. Second, in contrast to the interacting half-filled band, which exhibits one singlet-to-antiferromagnet (AFM) transition as the interchain coupling is increased from zero, there occur in the interacting quarter-filled band two distinct transitions: a similar singlet-to-antiferromagnet/spin-density wave (AFM/SDW) transition at small interchain coupling, giving rise to a bond-charge-spin density wave (BCSDW) state, followed by a second AFM/SDW-to-singlet transition at large interchain coupling. Third, we show that our conclusions remain unchanged if one assumes the conventional "effective 1/2-filled" lattice of dimer sites for the CTS's: the dimer lattice unconditionally dimerizes again to give the same BCDW found in the quarter-filled band. We make detailed comparisons to recent experiments in the tetramethyl-tetrathiafulvalene (TMTTF), tetramethyl-tetraselenafulvalene (TMTSF), bisethylenedithio-tetrathiafulvalene (BEDT-TTF) and bisethylenedithio-tetraselenafulvalene (BETS)-based CTS's. Our theory explains the mixed charge-spin density waves observed in TMTSF and certain BEDT-TTF systems, as well as the absence of antiferromagnetism in the BETS-based systems. An important consequence of this work is the suggestion that organic superconductivity is related to the proximate Coulomb-induced BCDW, with the SDW that coexists for large anisotropies being also a consequence of the BCDW, rather than the driver of superconductivity. We point out that the BCDW and BCSDW states are analogous to four different classes of "paired" semiconductors that are obtained within certain models of exotic superconductivity. That all four of these models can in principle give rise to superconductivity in the weakly incommensurate regime provides further motivation for the notion that the BCDW may be driving the superconductivity in the organics.
• Ogasawara, T., Ashida, M., Motoyama, N., Eisaki, H., Uchida, S., Tokura, Y., Ghosh, H., Shukla, A., Mazumdar, S., & Kuwata-Gonokami, M. (2000). Ultrafast optical nonlinearity in the quasi-one-dimensional Mott insulator Sr₂CuO₃. Physical Review Letters, 85(10), 2204-2207.
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  Abstract: The photoinduced absorption in the quasi-one-dimensional Mott insulator Sr2CuO3 in the infrared spectral region was studied. Theoretical calculations based on a two-band Hubbard model explain the experimental observations. Results indicated that the strong two-photon absorption is due to a very large dipole coupling. The potential of Sr2CuO3 for all-optical switching was demonstrated by the room temperature picosecond recovery of the optical transparency. The differential transmission was measured as a function of delay time. The relatively long-lived component of the transmission change is associated with the real excitation of the charge-transfer (CT) exciton.
• Chakrabarti, A., & Mazumdar, S. (1999). Theory of even-parity states in polyphenylenes. Physical Review B - Condensed Matter and Materials Physics, 59(7), 4839-4848.
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  Abstract: The diagrammatic exciton basis valence-bond approach, developed previously by us for obtaining physical, pictorial descriptions of correlated eigenstates of linear polyenes, is extended to the case of polyphenylenes. In linear chain polyenelike systems with large effective bond alternation, low-energy even-parity states are linear combinations of one-excitations that are charge transfer within the exciton representation and two-excitations that are composed of two triplets, which are localized on two different unit cells, and the spin angular momenta of which are coupled to give an overall singlet. This description of even-parity states gets modified substantially in the polyphenylenes, where the unit cell contains molecular orbitals that are delocalized over the entire unit cell as well as localized molecular orbitals with nonzero electron densities on only four of the six carbon atoms of each benzene molecule. We show that, as a consequence, there occur multiple classes of triplet-triplet two-excitations in the polyphenylenes, involving both the delocalized and localized molecular orbitals. We further show that all triplet-triplet two-excitations mix with the one-excitation charge-transfer type configurations that involve only the delocalized molecular orbitals, giving rise to different classes of two-photon allowed states. Low-energy two-photon states are predominantly composed of one-excitation charge-transfer configurations and triplet-triplet configurations involving the delocalized molecular orbitals. At higher energy, there occur two-photon states consisting of the same one-excitations, but triplet-triplet configurations that involve both delocalized and localized molecular orbitals. The high-energy two-photon states in polyphenylenes therefore cannot be obtained within effective linear chain models that retain only the delocalized molecular orbitals, and will also be missed unless electron correlations are taken into account to high order. Low- and high-energy photoinduced transitions in poly(para-phenylenevinylenes) are assigned to transitions from the optical exciton to these two distinct classes of two-photon states. The current assignment of the high-energy photoinduced absorption is in disagreement with our previous assignment of this transition to the biexciton, which occurs at still higher energy. © 1999 The American Physical Society.
• Chandross, M., Shimoi, Y., & Mazumdar, S. (1999). Diagrammatic exciton-basis valence-bond theory of linear polyenes. Physical Review B - Condensed Matter and Materials Physics, 59(7), 4822-4838.
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  Abstract: Understanding the photophysics of π-conjugated polymers requires a physical understanding of the excited states involved in the photophysics. Detailed physical understanding is difficult because of the extensive configuration interaction that occurs within realistic theoretical models for these systems. We develop a diagrammatic exciton-basis valence-bond representation that is particularly suitable for the intermediate magnitude of the Coulomb interactions in these systems. We present detailed comparisons of our exact exciton-basis treatment and previous approximate approaches, focusing on the specific many-body and single-particle interactions that have been ignored in the past, and the consequences thereof. Following this, we present the results of exact numerical calculations for the noninteracting band limit, the limit of isolated dimers interacting through Coulomb interactions, and for the Pariser-Parr-Pople Ohno Coulomb interactions with two different bond-alternation parameters for the ten-carbon linear polyene. Simple pictorial descriptions of the eigenstates relevant in photophysics are obtained in each case, and taken together, these results provide a systematic characterization of both low- and high-energy excited states in linear chain π-conjugated systems for realistic parameters. Two different quantities, the number of effective excitations within the exciton basis, and the particle-hole correlation length for the one-excitation eigenstates are defined and calculated for further quantitative comparisons between the eigenstates. A pictorial description of optical nonlinearity is obtained thereby. For both small and large bond alternation, it is found that the two-photon state that dominates third order optical nonlinearity in the low-energy region is the lowest even parity one-excitation state with a larger particle-hole correlation length than the 1Bu exciton. The reason for the dominance by this mAg state can be understood within the exciton basis from the nature of the current operator. It is shown that the relationship between the correlated mAg and the correlated 1Bu is identical to that between the uncorrelated 2Ag and the uncorrelated 1Bu . In the high-energy region of the spectrum evidence for stable biexcitons is found from the nature of the singlet-singlet two-excitation wave functions. © 1999 The American Physical Society.
• Mazumdar, S., Campbell, D., Clay, R. T., & Ramasesha, S. (1999). Comment on "Wigner crystal type of charge ordering in an organic conductor with a quarter-filled band: (DI-DCNQI) ₂Ag". Physical Review Letters, 82(11), 2411-.
• Mazumdar, S., Ramasesha, S., Clay, R. T., & Campbell, D. K. (1999). Theory of coexisting charge and spin-density waves in (TMTTF)₂Br, (TMTSF)₂PF₆ and α-(BEDT-TTF)₂MHg(SCN)₄. Synthetic Metals, 103(1-3), 1843-1844.
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  Abstract: In a previous study, a unified theory of spin density wave (SDW) formation in a large class of 2:1 cationic charge transfer solids (CTS) based on an anisotropic, 2D, lattice Hamiltonian with both Coulomb and e-ph interactions has been proposed. This article describes in the detail the proposed theory. The 1/4 -filled nature of these CTS is explicitly treated.
• Mazumdar, S., Ramasesha, S., Clay, R. T., & Campbell, D. K. (1999). Theory of coexisting charge and spin-density waves in (TMTTF)₂Br, (TMTSF)₂PF₆, and α-(BEDT-TTF)₂MHg(SCN)₄. Physical Review Letters, 82(7), 1522-1525.
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  Abstract: Recent experiments indicate that the spin-density waves (SDWs) in (TMTTF)2Br, (TMTSF)2PF6, and α-(BEDT-TTF)2MHg(SCN)4 are highly unconventional and coexist with charge-density waves (CDWs). We present a microscopic theory of this unusual CDW-SDW coexistence. A complete understanding requires the explicit inclusion of strong Coulomb interactions, lattice discreteness, the anisotropic two-dimensional nature of the lattice, and the correct band filling within the starting Hamiltonian.
• Shukla, A., & Mazumdar, S. (1999). Designing Emissive Conjugated Polymers with Small Optical Gaps: A Step Towards Organic Polymeric Infrared Lasers. Physical Review Letters, 83(19), 3944-3947.
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  Abstract: We show that chemical modification of the trans-polyacetylene structure that involves substitution of the backbone hydrogen atoms with conjugated side groups leads to reduction of the backbone bond alternation as well as screening of the effective Coulomb interaction. Consequently, the optical gap of the substituted material is smaller than the parent polyene with the same backbone length, and the excited state ordering is conducive to efficient photoluminescence. In the ideal long chain limit, the design of organic polymeric infrared lasers thereby becomes possible.
• Chakrabarti, A., & Mazumdar, S. (1997). Essential excitations in phenylene-based conjugated polymers. Synthetic Metals, 85(1-3), 1121-1122.
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  Abstract: We have developed a computational approach based on an exciton basis to calculate the excited state spectrum of long chain poly(paraphenylenes). Pictorial descriptions of the excited states relevant in the photophysics of these systems are obtained. Evidence for bound excitons has been found.
• Chandross, M., & Mazumdar, S. (1997). Coulomb interactions and linear, nonlinear, and triplet absorption in poly(para-phenylenevinylene). Physical Review B - Condensed Matter and Materials Physics, 55(3), 1497-1504.
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  Abstract: Within a model Hamiltonian with variable on-site and long-range Coulomb interactions between the π electrons for poly(para-phenylenevinylene), we conduct a thorough search in the parameter space to determine the magnitudes of the effective Coulomb interaction parameters necessary to fit all four absorption bands that are seen in the experimental absorption spectra of this material. We find best agreement between the calculated and experimental absorption spectra with Coulomb interactions that are slightly smaller than the standard Pariser-Parr-Pople parameters. For these values of the Coulomb parameters, the primary photoexcitation in poly(para-phenylenevinylene) is to an exciton with binding energy close to 0.9±0.2 eV. This result, obtained from fitting the linear absorption, is in agreement with nonlinear absorption studies, viz. electroabsorption, two-photon absorption, and picosecond photoinduced absorption, within our model. We have also calculated the energies of the lowest triplet state, and the final state to which triplet absorption occurs. The excited triplet state is an exciton. We show that the latter result, taken together with the known experimental triplet absorption energy, indicates that estimates of 0.2 eV or less for the binding energy are incorrect. We briefly discuss the possibility that the binding energy has an intermediate magnitude.
• Chandross, M., & Mazumdar, S. (1997). What do nonlinear optical spectroscopy and picosecond photinduced absorption have to say about excited states in conjugated polymers?. Synthetic Metals, 85(1-3), 1095-1096.
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  Abstract: We present a self-consistent picture of the excited state energy spectrum of poly(para-phenylene vinylene) based on theoretical calculations of exicted states and and experimental electroabsorption (EA) and picosecond photoinduced absorption measurements (PA). Our calculations place the one electron-one hole continuum (nBu) at 0.9 ± 0.2 eV above the 1Bu exciton. This is in agreement with EA and ps PA in the i.r., which finds an even-parity exciton (the mAg) about 0.7 eV away from the 1Bu. The high energy ps PA at 1.5 - 1.7 eV is to a biexciton whose energy is consistent with our estimates.
• Chandross, M., Mazumdar, S., Liess, M., Lane, P. A., Vardeny, Z. V., Hamaguchi, M., & Yoshino, K. (1997). Optical absorption in the substituted PPVs: Theory and experiment. Synthetic Metals, 84(1-3), 603-604.
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  Abstract: We investigate theoretically and experimentally the effects of (2,5) chemical substitution on the optical absorption in the phenylene-based conjugated polymers. The effects of the substituents on the optical absorption spectrum are weak. The 3.7 eV band is predicted to be polarized predominantly along the polymer chain axis. This prediction is confirmed experimentally.
• Chandross, M., Mazumdar, S., Liess, M., Lane, P. A., Vardeny, Z. V., Hamaguchi, M., & Yoshino, K. (1997). Optical absorption in the substituted phenylene-based conjugated polymers: Theory and experiment. Physical Review B - Condensed Matter and Materials Physics, 55(3), 1486-1496.
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  Abstract: We investigate theoretically and experimentally the effects of (2,5) chemical substitution on the optical absorption in the phenylene-based conjugated polymers. Theoretically, substitution destroys both the charge-conjugation symmetry and spatial symmetry that characterize the unsubstituted materials. Within Coulomb-correlated theoretical models, the effect of broken charge-conjugation symmetry alone on the underlying electronic structure and on the absorption spectrum is rather weak. When both broken spatial symmetry and broken charge-conjugation symmetry are taken into account, a strong effect on the electronic structure of polyphenylene derivatives is found. In spite of the strong effect of the broken symmetries on the electronic structure, the effect on the optical-absorption spectrum is weak. This surprising result is a consequence of the subtle nature of the configuration interaction in the substituted polyphenylenes within Coulomb-correlated models. We demonstrate numerically an approximate sum rule that governs the strength of an absorption band at 3.7 eV in the absorption spectra of poly(para-phenylene vinylene) (PPV) derivatives. Although substitution can make a previously forbidden transition weakly allowed, the latter acquires strength from a "finite-size band" at about the same energy, and not from a higher-energy band at 4.7 eV, as has been previously claimed. It is further predicted that the 3.7-eV band is polarized predominantly along the polymer-chain axis. We have measured the polarization dependence of the optical absorption in an oriented-substituted PPV film. We found that the two lowest-energy absorption bands are polarized predominantly parallel to the chain axis, while the band at 4.7 eV is polarized predominantly perpendicular to the chain axis. These results are in excellent agreement with the theory.
• Chandross, M., Shimoi, Y., & Mazumdar, S. (1997). Systematic characterization of excited states in conjugated polymers. Synthetic Metals, 85(1-3), 1001-1006.
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  Abstract: We present the first complete systematic characterization of the excited states in conjugated polymers. Our results are relevant for the understanding of the photophysics of these materials. We perform full configuration interaction calculations in an exciton basis within which a long chain polymer is considered as coupled molecular units. Complete pictorial descriptions of all excited states are obtained. In linear chain polymers such as the polyacetylenes and polydiacetylenes the 1Bu is an exciton, and the fundamental two-photon states can be broadly classified into triplet-triplet (TT), charge-transfer (CT) and singlet-singlet (SS) excitations. In the above CT refers to charge-transfer from one unit to another, and TT and SS are two electron-two hole excitations. In TT the spin angular momenta of two different triplet excitations combine to give an overall singlet, while the individual excitations are singlets in SS. The 2Ag is classified as TT. The mAg, an even parity state that plays a strong role in nonlinear optics, is a correlated CT state. The SS states occur higher in energy and for moderate exciton binding split into the biexciton and two-exciton continuum. The calculations can be easily extended to the polyphenylenes, for which the characterization of excited states continues to be possible. These theoretical results are useful in explaining a variety of third order nonlinear optical spectroscopic measurements as well as picosecond photoinduced absorption.
• Chandross, M., Shimoi, Y., & Mazumdar, S. (1997). Systematic characterization of excited states in π -conjugated polymers. Chemical Physics Letters, 280(1-2), 85-90.
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  Abstract: Exact finite chain calculations within a diagrammatic exciton basis give pictorial descriptions of all excited states of π-conjugated polymers. In linear chain polymers the 1Bu is an exciton, and the fundamental two-photon states can be broadly classified into triplet-triplet, charge-transfer and singlet-singlet excitations. The mAg, a two-photon state that plays a strong role in nonlinear optics, is a correlated charge-transfer state. The singlet-singlet states occur higher in energy and split into bound biexcitons and a continuum of free two-exciton states.
• Gallagher, F. B., & Mazumdar, S. (1997). Excitons and optical absorption in one-dimensional extended Hubbard models with short- and long-range interactions. Physical Review B - Condensed Matter and Materials Physics, 56(23), 15025-15039.
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  Abstract: We show that charge excitations within one-dimensional extended Hubbard models in the limit of large on-site Coulomb interaction can be mapped onto pairs of spin reversals from the ferromagnetic Ising-Heisenberg spin Hamiltonian. The exciton in the extended Hubbard models is equivalent to the bound two-magnon state of the spin Hamiltonian. From exact results for the spin Hamiltonian, for the intersite Coulomb interactions limited between nearest neighbors, the lowest optical exciton is separated in energy from the electron-hole continuum only for the intersite interaction larger than half the one-electron bandwidth. The commutation relationship between spin operators allows us to numerically investigate exciton binding energies and optical absorptions from the ground state in the limit of large on-site Coulomb interaction even for cases where analytic results are not available. For intersite Coulomb interaction limited between nearest neighbors, the binding energy of the lowest optical exciton changes very weakly for realistic Peierls bond dimerization, although there is a profound change in the optical absorption. While in the undimerized chain a single optical exciton can occur, two additional excitons that occur above the threshold of the electron-hole continuum become optically visible in the dimerized case. For long-range intersite Coulomb interactions, multiple excitons can occur even for the undimerized chain, and additional exciton states become optically visible upon dimerization. With the 1/r form for the intersite Coulomb interaction, our numerical calculations suggest that the formation of at least one optical exciton is unconditional, unlike the case of nearest-neighbor interaction. The binding energy of the lowest optical exciton again depends weakly on the dimerization for long-range interaction. Our theoretical results can explain the occurrence of two charge-transfer absorption bands in half-filled band segregated stack charge transfer solids below the spin-Peierls dimerization temperature. Our results also suggest that the lowest optical state of π-conjugated polymers, in which Coulomb interactions are long ranged, is necessarily an exciton.
• Shimoi, Y., & Mazumdar, S. (1997). Energy diagram of triplet and singlet excited states in conjugated polymers. Synthetic Metals, 85(1-3), 1027-1028.
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  Abstract: We have theoretically studied the triplet and singlet excited states of conjugated polymers for the purpose of further understanding the photophysics of these systems. Exact finite chain calculations within the Pariser-Parr-Pople and extended Hubbard models show that the triplet absorption energy is almost independent of the degree of bond alternation. Furthermore, the final state of this triplet-triplet transition lies below the so-called m1Ag charge-transfer exciton due to correlation effects higher than single configuration-interaction. This indicates that the final state in triplet absorption is an exciton. Based on the exciton nature of the excited triplet state, and energy relationships between the 11Bu, 21Ag, and triplet ground state, we are able to determine the absolute lower limit for the exciton binding energy in poly(p-phenylene vinylene).
• Mazumdar, S., & Guo, F. (1996). Theory of biexcitons in mixed and segregated stack charge-transfer solids, molecular crystals and π-conjugated polymers. Synthetic Metals, 78(3), 187-193.
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  Abstract: We discuss biexcitons within three different theoretical models in one dimension with excitonic interactions. We develop a general physical picture for optical absorption from the one-photon allowed exciton to the biexciton and to the two-exciton continuum in one dimension. Based on this picture, we propose a criterion of biexciton stability. The validity of our criterion of biexciton stability is proved in the strong-coupling limits of models describing mixed and segregated stack charge-transfer solids, while studies by other investigators have shown that the criterion also remains valid within the Frenkel exciton model with moderate exciton-exciton binding. Based on the same criterion of biexciton stability, we present theoretical evidence for stable biexcitons within the Coulomb-correlated model for π-conjugated polymers. High-energy picosecond photoinduced absorption in π-conjugated polymers is explained as a transition from the optical exciton to the biexciton.
• Mazumdar, S., Guo, F., Meissner, K., Fluegel, B., Peyghambarian, N., Kuwata-Gonokami, M., Sato, Y., Ema, K., Shimano, R., Tokihiro, T., Ezaki, H., & Hanamura, E. (1996). A new class of collective excitations: Exciton strings. Journal of Chemical Physics, 104(23), 9283-9291.
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  Abstract: Optical excitation in a strongly neutral quasi-one-dimensional mixed-stack charge-transfer solid results in an exciton state, in which the electron and the hole are bound by electrostatic Coulomb interactions that are large compared to the one-electron hopping. We present a joint theoretical-experimental demonstration of a new class of collective excitations, multiexcitons or exciton strings, consisting of a string of several (more than two) bound excitons, in a prototype neutral charge-transfer solid. The stability of the multiexciton states arise from the combined effects of one dimensionality and strong Coulomb interactions. Theoretically, we show that in narrow band one-dimensional semiconductors with long range Coulomb interactions, the occurrence of stable 2-exciton string (biexciton) necessarily implies stable higher multiexcitons. Experimentally, evidence for the multiexciton strings is demonstrated by femtosecond pump-probe spectroscopy of anthracene pyromellitic acid dianhydride. Excellent qualitative agreement is found between the calculated and the measured differential transmission spectra. Photoinduced absorptions to the 2-exciton string at low pump intensity and to the 3-exciton string at high pump intensity are observed, in agreement with the theory of excited state absorption. The 2-exciton string is confirmed also by a direct two-photon absorption measurement. The binding energies of the 2-exciton and the 3-exciton strings are obtained from the experimental data. The larger binding energy of the 3-exciton is in agreement with theory. © 1996 American Institute of Physics.
• Mazumdar, S., Guo, F., Meissner, K., Fluegel, B., Peyghambarian, N., Kuwata-Gonokami, M., Sato, Y., Ema, K., Shimano, R., Tokihiro, T., Ezaki, H., & Hanamura, E. (1996). Exciton-to-biexciton transition in quasi-one-dimensional organics. Journal of Chemical Physics, 104(23), 9292-9296.
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  Abstract: In the previous paper we demonstrated novel multiexcitons in a neutral mixed-stack charge-transfer solid. The lowest multiexciton, the biexciton, has recently been of interest also in the context of quasi-one-dimensional organic materials that are different from the mixed-stack solids. The nature and strength of the optical transition from the exciton to the two-exciton states is of importance in understanding photoinduced absorption as well as two-photon absorption. We show that within the diverse theoretical models that describe these different classes of materials, the excited state absorption from the optical exciton to the two-exciton states changes in a fundamental way upon the formation of the biexciton. The identical nature of the exciton absorption within these models is a consequence of one dimensionality. © 1996 American Institute of Physics.
• Chandross, M., Guo, F., & Mazumdar, S. (1995). Excitons and biexcitons in poly(para-phenylenevinylene). Synthetic Metals, 69(1-3), 625-628.
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  Abstract: The Hückel model fails to describe optical absorption in PPV and its derivatives, and explicit inclusion of moderate electron-electron interactions is essential for a proper theoretical description. The lowest optical state is an exciton with a binding energy estimated as 0.90 ± 0.15 eV, in excellent agreement with experiment. Such a large exciton binding energy strongly suggests stable biexcitons. The concept of biexcitons in π-conjugated polymers is introduced, and picosecond photoinduced absorption is explained as excited state absorption from the exciton to the biexciton. © 1995.
• Guo, F., Chandross, M., & Mazumdar, S. (1995). Stable biexcitons in conjugated polymers. Physical Review Letters, 74(11), 2086-2089.
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  Abstract: Theoretical evidence for stable biexcitons are found within the Coulomb-correlated model for π-conjugated polymers, for realistic Coulomb interaction parameters. High energy picosecond photoinduced absorption, not anticipated within one-electron theories of π-conjugated polymers, is explained as a transition from the optical exciton to the biexciton.
• Mazumdar, S., & Chandross, M. (1995). Theory of photoexcitations in phenylene-based polymers. Proceedings of SPIE - The International Society for Optical Engineering, 2528, 62-73.
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  Abstract: We show that optical absorption spectra of polyphenylenes can be explained only within theoretical models that explicitly include the Coulomb interaction among the π-electrons. We also show that the dominant effect of substitution on the electronic structure of polyphenylenes within Coulomb correlated models is broken spatial symmetry, while broken charge conjugation symmetry plays a rather weak role. The broken spatial symmetry has a subtle, and weak, effect on the optical absorption spectrum. Consequently, optical absorption spectra of unsubstituted polyphenylenes and the substituted derivatives are nearly identical. Comparison of theoretical and experimental absorption spectra leads to the conclusion that the exciton binding energy in a long chain of poly(para-phenylenevinylene) is about 0.9 eV. Such a large binding energy would be in agreement with nonlinear spectroscopic measurements and pump-probe experiments. However, the present work also indicates that the experimental polymers actually consist of short chains with the chain length distribution peaking at about 10 phenylene units. The gaps between the energy levels above the calculated continuum threshold are much too large for transport to be an intrachain process. Photoconductivity may be predominantly an interchain process, and probably measures the dissociation energy of the exciton which is different from the exciton binding energy.
• Chandross, M., Mazumdar, S., Jeglinski, S., Wei, X., Vardeny, Z. V., Kwock, E. W., & Miller, T. M. (1994). Excitons in poly(para-phenylenevinylene). Physical Review B, 50(19), 14702-14705.
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  Abstract: We show that one-electron band theory fails to describe optical absorption in PPV and the absorption spectrum can be described only within a Coulomb-correlated model. The lowest optical state is an exciton, whose binding energy is estimated theoretically to be 0.90±0.15 eV. Measurements of the photoconductivity quantum efficiency in poly[2-methoxy,5-(2'-ethyl-hexyloxy)-1, 4 phenylenevinylene] reveal a binding energy in good agreement with the theoretical value. © 1994 The American Physical Society.
• Guo, D., Mazumdar, S., & Dixit, S. N. (1994). Four-level `essential states' model of an optical nonlinearity in π-conjugated polymers. Molecular Crystals and Liquid Crystals Science and Technology Section B: Nonlinear Optics, 6(3-4), 337-350.
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  Abstract: It is shown that the theoretical description of optical nonlinearity in π-conjugated polymers must necessarily include both the on-site and intersite electron correlations. In spite of the infinite number of states that, in principle, contribute to the third-order optical nonlinearity in long chains, it is shown that only three dominant nonlinear optical channels, involving four `essential states', determine the bulk of the optical nonlinearity. A universality in one dimension is pointed out. The relative locations and natures of these four essential states are practically independent of the Coulomb interactions. In ideal isolated strands, third-harmonic generation experiments should find two three-photon resonances, and not merely one, as has been generally assumed. In systems with strong interchain interactions, band formation can lead to overlaps between the long wavelength three-photon resonance and a two-photon resonance. Two-photon resonances other than the one due to the essential two-photon excited state will have vanishingly weak intensities due to cancellations between nonlinear optical channels with opposite signs.
• Guo, F., Chandross, M., & Mazumdar, S. (1994). Even parity states in PPV, polythiophene and polysilanes. Materials Research Society Symposium Proceedings, 328, 787-792.
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  Abstract: Two photon absorptions (TPA) in conjugated polymers with large bond alternation are reinterpreted. TPA to the 2Ag in long chains should be tiny due to cancellation effects, and therefore the observed strong, narrow TPA is to a higher even parity state. We also present the first theoretical evidence for bound biexcitons, and explain recent photoinduced absorption data.
• Guo, F., Guo, D., & Mazumdar, S. (1994). Intensities of two-photon absorptions to low-lying even-parity states in linear-chain conjugated polymers. Physical Review B, 49(15), 10102-10112.
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  Abstract: Interest in the lowest even-parity two-photon states in linear-chain π-conjugated polymers has been largely limited to their energies relative to the lowest odd-parity one-photon state. We show that the intensities of the two-photon absorptions to even-parity states that occur below the lowest one-photon states should be tiny in the infinite chain, because of cancellations. This has important implications for the interpretation of third-harmonic generation spectra of polydiacetylenes. A resonance seen in third-harmonic generation of polydiacetylenes that has commonly been assigned as a two-photon resonance is interpreted here as a three-photon resonance. © 1994 The American Physical Society.
• Kuwata-Gonokami, M., Peyghambarian, N., Meissner, K., Fluegel, B., Sato, Y., Ema, K., Shimano, R., Mazumdar, S., Guo, F., Tokihiro, T., Ezaki, H., & Hanamura, E. (1994). Exciton strings in an organic charge-transfer crystal. Nature, 367(6458), 47-48.
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  Abstract: COLLECTIVE excitations resulting from many-body Coulomb interactions have been studied extensively in the solid state: for example, the exchange interaction between the electrons in two excitons (bound electron-hole pairs) can bind the excitons together, forming a biexciton. At the other extreme, if the number of excitons is sufficiently large (~106), they can condense into a degenerate 'liquid' phase known as an electron-hole drop. But in conventional semiconductors, intermediate bound states, consisting of more than two excitons, are not formed. We show here, both theoretically and experimentally, that bound states of multiple excitons can form in the organic charge-transfer solid anthracene-(pyromellitic acid dianhydride). Coulomb interactions along the one-dimensional stacks of this material can stabilize trains of several charge-transfer excitons, and we refer to the resulting collective excitations as exciton strings.
• Leng, J. M., Jeglinski, S., Wei, X., Benner, R. E., Vardeny, Z. V., Guo, F., & Mazumdar, S. (1994). Leng et al. reply. Physical Review Letters, 73(23), 3180-.
• Leng, J. M., Jeglinski, S., Wei, X., Benner, R. E., Vardeny, Z. V., Guo, F., & Mazumdar, S. (1994). Optical probes of excited states in poly(p-phenylenevinylene). Physical Review Letters, 72(1), 156-159.
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  Abstract: We have studied electronic excited states in films of poly(p-phenylenevinylene) using picosecond transient and cw photomodulation, photoluminescence, and their excitation spectra, as well as electroabsorption spectroscopy. We have determined all the important energy levels of singlet excitons with odd and even parity, the onset of the continuum band, the two-electron (biexciton) states, and the two relevant triplet states, and show that good agreement exists with models involving electron correlation.
• Mazumdar, S., & Guo, F. (1994). Observation of three resonances in the third harmonic generation spectrum of conjugated polymers: Evidence for the four-level essential states model. The Journal of Chemical Physics, 100(2), 1665-1672.
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  Abstract: Much of the theoretical effort to understand the mechanism of optical nonlinearity in π-conjugated polymers has been focused towards explaining the early observation of only two resonances in the third harmonic generation spectrum of trans-polyacetylene. A recent theory of third order optical nonlinearity predicts three, instead of two, resonances, with their intensities decreasing with increasing frequencies. This is a fundamental difference between the band and the exciton pictures of π-conjugated polymers. Very recent experimental results obtained with cis-polyacetylene, as well as earlier experiments on two blue forms of polydiacetylenes, are in complete agreement with this prediction. An isolated single strand of cis-polyacetylene should exhibit exciton behavior. We briefly discuss the latest experimental results for trans-polyacetylene. © 1994 American Institute of Physics.
• Peyghambarian, N., Fluegel, B., Meissner, K., Guo, F., Mazumdar, S., Kuwata-Gonokami, M., Sato, Y., Ema, K., Shimano, R., Tokihiro, T., Ezaki, H., & Hanamura, E. (1994). Observation of excitonic strings in a quasi-one dimensional charge transfer crystal. Proceedings of the International Quantum Electronics Conference (IQEC'94), 29-30.
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  Abstract: This paper demonstrates both experimental and theoretical analysis of the Coulomb attraction of CT excitons, assembling n-string excitons in a quasi-one-dimensional charge transfer (CT) organic crystal. The excitonic strings were experimentally investigated by applying an ultrafast pump-probe study on high caliber anthracine-PMDA crystals more than 50 μm thick. It was observed that an increase in pump intensity provoked another absorption band at even lower energy. Furthermore, the experimental and theoretical spectra were adequately in agreement with each other.
• Peyghambarian, N., Meissner, K., Fluegel, B., Mazumdar, S., Guo, F., Kuwata-Gonokami, M., Sato, Y., Ema, K., Shimano, R., Tokihiro, T., Ezaki, H., & Hanamura, E. (1994). Observation of excitonic n-strings in a quasi-one-dimensional charge-transfer crystal. Materials Research Society Symposium Proceedings, 328, 683-688.
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  Abstract: Collective excitations, such as plasmons, rotons, electron-hole liquid, and excitonic molecules, have been extensively studied in condensed matter. Wannier excitons in inorganic semiconductors are bound by the exchange interactions between two electrons of the exciton, and the bound state of more than two excitons is not possible. We demonstrate here a new type of collective excitations, bound states of multiple charge-transfer (CT) excitons. Coulomb interactions in one dimension are shown to bind a train of several (more than two) CT excitons. Experimental evidence for these new type of elementary excitations is reported in a quasi-one-dimensional CT crystal of anthracene PMDA. High density excitation by femtosecond light pulses generates multi-exciton chains, which we refer to as excitonic n-strings with n = 1, 2, 3, etc., along the stack axis of the crystal. Both the n = 2 excitonic string (biexciton) and the n = 3 string (tri-exciton) are observed. This report provides evidence for an n>2 exciton chain in this system. The stability of the n-string exciton is supported by our theoretical calculations based on the extended Hubbard Hamiltonian in one dimension.
• Ezaki, H., Tokihiro, T., Kuwata-Gonokami, M., Shimano, R., Ema, K., Hanamura, E., Fluegel, B., Meissner, K., Mazumdar, S., & Peyghambarian, N. (1993). Excitonic n-string in linear chains. Solid State Communications, 88(3), 211-216.
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  Abstract: The bound states of n Frenkel excitons (n = 2,3,...), which are called excitonic n-strings, are theoretically shown to exist in contrast with a system of Wannier excitons. The electronic structure and nonlinear optical responses of these excitonic n-strings are clarified. The first evidence for the excitonic 2- and 3-strings is provided from comparison of the calculated and observed differential transmission spectrum of an Anthracene-PMDA crystal pumped by femtosecond laser pulses. © 1993.
• Guo, D., Mazumdar, S., Dixit, S. N., Kajzar, F., Jarka, F., Kawabe, Y., & Peyghambarian, N. (1993). Role of the conduction band in electroabsorption, two-photon absorption, and third-harmonic generation in polydiacetylenes. Physical Review B, 48(3), 1433-1459.
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  Abstract: We report experimental and theoretical investigations of electroabsorption in a polydiacetylene, and determine the complete mechanism of third-harmonic generation (THG) and two-photon absorption (TPA) in linear-chain -conjugated polymers. The experimental electroabsorption is studied by transmission, rather than reflectance techniques. In addition to the Stark shift of the exciton, a significant feature is observed in the difference spectrum at a higher energy, where the linear absorption is negligible. The origin of this high-energy feature has been controversial. We report several extensive theoretical calculations within the extended Hubbard model, and are able to establish a universality that exists within one-dimensional Coulomb correlated models. We show that the high-energy oscillatory feature in the electroabsorption spectrum originates from the conduction-band threshold, which is separated from the exciton in polydiacetylenes. We also demonstrate that even-parity two-photon states that occur below the one-photon exciton are not observed in electroabsorption due to a cancellation effect. However, a dominant two-photon state that is predicted to occur in between the lowest optical exciton and the conduction-band threshold should be observable. The cancellation, which is only partial for the dominant two-photon state, can, however, reduce the intensity of the resonance due to the state. We show that the conduction-band threshold state, which is an odd-parity one-photon state, also plays an important role in other nonlinear optical processes such as third-harmonic generation and two-photon absorption. Third-order optical nonlinearity in linear-correlated chains is dominated by four essential states: the ground state, the lowest optical exciton and the conduction-band threshold states, and the two-photon state that lies in between the two excited odd-parity states. The two most important predictions of our theoretical work are (a) third-harmonic-generation experiments on ideal isolated strands should find two, not merely one, three-photon resonances, originating from the exciton and the conduction-band threshold states, and (b) only one dominant two-photon resonance in the infinite-chain limit should be observable in THG and TPA. Extensive comparisons between theoretical predictions and experiments are made to prove the validity of the theory. Conjugated polymers other than the polydiacetylenes are discussed briefly. © 1993 The American Physical Society.
• Mazumdar, S., Guo, D., & Dixit, S. N. (1993). A four-level "essential states" model of third order optical nonlinearity in π-conjugated polymers. Synthetic Metals, 57(1), 3881-3888.
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  Abstract: Theoretical description of optical nonlinearity in π-conjugated polymers must necessarily include both the on-site and the intersite electron correlations. We show that only three dominant nonlinear optical channels, involving four "essential states", determine the bulk of the optical nonlinearity. In ideal isolated strands, third harmonic generation experiments should find two three-photon resonances, and not merely one, as has been generally assumed. Two-photon resonances other than the one due to the essential two-phonon excited state will have vanishingly weak intensities due to cancellations between nonlinear optical channels with opposite signs. © 1993.
• Ung, K., Mazumdar, S., & Campbell, D. K. (1993). Coexisting CDW and BOW in organic conductors with non-half-filled bands. Solid State Communications, 85(11), 917-920.
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  Abstract: Commensurate 1 4 and 1 3-filled bands in which the electrons are coupled to both intersite phonons and intramolecular vibrations are investigated. Unlike in the 1 2-filled band, the bond order wave and the charge density wave coexist and interact cooperatively for all values of the two electron-phonon coupling constants. Despite the coexistence of the two kinds of density waves, solitons have charges that are rational fractions. The relevance of the cooperative interaction in non-half-filled bands to the metal-insulator transitions in segregated stack charge-transfer solids is discussed. © 1993.
• Ung, K., Mazumdar, S., Campbell, D. K., & Lin, H. Q. (1993). Metal-insulator transitions and electron-phonon interactions in organic conductors. Synthetic Metals, 57(2 -3 pt 6), 4660-4665.
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  Abstract: Commensurate 1/4 and 1/3-filled bands in which the electrons are coupled to both intersite phonons and intramolecular vibrations are investigated. Unlike in the 1/2-filled band, the bond order wave and the charge density wave coexist and interact cooperatively for all values of the two electron-phonon coupling constants. In spite of the coexistence of the two kinds of density waves, solitons have charges that are rational fractions. The relevance of the cooperative interaction in non-half-filled bands to the metal-insulator transitions in segregated stack charge-transfer solids is discussed.
• Ung, K., Mazumdar, S., Campbell, D. K., & Lin, H. Q. (1993). Metal-insulator transitions and electron-phonon interactions in organic conductors. Synthetic Metals, 57(2-3), 4660-4665.
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  Abstract: Commensurate 1/4 and 1/3-filled bands in which the electrons are coupled to both intersite phonons and intramolecular vibrations are investigated. Unlike in the 1/2-filled band, the bond order wave and the charge density wave coxexist and interact cooperatively for all values of the two electron-phonon coupling constants. In spite of the coexistence of the two kinds of density waves, solitons have charges that are rational fractions. The relevance of the cooperative interaction in non-half-filled bands to the metal-insulator transitions in segregated stack charge-transfer solids is discussed. © 1993.
• Guo, D., & Mazumdar, S. (1992). Comment on: Interchain dispersion and second hyperpolarizability of conjugated polymers. The Journal of Chemical Physics, 97(3), 2170-2171.
• Guo, D., & Mazumdar, S. (1992). On the tight-binding theory of C₆₀. Synthetic Metals, 49(1-3), 175-179.
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  Abstract: Optical absorption in a C60 molecule is calculated within a one-electron tight-binding model. It is seen that it is necessary to include non-zero second-neighbor hopping in order to explain the experimental absorption spectrum. Thus, the second-neighbor hopping plays a stronger role in C60 than in linear conjugated polymers. © 1992.
• Guo, D., Mazumdar, S., & Dixit, S. N. (1992). Microscopic theory of third-harmonic generation and electro-absorption in conjugated polymers. Synthetic Metals, 49(1-3), 1-11.
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  Abstract: We develop a physical, intuitive description of optical nonlinearity in conjugated polymers within a completely general microscopic theoretical model. It is shown that third-harmonic generation and electro-absorption give complimentary information on the electronic structures of π-conjugated systems. © 1992.
• Kawabe, Y., Jarka, F., Peyghambarian, N., Guo, D., Mazumdar, S., Dixit, S. N., & Kajzar, F. (1992). Electro-absorption in a polydiacetylene. Synthetic Metals, 50(1 -3 pt 2), 517-523.
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  Abstract: The electro-absorption in a DCH-polydiacetylene has been investigated both experimentally and theoretically. In addition to the red Stark shift of the exciton, a significant feature in the difference spectrum is observed at a higher energy. Within an interacting electron model (extended Peierls-Hubbard model), the roles played by the dipole-allowed one-photon states and the dipole-forbidden two-photon states are studied in detail. It is found that the high-energy signals in the difference spectrum are due to the strong field-induced mixing of the one-photon state at the edge of a continuum structure, or 'conduction band' in the language of band theory, with its neighboring two-photon states. Contributions to the electro-absorption from the one- and two-photon states cancel deep within the band. Three-photon resonance due to the band edge is expected in third-harmonic generation (THG) and has been seen in both a polydiacetylene and in polysilanes.
• Kawabe, Y., Jarka, F., Peyghambarian, N., Guo, D., Mazumdar, S., Dixit, S. N., & Kajzar, F. (1992). Electro-absorption in a polydiacetylene. Synthetic Metals, 50(1-3), 517-523.
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  Abstract: The electro-absorption in a DCH-polydiacetylene has been investigated both experimentally and theoretically. In addition to the red Stark shift of the exciton, a significant feature in the difference spectrum is observed at a higher energy. Within an interacting electron model (extended Peierls-Hubbard model), the roles played by the dipole-allowed one-photon states and the dipole-forbidden two-photon states are studied in detail. It is found that the high-energy signals in the difference spectrum are due to the strong field-induced mixing of the one-photon state at the edge of a continuum structure, or 'conduction band' in the language of band theory, with its neighboring two-photon states. Contributions to the electro-absorption from the one- and two-photon states cancel deep within the band. Three-photon resonance due to the band edge is expected in third-harmonic generation (THG) and has been seen in both a polydiacetylene and in polysilanes. © 1992.
• Mazumdar, S., Guo, D., & Dixit, S. N. (1992). High energy two-photon states in finite versus infinite polyenes. The Journal of Chemical Physics, 96(9), 6862-6867.
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  Abstract: Optical nonlinearity in π-conjugated polyenes depends crucially on a single two-photon state that is located above the one-photon allowed exciton. In the case of the infinite polyene this single state should be thought of as a collection of states with very narrow bandwidth. The location of this two-photon state in the infinite polyene is of considerable interest. We present numerical data for finite chains and discuss in detail why extrapolation of finite chain data to obtain the energies of the high energy states in the infinite polyene requires considerable care. Based on upper and lower bounds that we put on the energy of the two-photon state in question, we predict it to be extremely close to the one-photon excition in the infinite chain limit. We present recent experimental data for a polydiacetylene that is consistent with our prediction. © 1992 American Institute of Physics.
• Dixit, S. N., Guo, D., & Mazumdar, S. (1991). Essential-states mechanism of optical nonlinearity in -conjugated polymers. Physical Review B, 43(8), 6781-6784.
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  Abstract: We present a microscopic mechanism of optical nonlinearity in conjugated polymers. It is shown that the bulk of the nonlinearity is determined by only two well-defined channels, even though an infinite number of channels are possible in principle. We are able to explain the complete frequency dependence of the third-harmonic generation in both trans-polyacetylene and polydiacetylene within the same model without invoking weak Coulomb interactions between electrons or interchain interactions. © 1991 The American Physical Society.
• Gammel, J. T., Campbell, D., Mazumdar, S., Dixit, S., & Loh Jr., E. (1991). Determination of interaction parameters in peierls-hubbard models describing finite polyenes and polyacetylene. Synthetic Metals, 43(1-2), 3471-3475.
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  Abstract: We study the extent to which finite polyenes and polyacetylene can be described by the extended Peierls-Hubbard Hamiltonian (ePHH) in an internally consistent manner. Using Lanczos exact diagonalization methods (LEDM) on small systems, coupled with a novel boundary condition (b.c.) averaging technique, we investigate the excited state spectra of these models for the finite oligomer analogs of trans-polyacetylene, (CH)x. We search for sets of parameters describing the electron-phonon (e-p) and electron-electron (e-e) interactions which are consistent with all available experimental data on these systems, including the optical gap and bandwidth, the 21 Ag state, longitudinal optical phonon frequency ωLO2, triplet excitations, and the optical absorptions assigned to charged and neutral solitons and to polarons. We conclude that the e-p coupling is substantially weaker than that suggested by purely e-p models and that the e-e interaction parameters are in the intermediate coupling regime (U ∼ 2.5t0), consistent with values deduced from the finite polyenes. © 1991.
• Kawabe, Y., Jarka, F., Peygambarian, N., Guo, D., Mazumdar, S., Dixit, S. N., & Kajzar, F. (1991). Roles of band states and two-photon transitions in the electroabsorption of a polydiacetylene. Physical Review B, 44(12), 6530-6533.
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  Abstract: We report results of experimental and theoretical investigations of electroabsorption (EA) in a DCH polydiacetylene. In addition to the Stark shift of the exciton, a high-energy feature is seen in EA, whose origin has been the subject of controversy. Based on exact theoretical calculation of the EA within an extended Peierls-Hubbard model, we ascribe the high-energy signal to transitions to a continuum threshold. We briefly discuss the contributions of these band threshold states to the third-order optical nonlinearity, (3). © 1991 The American Physical Society.
• Lin, H., Campbell, D., & Mazumdar, S. (1991). Normal state broken symmetries in quasi-two-dimensional, quarter-filled charge-transfer solids. Synthetic Metals, 43(3), 3613-.
• Mazumdar, S., Campbell, D. K., & Lin, H. -. (1991). A novel valence-transition model for the pressure-induced metal-to-insulator transition in the organic conductor (2,5DM - DCNQI)₂Cu. Synthetic Metals, 42(1-2), 1859-.
• Mazumdar, S., Campbell, D. K., & Lin, H. -. (1991). Novel valence-transition model for the pressure-induced metal-to-insulator transition in the organic conductor (2,5DM-DCNQI)₂Cu. Synthetic Metals, 42(1-2), 1859-.
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  Abstract: Although metal-to-insulator transitions in organic charge-transfer solids are common, the transition in (2,5 DM - DCNQI)$-24/Cuis unique in the sense that this is the only case where the insulating form is the high-pressure phase. In all other known cases, pressure leads to a transition in the opposite direction. A theory of this novel phase transition is proposed that emphasizes integral, as opposed to mixed, valence, and make several experimental predictions. The model is analogus to one previously used to describe the neutral-to-ionic transition in donor-acceptor charge-transfer solids, such as TTF-Chloranil. The ionicity of a narrow band donor-acceptor system is essentially determined by the competition between the high ionization energy of the donor, which favors a lower ionicity, and the large Madelung energy, which favors a higher ionicity.
• Mazumdar, S., Guo, D., & Dixit, S. N. (1991). Excitonic optical nonlinearity in conjugated polymers and aromatic molecular chromophores - A theoretical approach. Synthetic Metals, 43(1-2), 3189-.
• Mazumdar, S., Guo, D., & Dixit, S. N. (1991). Microscopic mechanism of optical nonlinearity in conjugated polymers and other quasi-one-dimensional systems. Proceedings of SPIE - The International Society for Optical Engineering, 1436, 136-149.
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  Abstract: We present a microscopic mechanism of optical nonlinearity in quasi-one-dimensional semiconductors within the context of rigid band Peierls-extended Hubbard models. A detailed configuration space analysis is done to predict the dominant excitation paths. We show that only two channels contribute to the bulk of the optical nonlinearity, even though an infinite number of channels are possible in principle. Most importantly, these channels involve a virtual two photon excited state whose relative energy should be nearly parameter independent in the infinite chain limit. This would imply that the mechanism of optical nonlinearity, as well as the frequency dependence of the third order optical susceptibility, are also largely parameter independent. This universality is a consequence of the one dimensionality alone and remains valid for arbitrary convex Coulomb interactions. These conjectures are confirmed by exact numerical calculations on finite chains that do very careful analysis of finite size effects.
• Mazumdar, S., Lin, H. Q., & Campbell, D. K. (1991). The phase diagram of 2:1 cationic organic charge transfer salts. Synthetic Metals, 43(3), 4047-4050.
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  Abstract: We propose a new theoretical approach, stressing the role of Coulomb interactions and bandfilling effects, to describe the phase diagram of the 2:1 cationic organic charge transfer salts, including those exhibiting field induced spin density wave (FISDW) behavior. We predict three distinct nonsuperconducting phases. © 1991.
• Barlingay, C., García-Vázquez, V., Falco, C. M., Mazumdar, S., & Risbud, S. H. (1990). Effects of zinc substitution on the electron superconductor Nd1.85Ce0.15CuO4-. Physical Review B, 41(7), 4797-4800.
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  Abstract: We report the effects of Zn substitution for Cu on the superconducting properties of the electron superconductor Nd1.85Ce0.15CuO4-. Detailed comparisons are made to earlier work on Zn substitution effects on the single-CuO2-layer hole superconductor La1.85Sr0.15CuO4-. Theoretical implications are discussed briefly. © 1990 The American Physical Society.
• Mazumdar, S. (1989). Comment on "peierls instability in the two-dimensional half-filled Hubbard model". Physical Review B, 39(16), 12324-12326.
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  Abstract: The analyses of the two-dimensional Peierls-Hubbard model by Tang and Hirsch and by the present author are compared. While there is agreement with the principal conclusion of Tang and Hirsch, viz., that the effects of the Hubbard interaction on the Peierls bond alternation are different in one and two dimensions, I disagree on three related issues. First, the (, ) phonon which dominates in the uncorrelated limit is different from the one Tang and Hirsch believe wins [(, ) along x axis]. Second, the energy crossing between the (, ) and (, 0) phonons found by Tang and Hirsch for large magnitudes of the Hubbard interaction can occur only in the unrealistic limit of very strong bond alternation where the strength of the weak bond is close to zero. Finally, the disappearance of the bond alternation due to the Hubbard interaction is much more rapid than that calculated by Tang and Hirsch"the bond alternation should become weaker even at small U. This suppression of the bond alternation is indeed related to the long-range antiferromagnetism in two dimensions, as surmised by Tang and Hirsch. © 1989 The American Physical Society.
• Mazumdar, S. (1989). Experimental predictions for the normal state of electron-doped high-temperature superconductors. Physica C: Superconductivity and its applications, 161(3), 423-430.
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  Abstract: The discovery of n-type high-temperature superconductors implies an electron-hole symmetry that is missing in two-band descriptions of the copper oxides. At the same time, several peculiar differences between the new electron-carriers and the older hole-carriers cannot be explained within the Cu-based single-band Hubbard model. It is shown that these differences are expected within a dopant-induced valence transition mechanism proposed earlier, in which a first-order valence transition accompanies the antiferromagnetic-superconductor transition in the copper oxides. Based on this theoretical mechanism, a large number of explicit testable experimental predictions are made for the normal state of Nd(Ce)CuO. © 1989.
• Mazumdar, S., & Dixit, S. N. (1989). Electronic excited states in conjugated polymers. Synthetic Metals, 28(3), D463-D468.
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  Abstract: We re-examine the subgap neutral and charged soliton absorptions in polyacetylene and the 21 Ag state in finite polyenes within the context of a simple Peierls-Hubbard model. Our motivation is to determine the origin of the optical gap in trans-polyacetylene and to determine the magnitude of the effective on-site Coulomb repulsion. We conclude that this term lies between 2.25t0 and 2.75t0, where t0 is the nearest neighbor one-electron hopping integral. © 1989.
• Mazumdar, S. (1988). Coulomb correlations, broken symmetries and dimensionality effects. Synthetic Metals, 27(1-2), A127-A132.
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  Abstract: Within single particle theories occurrence of spatial broken symmetries depend only on nesting. This picture is shown to change dramatically for nonzero Coulomb interactions, where dimensionality plays a much stronger role. We examine the Peierls-Hubbard model in two dimension, and show that while the simple Peierls instability can occur in one, two or three dimension for perfect nesting, the Hubbard interaction destroys this instability for dimensionality greater than one. The spin-Peierls transition is a unique feature of one dimension. The above is also related to the occurrence of antiferromagnetism in two dimension. Recent observation of the vanishing of the spin-Peierls phase and the appearance of an antiferromagnetic phase in TMTTF-salts under pressure can be explained within the present theory. © 1988.
• Mazumdar, S., & Ramasesha, S. (1988). Organic superconductivity: An experiment based theory. Synthetic Metals, 27(1-2), A105-A113.
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  Abstract: Within the last decade the superconducting critical temperature in the organics has gone up from about 1K to greater than 10K, but the search for a comprehensive theory is still on. A completely experiment based theory is presented here. The only special features of the organic superconductors, as compared to other segregated stack charge-transfer solids, are quasi-two dimensionality (as opposed to the more common one dimensionality) and quarter-filled bands. We have shown earlier that there exists a single theoretical model which explains the experimental behavior of the complete family of one dimensional charge transfer solids, including enhanced as well as unenhanced magnetic susceptibility, occurrence as well as nonoccurrence of 4kF instability, - the actual behavior depending on the bandfilling. It will now be shown that the same theoretical model, when extended to two dimension, explains organic superconductivity. Structural distortion driven by Coulomb repulsion, and not magnetism, is the key to understanding exotic non-BCS superconductivity. Numerical results supporting the present model are presented. © 1988.
• Campbell, D. K., Baeriswyl, D., & Mazumdar, S. (1987). Electron-electron interaction effects in quasi-one-dimensional conducting polymers and related systems. Synthetic Metals, 17(1-3), 197-202.
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  Abstract: We review recent results on the role of electron-electron (e-e) interactions in quasi-one-dimensional conducting polymers and related systems. Within the Peierls-Hubbard model, the effects of both short-range (on-site U and nearest neighbor V1) and long-range (Vj, j ≥ 2) are examined, the former using quantum Monte Carlo and the latter with exact diagonalization of finite size systems and analytic arguments. We also discuss optical absorption properties in the presence of electronic correlation, focusing on the weak and strong coupling limits. © 1987.
• Mazumdar, S., & Dixit, S. N. (1987). Towards a unified theory of segregated stack organic charge-transfer solids. Synthetic Metals, 19(1-3), 93-98.
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  Abstract: Theoretical description of the complete family of quasi-one-dimensional segregated stack organic charge-transfer solids within a single unified model has remained elusive, largely due to the rich variety of behavior within the family. We claim that a previously proposed extended Hubbard model explains the variety in optical and magnetic behavior, as well as the irregular appearance of the 4kF x-ray scattering. We do not attempt to explain the superconductivity in the TMTSF and ET-salts, but we believe that theoretical modelling of even these latter class of materials must start from similar "correlated electron" models. © 1987.
• Baeriswyl, D., Campbell, D. K., & Mazumdar, S. (1986). Correlations and defect energies [1]. Physical Review Letters, 56(14), 1509-.
• Campbell, D. K., Baeriswyl, D., & Mazumdar, S. (1986). Coulomb correlation effects in quasi-one-dimensional conductors. Physica B+C, 143(1-3), 533-537.
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  Abstract: We review recent results on the role of electron-electron (e-e) interactions - "correlation effects" - in quasi-one-dimensional conductors. Within the Peierls-Hubbard model, we examine the consequences of short range (on-site U and nearest neighbor V) e-e interactions for ground state properties, nonlinear excitations, and optical absorption. Our techniques include quantum Monte Carlo and weak and strong coupling perturbative arguments. © 1986.
• Campbell, D. K., DeGrand, T. A., & Mazumdar, S. (1986). Quantum monte carlo studies of electron-electron interaction effects in conducting polymers. Journal of Statistical Physics, 43(5-6), 803-814.
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  Abstract: We discuss recent studies, using the quantum ensemble projector Monte Carlo (EPMC) method, of theoretical models of conducting polymers. Our focus is on the consequences of incorporating direct electron-electron interactions into the "standard" electron-phonon interaction models. Among the observables we examine one energetics of purely dimerized ground states, single solitons, soliton pairs, averaged spin and charge distributions, and local correlation functions. © 1986 Plenum Publishing Corporation.
• Mazumdar, S., & Campbell, D. (1986). Bond alternation in the infinite polyene: Effect of long-range Coulomb interactions. Synthetic Metals, 13(1-3), 163-172.
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  Abstract: We investigate the effects of long-range Coulomb interactions on bond and site dimerizations in a one-dimensional half-filled band. It is shown that the ground state broken symmetry is determined by two sharp inequalities involving the Coulomb parameters. Broken symmetry with periodicity 2kF is guaranteed only if the first inequality (downward convexity of the intersite potential) is obeyed, while the second inequality gives the phase boundary between the bond-dimerized and site-dimerized phases. Application of these inequalities to the Pariser-Parr-Pople model for linear polyenes shows that the infinite polyene has enhanced bond alternation for both Ohno and Mataga-Nishimoto parameterizations of the intersite Coulomb terms. The possible role of distant neighbour interactions in photogeneration experiments is discussed. © 1986.
• Mazumdar, S., & Dixit, S. N. (1986). Unified theory of segregated-stack organic charge-transfer solids: Magnetic properties. Physical Review B, 34(6), 3683-3699.
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  Abstract: A theoretical description of the complete family of quasi-one-dimensional segregated-stack charge-transfer solids within a single model has remained elusive mostly because of the rich variety of behavior within the family. In particular, materials with both strongly enhanced as well as almost unenhanced static magnetic susceptibilities are known. Furthermore, a large number of different interpretations of the static magnetic susceptibility data have led to an intense controversy over the role of Coulomb correlations in these narrow-band systems. By comparing structurally similar materials with different magnetic behavior we show that (a) these differences do not originate in differences in molecular properties or crystal structures and (b) none of the simple electron-electron or electron-phonon coupled models can explain the observed differences in susceptibility behavior. Within a previously proposed extended Hubbard model we then show that the susceptibility is expected to vary strongly and systematically as a function of the degree of charge transfer. Detailed comparisons of both the magnitude of the high-temperature susceptibilities as well as the temperatures at which the susceptibilities peak for a large number of materials are made with theoretical predictions to prove the validity of our model. In addition we discuss how other properties of the complete family can be explained and predicted within the present model, and show that the parameters of the model can be obtained from charge-transfer absorption data. Several of the newly synthesized materials have been suggested to have weak electron correlations based on weakly enhanced susceptibilities and the absence of the 4kF instability. We propose, however, that the above experimental features are consequences of the specific charge-transfer range within which these materials lie, and do not imply weak correlations. © 1986 The American Physical Society.
• Soos, Z. G., Kuwajima, S., & Mazumdar, S. (1986). MODIFIED PPP APPROACH TO POLYDIACETYLENE EXCITATIONS.. Polymeric Materials Science and Engineering, Proceedings of the ACS Division of Polymeric Material, 54, 497-501.
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  Abstract: A purely pi -electron description via the Pariser-Parr-Pople (PPP) model is introduced, and standard molecular parameters are adopted in application to polydiacetylene (PDA). Transfer integrals are presented to describe the single, double, and triple bonds of the pi network. The result of PPP, theory appears to be that Coulomb coupling of pi and pi prime electrons in PDA results in two low-energy absorptions with comparable intensity, rather than a single absorption as in polyacetylene.
• Soos, Z. G., Mazumdar, S., & Kuwajima, S. (1986). Extended PPP model: 2¹A and 1³B states of polydiacetylene fragments. Physica B+C, 143(1-3), 538-541.
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  Abstract: Coulomb correlations between an extended π subsystem and localized π′ dimers are found for the 21A and 13B states of polydiacetylene (PDA) fragments. In contrast to the red shift and splitting found previously for dipole-allowed 1B excitations, π-π′ coupling blue shifts 21A and 13B but hardly affects the 13B fine structure constants. Pseudocovalent contributions to correlated states rationalize these results. © 1986.
• Tripathy, S. K., Chen, Y. J., Rubner, M., Thakur, M. K., Carter, G. M., Mazumdar, S., Sandman, D. J., & Ahuja, R. (1986). MOLECULAR DESIGN AND INTERPLAY OF STRUCTURAL, OPTICAL AND ELECTRONIC PROPERTIES IN POLYDIACETYLENES.. Polymeric Materials Science and Engineering, Proceedings of the ACS Division of Polymeric Material, 55, 519-521.
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  Abstract: The combination of synthesis, thin film monomer single crystal growth, solid state polymerization, variation in the side group architecture and finally the novel optical properties make polydiacetylenes a suitable material of choice for many emerging optical technologies. This report discusses recent results of investigations of these systems. This includes growth and characterization of mono and multilayer film assemblies and thin film single crystal polydiacetylenes, measurement of linear and nonlinear optical properties of these systems, and design and synthesis of novel monomers expected to possess enhanced optical properties.
• Mazumdar, S., & Campbell, D. K. (1985). Broken Symmetries in a One-Dimensional Half-Filled Band with Arbitrarily Long-Range Coulomb Interactions. Physical Review Letters, 55(19), 2067-2070.
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  Abstract: The nature of the ground-state broken symmetry in a one-dimensional half-filled band with arbitrarily long-range Coulomb interactions between electrons is shown to be precisely given by two simple inequalities involving the Coulomb parameters. It is further shown that non-2kF periodicity may occur if the first of these inequalities is violated. © 1985 The American Physical Society.
• Campbell, D. K., DeGrand, T. A., & Mazumdar, S. (1984). Soliton energetics in Peierls-Hubbard models. Physical Review Letters, 52(19), 1717-1720.
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  Abstract: We study the effect of electron-electron correlations on the energetics of solitons in electron-phonon models of quasi one-dimensional materials. In these combined Peierls-Hubbard models, by use of quantum Monte Carlo techniques, we (1) establish that the ground state of an odd chain, singly charged or neutral, is a soliton; (2) calculate neutral-soliton creation energies; and (3) prove that "soliton doping" persists in the presence of correlations. We discuss the relevance of our results for trans-polyacetylene. © 1984 The American Physical Society.
• Dixit, S. N., & Mazumdar, S. (1984). Electron-electron interaction effects on Peierls dimerization in a half-filled band. Physical Review B, 29(4), 1824-1839.
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  Abstract: A novel real-space approach to dimerization in a half-filled band is developed to investigate effects of electron-electron interactions on the Peierls instability. Dimerization is shown to be a result of imperfect resonance between pairs of electron configurations related to each other by a mirror-plane symmetry passing through the longest diagonal of the infinite ring, and the kinetic- and potential-energy contributions to the barrier to resonance are identified separately. The effects of including the on-site, nearest-neighbor, and next-nearest-neighbor interactions are investigated, and in each case it is shown that the enhancement or reduction in dimerization can be predicted from elementary physical arguments. These predictions are then substantiated by exact numerical calculations on a ten-site ring, and finite-size effects are shown to be small. The principal results that are obtained are the following: (i) The on-site correlation U strongly enhances the dimerization, the enhancement being strongest for U4t0, where 4t0 is the bandwidth of the uniform chain; (ii) the nearest-neighbor interaction V1 further enhances the dimerization until V112U, while V1>12U favors a uniform chain with a different broken-symmetry ground state, an on-site charge-density wave; (iii) for V112U, the second-neighbor interaction V2 reduces the dimerization slightly, although the dimerization is still stronger than that with an effective nearest-neighbor interaction V1-V2; (iv) for V1>12U, V2 destroys the on-site charge-density wave and the ground state is strongly dimerized again. The complete Parisier-Parr-Pople (PPP) Hamiltonian is discussed, and it is pointed out that the above results, together with the excited-state orderings in the PPP Hamiltonian, strongly indicate that the ground state of the PPP Hamiltonian is the dimerized state. The excited-state orderings in finite polyenes, spin-density distributions in polyacetylene, and our theoretical results all indicate then that explicit inclusion of Coulomb interactions may be necessary for an accurate description of the ground and excited states in polyacetylene. © 1984 The American Physical Society.
• Mazumdar, S., & Dixit, S. N. (1984). Electron-electron interactions and broken symmetry in one dimension. Synthetic Metals, 9(2), 275-281.
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  Abstract: A novel real-space approach to dimerization in a half-filled band is developed to investigate effects of electron-electron interactions on the Peierls instability. Dimerization is shown to be a result of imperfect resonance between pairs of electron configurations related to each other by a mirror plane symmetry passing through the longest diagonal of the infinite polygon. Coulomb interactions enhance the barrier to perfect resonance, so that dimerization itself is enhanced for moderate values of the correlation parameter. Recent experimental findings and the present theoretical work then suggest that explicit inclusion of Coulomb interactions between electrons may be necessary for an accurate description of ground and excited states in polyacetylene. © 1984.
• Mazumdar, S., & Dixit, S. N. (1984). Interacting charge-ordered states in a correlated diatomic polymer. Physical Review B, 29(4), 2317-2320.
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  Abstract: Using physical arguments based on a real-space view of Peierls dimerization, we predict that site energy differences in a diatomic polymer can increase the dimerization in the presence of strong Coulomb correlations. This increase in dimerization continues until the difference in site energies reaches a maximum value, which itself is an increasing function of the correlation parameter. The effect of the site energy differences in the strongly correlated region is therefore opposite to that in the uncorrelated limit. Exact numerical calculations are presented to substantiate this surprising conclusion. © 1984 The American Physical Society.
• Mazumdar, S., Dixit, S. N., & Bloch, A. N. (1984). Correlation effects on charge-density waves in narrow-band one-dimensional conductors. Physical Review B, 30(8), 4842-4845.
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  Abstract: We show quantitatively that the conditions for charge ordering in one-dimensional metals depend strongly on the band filling and the relative magnitudes of intermolecular electronic correlations. We thereby explain the irregular appearance of 4kF charge-density waves in conducting organic charge transfer salts. For band fillings incrementally larger than one-fourth, we predict a new kind of Coulomb-induced defect. © 1984 The American Physical Society.
• Mazumdar, S., & Bloch, A. N. (1983). Systematic trends in short-range coulomb effects among nearly one-dimensional organic conductors. Physical Review Letters, 50(3), 207-211.
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  Abstract: A resolution to the controversy over the magnitude of the effective short-range Coulomb interaction in molecular conductors is proposed by showing that it depends very strongly on band filling because of intermolecular correlations. © 1983 The American Physical Society.
• Mazumdar, S., & Dixit, S. N. (1983). Coulomb effects on one-dimensional peierls instability: The peierls-hubbard model. Physical Review Letters, 51(4), 292-295.
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  Abstract: A novel approach to dimerization is developed, based on a many-body valence-bond approach, that predicts enhancement of the dimerization on inclusion of the on-site electron repulsion. Exact numerical calculations are presented to substantiate this conclusion. © 1983 The American Physical Society.
• Mazumdar, S., & Soos, Z. G. (1981). Valence-bond analysis of extended Hubbard models: Charge-transfer excitations of molecular conductors. Physical Review B, 23(6), 2810-2823.
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  Abstract: The low-energy charge transfer (CT) excitation characteristic of both -molecular conductors and complex-ion-radical salts is interpreted as a nearest-neighbor Coulomb interaction V that is comparable to the bandwidth, 4|t|. Partly filled segregated regular stacks in organic conductors are represented by extended Hubbard models, whose exact CT energies and intensities are obtained by diagrammatic valence-bond (VB) methods for four electrons on finite rings and chains, together with an approximate treatment of V in partly filled infinite stacks for infinite on-site correlations U. Finite V4|t| yields an intense low-lying CT band, containing V and U-2V excitations, that depends weakly on the band filling. Finite V also splits the usual CT absorption around U for half filled bands into strong absorptions around U-V, weak ones around U, and much weaker bands around U+V and U+2V. The CT spectra of mixed-valence tetrathiofulvalene (TTF) salts are modeled with V0.4 eV, U1.4 eV, and |t|0.10-0.13 eV. Similar CT transitions in complex tetracyanoquinodimethane (TCNQ) salts are consistent with the insensitivity of the V peak's position to the filling or the structure. Restricting the basis to one valence state per site produces several general consequences for dipole-allowed optical transitions. © 1981 The American Physical Society.
• Mazumdar, S., & Soos, Z. G. (1979). Valence bond theory of narrow-band conductors. Synthetic Metals, 1(1), 77-94.
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  Abstract: A diagrammatic valence bond (VB) theory is introduced for solids with one valence state per site, Coulomb intersite interactions, and nearest-neighbor electron transfers leading to a bandwidth 4 |t|. The VB method is applied to partly-filled regular segregated stacks of acceptors (A) and donors (D), which occur in all π-molecular organic conductors. The dimensions of the site representation for Ne = γN electrons or holes on N > Ne sites are found, each VB assignment is represented by a diagram, and the interconnection of these many-electron site functions due to electron transfers is also obtained diagrammatically. The resulting configuration interaction (CI) between all orbital assignments is solved exactly in subspaces of total spin, S. The complete thermodynamics of Ne = 4 electrons on chains of N = 4, 5, 6, and 7 sites are found for the Hubbard choice of on-site repulsion, U. The VB method extends the computation of the susceptibility χ(T) of partly-filled Hubbard models to the previously inaccessible regime U ∼ 4|t| which, however, is suggested by experiment. Comparison with absolute χ(T) data for TTF-TCNQ is slightly improved for U ∼ 4|t| = 0.56 eV. Extensions of the VB method to more realistic potentials, to other data, and to N → ∞ extrapolations are discussed. © 1979.
• Soos, Z. G., Bondeson, S. R., & Mazumdar, S. (1979). Magnetic analog of the mott transition. Chemical Physics Letters, 65(2), 331-334.
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  Abstract: A model hamiltonian with only on-site interactions is introduced and solved for a hypothetical ... LiHLiH... array with variable lattice spacing R. The paramagnetic (large R) system becomes diamagnetic, with a finite magnetic gap, as the bandwidth is increased. The sharp transition is marked by a symmetry change of the exact ground state | ψ〉. © 1979.
• Nöthe, D., Moroni, W., Keller, H. J., Soos, Z. G., & Mazumdar, S. (1978). Triplet spin excitons in organic charge transfer complexes. Solid State Communications, 26(11), 713-716.
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  Abstract: Electron paramagnetic resonance (EPR) spectra of single crystals of the 1 : 1 charge transfer (CT) complex of N, N'-dimethyldihydrophenazine (M2P) and of tetracyanoquinodimethane (TCNQ) provide the first case of thermally activated, with ΔEp = 0.60 ± 0.06 eV, triplet spin excitons observed in an organic CT solid. The narrow EPR lines and resolved fine structure splittings indicate mobile triplets based on adjacent M2P+TCNQ- ion radicals above a largely neutral ground state. © 1978.
• Soos, Z. G., & Mazumdar, S. (1978). Magnetic gap and partial charge transfer in mixed donor-acceptor stacks. Chemical Physics Letters, 56(3), 515-518.
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  Abstract: The ground state charge transfer γ(z) and magnetic gap ΔEm(z) of organic donor-acceptor complexes with one dimensional⋯D+γA-γD+γA+γ⋯ stacks are computed for arbitrary ratios of z = δ/2 1 2|t|where |t| is the charge-transfer integral and -2δ is the excitation energy for a D+A- electron-hole pair. Valence bond techniques give exact results for rings of N = 4, 6, 8, and 10 sites, which suffice to show that the neutral-ionic transition and ΔEm(z) → 0 occur at zc = 0.53 ± 0.01 with γ(zc) = 0.68 ± 0.01. © 1978.
• Soos, Z. G., & Mazumdar, S. (1978). Neutral-ionic interface in organic charge-transfer salts. Physical Review B, 18(4), 1991-2003.
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  Abstract: Charge transfer (CT) stabilization in linear stacks D+A-D+A- of -electron donors (D) and acceptors (A) involve spin-dependent configuration interactions that are treated exactly in rings of N=4, 6, 8, 10 sites, and extrapolated to N, by adapting valence-bond techniques to electron-hole excitations. The ground state CT (z) and the magnetic gap Em(z)2|t| to the lowest triplet are computed for arbitrary z=2|t|, where -2 is the energy for DAD+A-, -|t|=D+A-|H|DA is the Mulliken CT integral, and D2+, A2- sites are excluded. The spin degeneracy of A- and D+ ion radicals is treated exactly. Instead of the discontinuous change from =0 to =1 in the limit |t|0, finite overlap gives a continuous (z) and (zc)=0.680.01 at the neutral-ionic interface zc=0.530.01. The magnetic gap Em is finite for zzc, where there is a diamagnetic to paramagnetic transition and the ground state switches from k=0 to k= symmetry. Collective effects due to long-range three-dimensional Coulomb interactions are included in a Hartree approximation and produce a first-order transition, with discontinuous (z), when the critical value m2|t|=1.40.1 of the Madelung stabilization m of a dimer is exceeded. The puzzling magnetic gaps in paramagnetic organic CT salts with mixed regular stacks arise naturally for partial CT and z
• Soos, Z. G., Mazumdar, S., & Cheung, T. T. (1978). VALENCE BOND THEORY OF ORGANIC CHARGE-TRANSFER SALTS.. Mol Cryst Liq Cryst, 52(1-4), 93-102.
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  Abstract: A valence bond (VB) description is introduced for organic charge-transfer (CT) salts based on mixed stacks . . . D** plus **yA** minus **yD** plus **yA** minus **y . . . of donors (D) and acceptors (A). Extrapolating exact results for finite rings shows that the magnetic gap DELTA E//m is finite for gamma less than gamma //c equals 0. 68 plus or minus 0. 01 and vanishes for gamma greater than gamma //c as expected in the paramagnetic ionic limit gamma equals 1. The small finite DELTA E//m in the ″ionic″ TMPD-TCNQ complex indicates incomplete CT, in agreement with the resonant Raman result of gamma similar 0. 7. The extension of diagrammatic VB techniques to partly-filled segregated ion-radical stacks like . . . A** minus **yA** minus **yA** minus **yA** minus **y or D** plus **yD** plus **yD** plus **yD** plus **y . . . is discussed.

Presentations

• Clay, R. T. (2019, September). Valence-Bond theory of antiferromagnetism, charge-ordering and correlated-electron superconductivity in organic charge-transfer solids. International Symposium on Crystalline Organic Materials. Tomar, Portugal: Instituto Superior Técnico, Portugal.
• Khan, S. (2019, July). THE ROLE OF THE QUANTUM-ENTANGLED TRIPLET-TRIPLET BIEXCITON IN SINGLET FISSION: RECENT THEORETICAL DEVELOPMENTS. 13th International Conference on Optical Probes of Organic and Hybrid Optoelectronic Materials and Applications. Vilnius, Lithuania: Lithuanian Physical Society and Vilnius University.
• Khan, S. (2018, June). Optical Probes of the Quantum-Entangled Triplet-Triplet State in Singlet Fission Materials. 2018 Center for Nonlinear Studies Conference on Excited State Processes. Santa Fe, NM: CNLS, Los Alamos National Laboratory.
• Mazumdar, S. (2017, June). Theory of Transient Excited State Absorptions in Solid Pentacene. Optical Probes in Conjugated Polymers 2017. Quebec City, Canada.
• Mazumdar, S., & Khan, S. (2017, summer). Photophysics of Pentacene Dimers. Singlet Fission Workshop. Lyons, CO: University of Colorado.
• Mazumdar, S. (2016, June). Theory of Primary Photoexcitations in Low Bandgap Donor-Acceptor Copolymers. Excited State Processes 2016. Santa Fe, NM: Los Alamos National Laboratory.
• Mazumdar, S. (2016, June-July). Theory of Primary Photoexcitations in Low Bandgap Donor-Acceptor Copolymers. International Conference on Synthetic Metals 2016. Guangzhou, China: ICSM.
• Mazumdar, S. (2015, August). Theory of Singlet Fission in Carotenoid and Acene Crystals, and in Low Band Gap Donor-Acceptor Polymers. Solar Solutions to Energy and Environmental Problems. Telluride, Colorado: Telluride Science Research Center.
• Mazumdar, S. (2015, August). Unique Coulomb enhancement of superconducting correlations in the frustrated quarter-filled band. Materials and Mechanisms of Superconductivity, M2S2015. Geneva, Switzerland: M2S.
• Mazumdar, S. (2015, June). Theory of Singlet Fission in Polyenes, Acene Crystals, and Covalently Linked Acene Dimers. Singlet Fission Workshop. Lyons, Colorado: University of Colorado, Boulder.
• Mazumdar, S. (2015, September). ELECTRON CORRELATION INDUCED SUPERCONDUCTIVITY IN THE QUARTER-FILLED BAND. International Symposium on Crystalline Organic Materials. Bad Gögging, Germany: ISCOM.
• Dutta, T. (2014, July). Unified theoretical approach to superconductivity in organic charge-transfer solids, phenacenes and fullerides.. Gordon Research Conference on Conductivity & Magnetism in Molecular Materials. Lewiston, Maine: Gordon Research Conference.
• Dutta, T. (2014, June). What is special with molecular valence 3 in metal-intercalated phenacenes?. International Conference on Science and Technology of Synthetic Metals 2014. Turku, Finland.
• Dutta, T. (2014, November). Unified theoretical approach to superconductivity in organic charge-transfer solids, phenacenes and fullerides.. Ushimado International Workshop on Physics and Chemistry of Novel Superconductors and Related Materials. Ushimado, Okayama Prefecture, Japan.: Okayama Univ. and EU-Japan superconductivity project.
• Mazumdar, S. (2014, July). The Chemical Physics of Unconventional Superconductivity. The VIIIth Congress of the International Society of Theoretical Chemical Physics. Budapest, Hungary: International Society of Theoretical Chemical Physics..