Sumitendra Mazumdar
 Department Head, Physics
 Professor, Physics
 Professor, Optical Sciences
 Professor, Chemistry and Biochemistry
Contact
 (520) 6216803
 PhysicsAtmospheric Sciences, Rm. 000375
 Tucson, AZ 85721
 sumit@physics.arizona.edu
Degrees
 Ph.D. Chemistry
 Princeton University, Princeton, New Jersey
 Valence bond theory of organic chargetransfer solids
 M.S. Chemistry
 Indian Institute of Technology, Kanpur, India.
Interests
No activities entered.
Courses
202021 Courses

Hnrs Intr Optics+Thermod
PHYS 162H (Spring 2021) 
Intro Optics + Thermodyn
PHYS 142 (Spring 2021) 
Independent Study
PHYS 599 (Fall 2020)
201920 Courses

Independent Study
PHYS 599 (Spring 2020) 
SolidState Physics
PHYS 460 (Spring 2020) 
Independent Study
PHYS 599 (Fall 2019)
201819 Courses

Hnr Intro Rel+Quant Phys
PHYS 263H (Spring 2019) 
Independent Study
PHYS 599 (Spring 2019) 
Dissertation
PHYS 920 (Fall 2018) 
Independent Study
PHYS 599 (Fall 2018)
201718 Courses

Condensed Matter Physics
PHYS 560A (Spring 2018) 
Dissertation
PHYS 920 (Spring 2018) 
Dissertation
PHYS 920 (Fall 2017)
201617 Courses

Dissertation
PHYS 920 (Spring 2017) 
Honors Independent Study
PHYS 199H (Spring 2017) 
Independent Study
PHYS 499 (Spring 2017) 
Intro Optics + Thermodyn
PHYS 142 (Spring 2017) 
Intro Optics + Thermodyn
PHYS 143 (Spring 2017) 
Directed Research
PHYS 492 (Fall 2016) 
Dissertation
PHYS 920 (Fall 2016) 
Independent Study
PHYS 599 (Fall 2016) 
Internship
PHYS 593 (Fall 2016)
201516 Courses

Internship
PHYS 593 (Summer I 2016) 
Dissertation
PHYS 920 (Spring 2016) 
Hnrs Intr Optics+Thermod
PHYS 162H (Spring 2016)
Scholarly Contributions
Journals/Publications
 Khan, S. (2020). Free Triplets Versus Bound TripletTriplet Biexciton in Intramolecular Singlet Fission Materials: StructureProperty Correlations. JOURNAL OF PHYSICAL CHEMISTRY C, 124, 11711177. doi:10.1021/acs.jpcc.9b09831
 Clay, R. (2019). Theory of triangular lattice quasionedimensional chargetransfer solids. Physical Review B, 100, 115158. doi:DOI10.1103/PhysRevB.100.115158
 Mazumdar, S., & Clay, R. T. (2019). From charge and spinordering to superconductivity in the organic chargetransfer solids R. T. Clay, S. Mazumdar. Physics Reports, 788, 189.More infoSolicited review article, 104 pages, 63 figures
 Pradhan, S., & Ramasesha, S. (2019). Correlated Electronic Properties of a Graphene Nanoflake: Coronene. MOLECULES, 24, 730 [14]. doi:10.3390/molecules24040730
 Khan, S. (2018). Optical probes of the quantumentangled triplettriplet 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 electrondoped and holedoped copper oxidesed. Physical Review B, 98, 205153 (128). doi:https://doi.org/10.1103/PhysRevB.98.205153
 Mazumdar, S. (2017). Bond patterns and charge order amplitude in 1/4filled chargetransfer 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, 44684478. doi:DOI: 10.1021/acs.jpclett.7b01829
 Mazumdar, S., & Khan, S. (2017). Theory of Transient Excited State Absorptions in Pentacene and Derivatives: TripletTriplet Biexciton versus Free Triplets. JOURNAL OF PHYSICAL CHEMISTRY LETTERS, 8, 59435948. 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 piConjugated DonorAcceptor 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 pairpair correlations in a 3/4filled model for kappa(BEDTTTF)(2)X. Physical Review B, 93(20), 205111. doi:10.1103/PhysRevB.93.205111
 Mazumdar, S. (2016). Coulombenhanced superconducting pair correlations and pairedelectron liquid in the frustrated quarterfilled 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 DonorAcceptor 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), 69666979.
 Mazumdar, S., Aryanpour, K., Dutta, T., Huynh, U., & vardeny, Z. (2015). Theory of Primary Photoexcitations in DonorAcceptor 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 twophoton states in polycyclic aromatic hydrocarbons: Evidence for strong electron correlations. Journal of Physical Chemistry C, 118(6), 33313339.More infoAbstract: Strong electron correlation effects in the photophysics of quasionedimensional πconjugated organic systems such as polyenes, polyacetylenes, polydiacetylenes, etc., have been extensively studied. Far less is known on correlation effects in twodimensional πconjugated systems. Here we present theoretical and experimental evidence for moderate repulsive electronelectron 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 oneelectron and meanfield theories. Our results reflect similarities as well as differences in the role and magnitude of electron correlation effects in these twodimensional molecules compared to those in polyenes. © 2014 American Chemical Society.
 Aryanpour, K., Shukla, A., & Mazumdar, S. (2014). Electron correlations and twophoton states in polycyclic aromatic hydrocarbon molecules: A peculiar role of geometry. Journal of Chemical Physics, 140(10), 104301.More infoAbstract: We present numerical studies of one and twophoton excited states ordering in a number of polycyclic aromatic hydrocarbon molecules: coronene, hexaperihexabenzocoronene, and circumcoronene, all possessing D6h point group symmetry versus ovalene with D2h symmetry, within the PariserParrPople model of interacting πelectrons. The calculated energies of the twophoton states as well as their relative twophoton absorption crosssections within the interacting model are qualitatively different from singleparticle descriptions. More remarkably, a peculiar role of molecular geometry is found. The consequence of electron correlations is far stronger for ovalene, where the lowest spinsinglet twophoton 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 twophoton states in D2h ovalene but not in those with D6h symmetry. © 2014 AIP Publishing LLC.
 Dutta, T., & Mazumdar, S. (2014). Theory of metalintercalated phenacenes: Why molecular valence 3 is special. Physical Review B, 89(24), 245129.More infoWe develop a correlatedelectron 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 correlatedelectron 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 manybodyfiniteclustercalculationsshowthatwhilethesystemswithmolecularcharges of\ − 1and −2 are one and twoband MottHubbard semiconductors, respectively, molecular charge −3 gives two nearly 3/4filled 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 chargetransfer 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), 10531059.
 Mazumdar, S., & Dutta, T. (2014). Theory of metalintercalated phenacenes: Why molecular valence 3 is special. PHYSICAL REVIEW B, 89(24), 245129.
 Mazumdar, S., & Dutta, T. (2014). Theory of metalintercalated 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), 49714979.More infoAbstract: Singlet fission, in which the optical spinsinglet 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 donoracceptor interface must satisfy specific requirements for the solar cell performance to be enhanced by this process. We focus on the pentaceneC60 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 donoracceptor 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 HubbardHeisenberg model for organic chargetransfer solids. Journal of Physics Condensed Matter, 25(38).More infoPMID: 23995074;Abstract: A frustrated, effective 1/2filled band HubbardHeisenberg model has been proposed for describing the strongly dimerized chargetransfer solid families κ(ET)2X and Z[Pd(dmit)2]2. In addition to showing unconventional superconductivity, these materials also exhibit antiferromagnetism, candidate spinliquid phases, and, in the case of Z = EtMe3P, a spingapped 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 HubbardHeisenberg 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).More infoAbstract: We show that the energy differences between the lowest optical singlet exciton and the lowest triplet exciton in semiconducting singlewalled carbon nanotubes with diameter ∼1 nm and graphene nanoribbons with widths ∼2 nm are an order of magnitude smaller than in the πconjugated polymer poly(paraphenylenevinylene). 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 πconjugated 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 singlettriplet 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), 991994.More infoAbstract: The occurrence of antiferromagnetism (AFM) in κ(ET) 2X can be understood within an effective 1/2filled 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 onehalf per molecule. For dimerized and strongly frustrated 1/4filled lattices we show that a singletpaired statecoexisting with charge ordering occurs that we have termed the paired electron crystal (PEC). Here we investigate the 1/4filled model on a dimerized lattice, showing regions where AFM, PEC, and the Wignercrystal 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 WILEYVCH Verlag GmbH & Co. KGaA, Weinheim.
 Clay, R. T., Song, J., Dayal, S., & Mazumdar, S. (2012). Ground state and finite temperature behavior of 1/4filled band zigzag ladders. Journal of the Physical Society of Japan, 81(7).More infoAbstract: We consider the simplest example of lattice frustration in the 1/4filled band, a onedimensional chain with nextnearest neighbor interactions. For this zigzag ladder with electronelectron as well as electronphonon 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 electronelectron interaction strength. The spin gap from the ground state of the zigzag ladder increases with the degree of frustration. Unlike in onedimension, where the spingap 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 1/4filled 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 longrange superconducting correlations in the frustrated halffilledband Hubbard model. Physical Review B  Condensed Matter and Materials Physics, 85(16).More infoAbstract: We present manybody calculations of superconducting pairpair correlations in the ground state of the halffilledband 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 pairpair 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 12filled band Hubbard model, suggested by many authors to be appropriate for the κ(BEDTTTF)based organic chargetransfer 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 correlatedelectron superconductivity in organic chargetransfer solids, cobaltates, spinels, and fullerides?. Physica Status Solidi (B) Basic Research, 249(5), 995998.More infoAbstract: We posit that there exist deep and fundamental relationships between the above seemingly very different materials. The carrier concentrationdependences of the electronic behavior in the conducting organic chargetransfer 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 onequarter, as in the organics. We show that dynamic JahnTeller effects and the resultant orbital ordering can lead to 1/4filled band descriptions for both superconducting spinels and fullerides, which show evidence for both strong electronelectron and electronphonon interactions. The orbital orderings in antiferromagnetic latticeexpanded bcc M 3C 60 and the superconductor are different in our model. Strong correlations, quarterfilled band and lattice frustration are the common characteristics shared by these unusual superconductors. © 2012 WILEYVCH Verlag GmbH & Co. KGaA, Weinheim.
 Mazumdar, S., Clay, R. T., & Hongtao, L. i. (2012). Similarities in electronic properties of organic chargetransfer solids and layered cobaltates. Physica B: Condensed Matter, 407(11), 17221724.More infoAbstract: The apparently counterintuitive carrier concentrationdependent electronic properties of layered cobaltates have attracted wide interest. Here we point out that very similar carrierconcentration dependence has previously been noted in strongly correlated quasione dimensional (quasi1D) organic chargetransfer 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 chargetransfer solids, superconductivity in the cobaltates appears to be limited to bandfilling of onequarter. 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 onequarter 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 πconjugated polymer film. Physical Review B  Condensed Matter and Materials Physics, 83(15).More infoAbstract: We report pressuredependent transient picosecond and continuouswave photomodulation studies of disordered and ordered films of 2methoxy5(2 ethylhexyloxy) poly(paraphenylenevinylene). 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 highorder 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 quarterfilled band. Physical Review B  Condensed Matter and Materials Physics, 83(24).More infoAbstract: We present a study of the effects of simultaneous charge and spin frustration on the twodimensional strongly correlated quarterfilled band on an anisotropic triangular lattice. Our conclusions are based on exact diagonalization studies that include electronelectron interactions as well as adiabatic electronphonon coupling terms treated selfconsistently. The brokensymmetry states that dominate in the weakly frustrated region near the rectangular lattice limit are the well known antiferromagnetic state with inphase 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 spinsinglet pairedelectron crystal (PEC), consisting of pairs of chargerich sites separated by pairs of chargepoor sites. The PEC, with coexisting charge order and spin gap in two dimensions, is the quarterfilled band equivalent of the valence bond solid (VBS) that can appear in the frustrated halffilled band within antiferromagnetic spin Hamiltonians. We discuss the phase diagram as a function of onsite and intersite Coulomb interactions as well as electronphonon coupling strength. We speculate that the spinbonded pairs of the PEC can become mobile for even stronger frustration, giving rise to a pairedelectron liquid. We discuss the implications of the PEC concept for understanding several classes of quarterfilled 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 chargetransfer solids such as κ(BEDTTTF)2Cu 2(CN)3 and EtMe3Sb[Pd(dmit)2] 2, which we believe show frustrationinduced charge disproportionation at low temperatures. We discuss possible application to layered cobaltates and quarterfilled band spinels. © 2011 American Physical Society.
 Li, H., Clay, R. T., & Mazumdar, S. (2011). Theory of carrier concentrationdependent electronic behavior in layered cobaltates. Physical Review Letters, 106(21).More infoPMID: 21699322;Abstract: A natural explanation for the carrier concentrationdependent electronic behavior in the layered cobaltates emerges within correlatedelectron Hamiltonians with finite onsite and significant nearest neighbor holehole 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 singleband and threeband 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 chargetransfer complex photophysics in π conjugated polymerfullerene blends. Physical Review B  Condensed Matter and Materials Physics, 81(8).More infoAbstract: We present a theory of the electronic structure and photophysics of 1:1 blends of derivatives of polyparaphenylenevinylene and fullerenes. Within the same Coulombcorrelated Hamiltonian applied previously to interacting chains of singlecomponent 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 abovegap and belowgap photoexcitations, within our theory. Photoinduced absorptions for abovegap photoexcitation are from the optical exciton as well as the exciplex while for belowgap 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 singlecomponent π conjugated polymers and blends can both be understood by extending Mulliken's theory of groundstate charge transfer to the case of excitedstate charge transfer. © 2010 The American Physical Society.
 Clay, R. T., Li, H., & Mazumdar, S. (2010). Bipolaron densitywave driven by antiferromagnetic correlations and frustration in organic superconductors. Physica B: Condensed Matter, 405(11 SUPPL.), S253S255.More infoAbstract: We describe the paired electron crystal (PEC) which occurs in the interacting frustrated twodimensional 1/4 filled band. The PEC is a chargeordered state with nearestneighbor 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 chargetransfer solids. Increased frustration drives a PECtosuperconductor transition in these systems. © 2010 Elsevier B.V. All rights reserved.
 Clay, R. T., Li, H., Sarkar, S., Mazumdar, S., & SahaDasgupta, 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).More infoAbstract: It has been suggested that the metalinsulator 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 twodimensional checkerboard lattice with twodirectional metal orbitals per atomic site. We include orbital ordering and interatom electronphonon interactions selfconsistently within a semiclassical approximation, and onsite intraorbital and interorbital electronelectron interactions at the HartreeFock level. We find a stable, orbitally induced Peierls bonddimerized 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 onedimensional band is significantly smaller than 1. In contrast, for carrier density of half an electron per atom the Peierls instability is absent within oneelectron theory as well as meanfield theory of electronelectron 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 pairedelectron crystal in the twodimensional frustrated quarterfilled band. Journal of Physics Condensed Matter, 22(27).More infoPMID: 21399248;Abstract: The competition between antiferromagnetic and spinsinglet ground states within quantum spin models and the 1/2 filled band Hubbard model has received intense scrutiny. Here we demonstrate a frustrationinduced transition from Néel antiferromagnetism to a spinsinglet 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 spinsinglet state is a pairedelectron crystal, with pairs of chargerich sites separated by pairs of chargepoor sites. The pairedelectron crystal provides a natural description of the spingapped state proximate to superconductivity in many organic charge transfer solids. Pressureinduced superconductivity in these correlatedelectron 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 chargetransfer solids. Physica B: Condensed Matter, 404(34), 487489.More infoAbstract: We suggest that superconductivity (SC) in the organic chargetransfer solids (CTS) is reached from a bondcharge 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 chargetransfer solids. Synthetic Metals, 159(2122), 24192421.More infoAbstract: We show that superconductivity is absent within the frac(1, 2)filled band triangular lattice repulsive Hubbard model that has been proposed for organic chargetransfer solids. We posit that organic superconductivity is rather reached from a bondcharge 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 bondcharge density wave can be described within an effective attractive U extended Hubbard Hamiltonian with repulsive nearest neighbor interaction V. A firstorder transition from the insulating to the superconducting state occurs within the model with increasing frustration. © 2009 Elsevier B.V.
 Psiachos, D., & Mazumdar, S. (2009). Correlatedelectron description of the photophysics of thin films of π conjugated polymers. Physical Review B  Condensed Matter and Materials Physics, 79(15).More infoAbstract: We extend the Mulliken theory of groundstate charge transfer in a donoracceptor complex to excitedstate 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 chargetransfer exciton. Within our theory, optical absorptions from the chargetransfer exciton should include a lowenergy intermolecular chargetransfer excitation, as well as distinct intramolecular excitations from both the neutral delocalized exciton component and the Coulombically bound polaronpair component of the chargetransfer exciton. We report highorder configurationinteraction calculations for pairs of oligomers of polyparaphenylenevinylene (PPV) that go beyond our previous single configurationinteraction calculation and find all five excitedstate absorptions predicted using heuristic arguments based on the Mulliken concept. Our calculated excitedstate absorption spectrum exhibits strong qualitative agreement with the complete wavelengthdependent ultrafast photoinduced absorption in films of PPV derivatives, suggesting that a significant fraction of the photoinduced absorption here is from the chargetransfer 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). Electronelectron interaction effects on the photophysics of metallic singlewalled carbon nanotubes. Journal of Physics Condensed Matter, 21(9).More infoPMID: 21817382;Abstract: Singlewalled 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 πelectron systems, we show that optical excitations polarized parallel to the nanotube axes in the socalled metallic singlewalled carbon nanotubes are excitons. Our calculated absolute exciton energies in twelve different metallic singlewalled carbon nanotubes, with diameters in the range 0.81.4 nm, are in nearly quantitative agreement with experimental results. We have also calculated the absorption spectrum for the (21, 21) singlewalled 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 onsite Coulomb interactions within theories based on the density functional approach. As in the semiconducting nanotubes we predict in the metallic nanotubes a twophoton exciton above the lowest longitudinally polarized exciton that can be detected by ultrafast pumpprobe spectroscopy. We also predict a subgap absorption polarized perpendicular to the nanotube axes below the lowest longitudinal exciton, blueshifted from the exact midgap by electronelectron interactions. © 2009 IOP Publishing Ltd.
 Clay, R. T., Li, H., & Mazumdar, S. (2008). Absence of superconductivity in the halffilled band hubbard model on the anisotropic triangular lattice. Physical Review Letters, 101(16).More infoAbstract: We report exact calculations of magnetic and superconducting pairpair correlations for the halffilled band Hubbard model on an anisotropic triangular lattice. Our results for the magnetic phases are similar to those obtained with other techniques. The superconducting pairpair correlations at distances beyond nearest neighbor decrease monotonically with increasing Hubbard interaction U for all anisotropy, indicating the absence of frustrationdriven superconductivity within the model. © 2008 The American Physical Society.
 Mazumdar, S., & Clay, R. T. (2008). Quantum critical transition from chargeordered to superconducting state in the negative U extended Hubbard model on a triangular lattice. Physical Review B  Condensed Matter and Materials Physics, 77(18).More infoAbstract: We demonstrate a robust frustrationdriven charge order to superconductivity transition in the halffilled 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 chargetransfer solids and other quarterfilled systems. © 2008 The American Physical Society.
 Musfeldt, J. L., Brown, S., Mazumdar, S., Clay, R. T., MasTorrent, 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)_{2}Cu(mnt)_{2}. Solid State Sciences, 10(12), 17401744.More infoAbstract: 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 metalinsulator 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 onsite charge asymmetry.
 Wang, Z., Mazumdar, S., & Shukla, A. (2008). Photophysics of chargetransfer excitons in thin films of π conjugated polymers. Physical Review B  Condensed Matter and Materials Physics, 78(23).More infoAbstract: 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 chargetransfer excitons are generated in films. We extend existing theories of the lowest polaron pair and chargetransfer 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 chargetransfer exciton. Our proposed mechanism explains the simultaneous observation of polaronlikeinduced 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 lowenergy electronic excitation in metalexchanged vanadium oxide nanoscrolls. Nano Letters, 7(8), 23512355.More infoPMID: 17628114;Abstract: We measured the optical properties of mixed valent vanadium oxide nanoscrolls and their metalexchanged derivatives in order to investigate the charge dynamics in these compounds. In contrast to the prediction of a metallic state for the metalexchanged derivatives within a rigid band model, we find that the injected charges in Mn2+exchanged vanadium oxide nanoscrolls are pinned. A lowenergy 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). Temperaturedriven transition from the Wigner crystal to the bondchargedensity wave in the quasionedimensional quarterfilled band. Physical Review B  Condensed Matter and Materials Physics, 76(20).More infoAbstract: It is known that within the interacting electron model Hamiltonian for the onedimensional 1 4 filled band, the singlet ground state is a Wigner crystal only if the nearestneighbor electronelectron repulsion is larger than a critical value. We show that this critical nearestneighbor 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 chargeordered state to a spinPeierls state that is a bondchargedensity wave with charge occupancies different from the Wigner crystal. This transition is possible because spin excitations from the spinPeierls 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 quasionedimensional organic chargetransfer 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 bondchargedensity Wave transition in several members of the TMTTF family. We explain the occurrence of two different antiferromagnetic phases but a single spinPeierls state in the generic phase diagram for the 2:1 cationic solids. The antiferromagnetic phases can have either the Wigner crystal or the bondchargespindensity wave charge occupancies. The spinPeierls state is always a bondchargedensity wave. © 2007 The American Physical Society.
 Stafford, C. A., Cardamone, D. M., & Mazumdar, S. (2007). The quantum interference effect transistor. Nanotechnology, 18(42).More infoPMID: 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 singlewalled carbon nanotubes. Physical Review B  Condensed Matter and Materials Physics, 76(11).More infoAbstract: We present a quantitative theory of optical absorption polarized transverse to the tube axes in semiconducting singlewalled carbon nanotubes. Within oneelectron 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. Electronelectron 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 singlewalled carbon nanotubes by electroabsorption spectroscopy. Physical Review Letters, 98(16).More infoAbstract: We report benchmark calculations of electroabsorption in semiconducting singlewalled 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 Fanotype 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), 24222426.More infoPMID: 17090067;Abstract: We investigate multiterminal quantum transport through single monocyclic aromatic annulene molecules, and their derivatives, using the nonequilibrium Green function approach within the selfconsistent HartreeFock 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 quarterfilled ladders. Journal of Low Temperature Physics, 142(34), 365370.More infoAbstract: Several new families of charge transfer solids with ladderlike structure show unusually high spin gap transition temperatures. We argue that these materials are best described by a 1/4filled ladder with zigzag interstack bonds. We find that in the 1/4filled zigzag ladder, a cooperative bondcharge 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 singlewalled carbon nanotubes within a π electron model. Physical Review B  Condensed Matter and Materials Physics, 74(19).More infoAbstract: 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 singlewalled carbon nanotubes (SSWCNTs), we are able to determine quantitatively the exciton energies and exciton binding energies of 29 SSWCNTs 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 SSWCNTs. © 2006 The American Physical Society.
 Yan, Y., Mazumdar, S., & Ramasesha, S. (2006). Dimerizationinduced enhancement of the spin gap in the quarterfilled twoleg rectangular ladder. Physical Review B  Condensed Matter and Materials Physics, 73(22).More infoAbstract: We report densitymatrix renormalization group calculations of spin gaps in the quarterfilled correlated twoleg rectangular ladder with bonddimerization along the legs of the ladder. In the small rungcoupling region, dimerization along the leg bonds can lead to large enhancement of the spin gap. Electronelectron interactions further enhance the spin gap, which is nonzero for all values of the rung electron hopping and for arbitrarily small bonddimerization. Very large spin gaps, as are found experimentally in quarterfilled band organic chargetransfer solids with coupled pairs of quasionedimensional 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 quasionedimensional organic semiconductors: Singlewalled carbon nanotubes and π conjugated polymers. Physical Review B  Condensed Matter and Materials Physics, 73(7).More infoAbstract: The nature of the primary photoexcitations in semiconducting singlewalled carbon nanotubes (SSWCNTs) is of strong current interest. We have studied the emission spectra of SSWCNTs and two different π conjugated polymers in solutions and films, and have also performed ultrafast pumpprobe 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 SSWCNTs 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 SSWCNTs. 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 SSWCNTs within the same correlated electron Hamiltonian. We find strong similarities in the excitonic energy spectra of these two classes of quasionedimensional materials, although there exist also subtle differences such as the occurrence of dark excitons below the optical excitons in the SSWCNTs. In the polymers PA1 is an excited state absorption from the optical exciton to a twophoton exciton that occurs below the continuum band threshold. In the SSWCNTs 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 SSWCNTs. The binding energy of lowest exciton that belongs to the widest SSWCNTs with diameters ≥1 nm in films is 0.30.4 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 quarterfilled zigzag electron ladder. Physical Review Letters, 94(20).More infoAbstract: Strong cooperative interactions occur between four different broken symmetries involving charge ordering and bond distortions in the quarterfilled correlated zigzag electron ladder. The ground state is singlet, with spin gap several times larger than in the spinPeierls state of the onedimensional quarterfilled chain with the same parameters. We propose the quarterfilled zigzag electron ladder model for several different organic charge transfer solids with coupled pairs of quasionedimensional stacks, in which the spingap transition temperatures are unusually high. © 2005 The American Physical Society.
 Clay, R. T., & Mazumdar, S. (2005). Magnetism in BEDTTTF materials. Synthetic Metals, 153(13), 445448.More infoAbstract: 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 spinspin correlations within the effective halffilled band triangular lattice proposed for the κET, 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 κET, this cooperative 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 electronelectron interactions on spin effects in electronhole recombination in organic light emitting diodes. Synthetic Metals, 155(2), 270273.More infoAbstract: We investigate the possibility that highenergy singlet and triplet excited states with large electronhole separations are generated in electronhole recombination process, over and above the lowest singlet and triplet excitons. Using a timedependent calculation of the interchain/intermolecular chargetransfer within model Hamiltonians that explicitly include electronelectron interactions between the πelectrons, we show that there occurs a bifurcation of the electronhole recombination path in each of the two spin channels that leads to the generation of both the lowest energy exciton and a specifichighenergy chargetransfer 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 localsinglet formation in quarterfilled band chargetransfer solids and oxides of early transition metals. Journal De Physique. IV : JP, 131, 63.More infoAbstract: In many 1/4filled band nonsuperconducting chargetransfer solids (CTS), such as the θ(ET)2X and the "ladder" compounds (DTTTF)2Au(mnt)2 a chargeordering (CO) transition is followed by a transition to a spingap (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 1/4filled band oxides of early transition metals such as Ti 4O7 and αNaV2O5. The mechanisms of the SG formations are not understood and are often controversial. We present a global perspective of the CO transitions in 1/4filled band systems. We report calculations incorporating both electronelectron and electronphonon interactions in 1/4filled band (a) 1D chains (b) 2D rectangular and triangular lattices, and (c) zigzag and rectangular ladders. We show that CO in many 1/4filled 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 metalsemiconductor 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 Voxides. © EDP Sciences.
 Yan, Y., & Mazumdar, S. (2005). Density matrix renormalization group study of conjugated polymers with transverse πconjugation. Physical Review B  Condensed Matter and Materials Physics, 72(21).More infoAbstract: We report accurate numerical studies of excited state orderings in long hypothetical πconjugated 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 πconjugated polymers with transverse conjugation and small optical gaps. © 2005 The American Physical Society.
 Zhao, H., & Mazumdar, S. (2005). Excitons in semiconducting singlewalled carbon nanotubes. Synthetic Metals, 155(2), 250253.More infoAbstract: We report correlatedelectron calculations of optically excited states in 10 semiconducting singlewalled 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 singleparticle theory. The experimentally observed weak photoluminescence is an intrinsic feature of semiconducting nanotubes, and is consequence of dipoleforbidden 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 phenylenebased conjugated polymers. Synthetic Metals, 141(12), 5965.More infoAbstract: Ultrafast pumpprobe spectroscopy of phenylenebased πconjugated polymers give low and high energy photoinduced absorptions (PAs) to final states with very different relaxation behavior. While the low energy twophoton state that is reached in PA relaxes to the optical exciton in ultrafast times, the higher energy twophoton state violates the VavilovKasha 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 polyparaphenylene and polyparaphenylenevinylene within a rigid band correlated electron Hamiltonian, using the complete πelectron basis and the powerful multireference 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 phenylenebased systems within oneelectron theory. We speculate that the violation of the VavilovKasha rule and charge separation into polaron pairs upon excitation to the high energy twophoton state is a consequence of its specific electronic structure, and discuss an earlier work on wavelength dependent photoconductivity of polyparaphenylenevinylene to support this conjecture. © 2003 Elsevier B.V. All rights reserved.
 Zhao, H., & Mazumdar, S. (2004). Electronelectron interaction effects on the optical excitations of semiconducting singlewalled carbon nanotubes. Physical Review Letters, 93(15), 15740211574024.More infoPMID: 15524940;Abstract: Correlatedelectron calculations of optically excited states in ten semiconducting singlewalled 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 singleparticle 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 quasionedimensional organic chargetransfer solids. Synthetic Metals, 135136, 681682.More infoAbstract: We examine two proposed models of the observed charge ordering (CO) in the quasi1D, 1/4filled organic charge transfer solids. The models are characterized by the site charge densities ..1010.. and ..1100.. respectively. Using a Peierlsextended Hubbard model to include both electronelectron and electronphonon interactions, we show that 1) the ..1010..CO can occur only when the nearestneighbor Coulomb repulsion is sufficiently strong; 2) a spinPeierls 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 quasionedimensional organic chargetransfer solids. Physical Review B  Condensed Matter and Materials Physics, 67(11), 11512111151219.More infoAbstract: We examine two recently proposed models of charge ordering (CO) in the nominally 1/4filled, quasionedimensional (1D) organic chargetransfer solids (CTS). The two models are characterized by site charge density "cartoons" ... 1010 ... and ... 1100 ..., respectively. We use the Peierlsextended Hubbard model to incorporate both electronelectron (ee) and electronphonon (eph) interactions. We first compare the results, for the purely electronic Hamiltonian, of exact manybody calculations with those of HartreeFock (HF) meanfield theory. We find that HF gives qualitatively and quantitatively incorrect values for the critical nearestneighbor Coulomb repulsion (Vc) necessary for ... 1010 ... order to become the ground state. Second, we establish that spinPeierls order can occur in either the ... 1100 ... and ... 1010 ... states and calculate the phase diagram including both onsite and intrasite eph interactions. Third, we discuss the expected temperature dependence of the CO and metalinsulator 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 oneelectron bandwidths are more ambiguous, likely because the nearestneighbor Coulomb interaction in these materials is near Vc.
 Dallakyan, S., & Mazumdar, S. (2003). SubPoissonian shot noise in molecular wires. Applied Physics Letters, 82(15), 24882490.More infoAbstract: 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 subPossonian behavior in the shot noise as a function of applied voltage.
 Dallakyan, S., Chandross, M., & Mazumdar, S. (2003). Infrared light emission from πconjugated polymers: A diagrammatic exciton basis valence bond theory. Physical Review B  Condensed Matter and Materials Physics, 68(7), 7520417520410.More infoAbstract: There is currently a great need for solidstate lasers that emit in the infrared, as this is the operating wavelength regime for applications in telecommunications. Existing πconjugated polymers all emit in the visible or ultraviolet, and whether or not πconjugated polymers that emit in the infrared can be designed is an interesting challenge. On the one hand, the excitedstate ordering in transpolyacetylene, the πconjugated polymer with a relatively small optical gap, is not conducive to light emission because of electronelectron interaction effects. On the other hand, excitedstate ordering opposite to that in transpolyacetylene 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 πconjugated polymer that is obtained by replacing the hydrogen atoms of transpolyacetylene with transverse conjugated groups and show that the effective onsite correlation in this system is smaller than the bare correlation in the unsubstituted system. An optical gap in the infrared as well as excitedstate 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 chargetransfer solids. Synthetic Metals, 137(13), 13171319.More infoAbstract: Charge and spinorderings in the 1/4filled 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 metalinsulator and magnetic insulatorinsulator transitions in the θET 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 πconjugated polymers with light emission in the infrared. Synthetic Metals, 139(3), 769772.More infoAbstract: 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 πconjugation 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 electronhole recombination and triplettriplet scattering in organic light emitting diodes. Synthetic Metals, 139(3), 917920.More infoAbstract: Using a timedependent model manybody formulation, we follow the electronhole recombination between a pair of oppositely charged polyene chains and triplettriplet (TT) scattering in πconjugated 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 triplettriplet 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 excitedstate absorption in phenylenebased πconjugated polymers. Physical Review B  Condensed Matter and Materials Physics, 67(24), 24520312452039.More infoAbstract: Within a rigidband correlated electron model for oligomers of poly(paraphenylene) (PPP) and poly(paraphenylenevinylene) (PPV), we show that there exist two fundamentally different classes of twophoton 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 electronone hole (1e1h) and two electrontwo hole (2e2h) excitations, that are both comprised of the highest delocalized valenceband and the lowest delocalized conductionband 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 2e2h 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 highenergy PA's in PPV. The calculated relative intensities of the two PA's are also in qualitative agreement with experiment. In the case of laddertype 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 chargecarrier generation occurs upon excitation to oddparity 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 electronhole recombination in organic lightemitting diodes. Physical Review B  Condensed Matter and Materials Physics, 67(4), 4510914510919.More infoAbstract: We develop a general theory of electronhole recombination in organic lightemitting 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 longrange 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 bondcharge repulsion, X⊥, is included in the interchain part of the Hamiltonian for the twochain system. The larger (smaller) yield of the singlet (triplet) exciton in carbonbased longchain 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 timedependent approach developed here for describing intermolecular chargetransfer 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., & KuwataGonokami, M. (2002). Dimensionality dependence of optical nonlinearity and relaxation dynamics in cuprates. Europhysics Letters, 58(3), 455461.More infoAbstract: Femtosecond pumpprobe measurements find pronounced dimensionality dependence of the optical nonlinearity in cuprates. Although the coherent twophoton absorption (TPA) and linear absorption bands nearly overlap in both quasione and twodimensional (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 twoband extended Hubbard model.
 Clay, R. T., Mazumdar, S., & Campbell, D. K. (2002). Charge Ordering in θ(BEDTTTF)_{2}X Materials. Journal of the Physical Society of Japan, 71(8), 18161819.More infoAbstract: We theoretically investigate charge ordered states on the anisotropic triangular lattice characteristic of the θ(BEDTTTF)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 (pdirections), and ... 1010... CO along the third direction (cdirection). The CO is accompanied by cooperative bond dimerizations along all three directions in the highest spin state. In the lowest spin state bonds along the pdirections are tetramerized. Our theory explains the occurrence of a chargeinduced 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 chargetransfermott insulators: A new paradigm for nonlinear optics" [1] (multiple letters). Physical Review Letters, 89(3), 397011397021.
 Ashida, M., Ogasawara, T., Motoyama, N., Eisaki, H., Uchida, S., Taguchi, Y., Tokura, Y., KuwataGonokami, M., Ghosh, H., Shukla, A., & Mazumdar, S. (2001). Interband twophoton transition in Mott insulator as a new mechanism for ultrafast optical nonlinearity. International Journal of Modern Physics B, 15(2830), 36283632.More infoAbstract: The dimensionality dependence of optical nonlinearity in cuprates was investigated by subpicosecond pumpprobe transmission measurements in the nearinfrared region. It was found that cuprates with onedimensional CuO networks show nonlinearity one order of magnitude larger than that of conventional band semiconductors and picosecond relaxation of the excited state. In contrast, a twodimensional 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 alloptical switching devices is also discussed.
 Ashida, M., Ogasawara, T., Tokura, Y., Uchida, S., Mazumdar, S., & KuwataGonokami, M. (2001). Onedimensional cuprate as a nonlinear optical material for ultrafast alloptical switching. Applied Physics Letters, 78(19), 28312833.More infoAbstract: We examine the feasibility of ultrafast alloptical switching utilizing a onedimensional cuprate Sr2CuO3 operable within the optical fiber communication window at room temperature by using the pumpprobe spectroscopy and Zscan measurements. The strength of the interband twophoton absorption in Sr2CuO3 is much larger than that of conventional semiconductors, and is comparable to the largest reported values in πconjugated polymers. The intensitydependent 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 onedimensional cuprate a potential material for multiterabits/second rate alloptical switch. © 2007 American Institute of Physics.
 Ashida, M., Ogasawara, T., Uchida, S., Tokura, Y., KuwataGonokami, M., & Mazumdar, S. (2001). Lowdimensional perovskite copper oxide as a nonlinear optical material for alloptical switching. Conference on Lasers and ElectroOptics Europe  Technical Digest, 215.More infoAbstract: The use of low dimensional perovskite copper oxide as a nonlinear optical material for alloptial switching (AOS) was demonstrated. Subpicosecond photoinduced absorption change was measured for samples with different copper and oxygen networks at room temperature. The photoinduced absorption changes were found to consist of a prompt component and an exponentially decaying tail. The lowdimensional cuprates showed strong twophoton absorption (TPA) and picosecond ground state recovery at room temperature.
 Clay, R. T., Mazumdar, S., & Campbell, D. K. (2001). Reintegerization of fractional charges in the correlated quarterfilled band. Physical Review Letters, 86(18), 40844087.More infoPMID: 11328101;Abstract: A study was conducted to demonstrate that charge fractionalization does not occur in the low temperature 2kF spinPeierls (SP) phase of the interacting 1D 1/4filled band. It was shown that the charge ordering associated with the SP tetramerization causes a "reintegerization" 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(1920), 27932798.More infoAbstract: We investigate theoretically photoluminescence (PL) properties of mono and diphenyl substituted transpolyacetylene (tPA), namely, polyphenylacetylene (PPA) and polydiphenylacetylene (PDPA), respectively. PL is a consequence of the occurrence of the twophoton 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 phenylsubstituted polyacetylenes electronelectron 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 xpolarized whereas the high energy absorption is predominantly ypolarized. In contrast, in presence of many body electron interaction both of them (low and high energy absorptions) are xpolarized. 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(13), 679682.More infoAbstract: We review our recent proposal for a unified, microscopic, manybody 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 bondcharge density wave (BCDW) ground state in the limit of exact 1/4filling for all anisotropies, consistent with experimental observations of coexisting density waves in the real materials. For weakly 2D materials, we find that a spindensity 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 1/4filling, commensurability defects can form pairs leading to superconductivity.
 Shukla, A., Ghosh, H., & Mazumdar, S. (2001). Photophysics of phenylsubstituted polyacetylenes, theory. Synthetic Metals, 116(13), 8790.More infoAbstract: We present a theoretical study of the electronic structures of polyphenylacetylene (PPA) and polydiphenylacetylene (PDPA). Strong photoluminescence (PL) in PDPA's is a consequence of the occurrence of the 2Ag twophoton state above the optical 1Bu, in spite of the polyene backbone. This reversed energy ordering (as compared to linear polyenes and transpolyacetylene, tPA) 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 crosssections of singlet and triplet excitons in πconjugated polymers (Nature (2001) 409 (494497)). Nature, 411(6837), 617.
 Wohlgenannt, M., Tandon, K., Mazumdar, S., Ramasesha, S., & Vardeny, Z. V. (2001). Formation crosssections of singlet and triplet excitons in πconjugated polymers. Nature, 409(6819), 494497.More infoPMID: 11206541;Abstract: Electroluminescence in organic lightemitting diodes arises from a chargetransfer 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 crosssection of singlet excitons, σS, is approximately the same as that of any one of the three equivalent nonradiative 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 πconjugated 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 chargetransfer reaction for correlated πelectrons, 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., KuwataGonokami, 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, 183184.More infoAbstract: Discussed as peculiar optical response observed in quasionedimensional (1D) cuprates, Sr2CuO3 and SrCuO2. Femtosecond pumpprobe transmission spectra in the 1D cuprates are measured at 10300 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 0.320 μm.
 Ghosh, H., Shukla, A., & Mazumdar, S. (2000). Electroncorrelationinduced transverse delocalization and longitudinal confinement in excited states of phenylsubstituted polyacetylenes. Physical Review B  Condensed Matter and Materials Physics, 62(19), 1276312774.More infoAbstract: Electronelectron interactions in general lead to both groundstate and excitedstate confinement. We show, however, that in phenylsubstituted polyacetylenes electronelectron 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 onephoton state and raises the relative energy of the lowest twophoton state. The twophoton state is slightly below the optical state in monophenylsubstituted polyacetylenes, but above the optical state in diphenylsubstituted 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 twophoton states in these systems. In addition, we calculate the opticalabsorption spectra over a wide wavelength region, and make specific predictions for the polarizations of low and highenergy transitions that can be tested on oriented samples. Within existing theories of light emission from πconjugated 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), 630631.
 Mazumdar, S., Clay, R. T., & Campbell, D. K. (2000). Bondorder and chargedensity waves in the isotropic interacting twodimensional quarterfilled band and the insulating state proximate to organic superconductivity. Physical Review B  Condensed Matter and Materials Physics, 62(20), 1340013425.More infoAbstract: We report three surprising results regarding the nature of the spatial broken symmetries in the twodimensional (2D), quarterfilled band with strong electronelectron interactions that provides a microscopic model of the 2:1 cationic organic chargetransfer solids (CTS's). First, in direct contradiction to the predictions of oneelectron theory, we find a coexisting "bondorder and chargedensity wave" (BCDW) insulating ground state in the 2D rectangular lattice for all anisotropies, including the isotropic limit. Second, in contrast to the interacting halffilled band, which exhibits one singlettoantiferromagnet (AFM) transition as the interchain coupling is increased from zero, there occur in the interacting quarterfilled band two distinct transitions: a similar singlettoantiferromagnet/spindensity wave (AFM/SDW) transition at small interchain coupling, giving rise to a bondchargespin density wave (BCSDW) state, followed by a second AFM/SDWtosinglet transition at large interchain coupling. Third, we show that our conclusions remain unchanged if one assumes the conventional "effective 1/2filled" lattice of dimer sites for the CTS's: the dimer lattice unconditionally dimerizes again to give the same BCDW found in the quarterfilled band. We make detailed comparisons to recent experiments in the tetramethyltetrathiafulvalene (TMTTF), tetramethyltetraselenafulvalene (TMTSF), bisethylenedithiotetrathiafulvalene (BEDTTTF) and bisethylenedithiotetraselenafulvalene (BETS)based CTS's. Our theory explains the mixed chargespin density waves observed in TMTSF and certain BEDTTTF systems, as well as the absence of antiferromagnetism in the BETSbased systems. An important consequence of this work is the suggestion that organic superconductivity is related to the proximate Coulombinduced 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., & KuwataGonokami, M. (2000). Ultrafast optical nonlinearity in the quasionedimensional Mott insulator Sr_{2}CuO_{3}. Physical Review Letters, 85(10), 22042207.More infoAbstract: The photoinduced absorption in the quasionedimensional Mott insulator Sr2CuO3 in the infrared spectral region was studied. Theoretical calculations based on a twoband Hubbard model explain the experimental observations. Results indicated that the strong twophoton absorption is due to a very large dipole coupling. The potential of Sr2CuO3 for alloptical 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 longlived component of the transmission change is associated with the real excitation of the chargetransfer (CT) exciton.
 Chakrabarti, A., & Mazumdar, S. (1999). Theory of evenparity states in polyphenylenes. Physical Review B  Condensed Matter and Materials Physics, 59(7), 48394848.More infoAbstract: The diagrammatic exciton basis valencebond 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, lowenergy evenparity states are linear combinations of oneexcitations that are charge transfer within the exciton representation and twoexcitations 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 evenparity 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 triplettriplet twoexcitations in the polyphenylenes, involving both the delocalized and localized molecular orbitals. We further show that all triplettriplet twoexcitations mix with the oneexcitation chargetransfer type configurations that involve only the delocalized molecular orbitals, giving rise to different classes of twophoton allowed states. Lowenergy twophoton states are predominantly composed of oneexcitation chargetransfer configurations and triplettriplet configurations involving the delocalized molecular orbitals. At higher energy, there occur twophoton states consisting of the same oneexcitations, but triplettriplet configurations that involve both delocalized and localized molecular orbitals. The highenergy twophoton 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 highenergy photoinduced transitions in poly(paraphenylenevinylenes) are assigned to transitions from the optical exciton to these two distinct classes of twophoton states. The current assignment of the highenergy 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 excitonbasis valencebond theory of linear polyenes. Physical Review B  Condensed Matter and Materials Physics, 59(7), 48224838.More infoAbstract: Understanding the photophysics of πconjugated 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 excitonbasis valencebond representation that is particularly suitable for the intermediate magnitude of the Coulomb interactions in these systems. We present detailed comparisons of our exact excitonbasis treatment and previous approximate approaches, focusing on the specific manybody and singleparticle 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 PariserParrPople Ohno Coulomb interactions with two different bondalternation parameters for the tencarbon 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 highenergy excited states in linear chain πconjugated systems for realistic parameters. Two different quantities, the number of effective excitations within the exciton basis, and the particlehole correlation length for the oneexcitation 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 twophoton state that dominates third order optical nonlinearity in the lowenergy region is the lowest even parity oneexcitation state with a larger particlehole 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 highenergy region of the spectrum evidence for stable biexcitons is found from the nature of the singletsinglet twoexcitation 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 quarterfilled band: (DIDCNQI) _{2}Ag". Physical Review Letters, 82(11), 2411.
 Mazumdar, S., Ramasesha, S., Clay, R. T., & Campbell, D. K. (1999). Theory of coexisting charge and spindensity waves in (TMTTF)_{2}Br, (TMTSF)_{2}PF_{6} and α(BEDTTTF)_{2}MHg(SCN)_{4}. Synthetic Metals, 103(13), 18431844.More infoAbstract: 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 eph 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 spindensity waves in (TMTTF)_{2}Br, (TMTSF)_{2}PF_{6}, and α(BEDTTTF)_{2}MHg(SCN)_{4}. Physical Review Letters, 82(7), 15221525.More infoAbstract: Recent experiments indicate that the spindensity waves (SDWs) in (TMTTF)2Br, (TMTSF)2PF6, and α(BEDTTTF)2MHg(SCN)4 are highly unconventional and coexist with chargedensity waves (CDWs). We present a microscopic theory of this unusual CDWSDW coexistence. A complete understanding requires the explicit inclusion of strong Coulomb interactions, lattice discreteness, the anisotropic twodimensional 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), 39443947.More infoAbstract: We show that chemical modification of the transpolyacetylene 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 phenylenebased conjugated polymers. Synthetic Metals, 85(13), 11211122.More infoAbstract: 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(paraphenylenevinylene). Physical Review B  Condensed Matter and Materials Physics, 55(3), 14971504.More infoAbstract: Within a model Hamiltonian with variable onsite and longrange Coulomb interactions between the π electrons for poly(paraphenylenevinylene), 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 PariserParrPople parameters. For these values of the Coulomb parameters, the primary photoexcitation in poly(paraphenylenevinylene) 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, twophoton 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(13), 10951096.More infoAbstract: We present a selfconsistent picture of the excited state energy spectrum of poly(paraphenylene vinylene) based on theoretical calculations of exicted states and and experimental electroabsorption (EA) and picosecond photoinduced absorption measurements (PA). Our calculations place the one electronone 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 evenparity 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(13), 603604.More infoAbstract: We investigate theoretically and experimentally the effects of (2,5) chemical substitution on the optical absorption in the phenylenebased 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 phenylenebased conjugated polymers: Theory and experiment. Physical Review B  Condensed Matter and Materials Physics, 55(3), 14861496.More infoAbstract: We investigate theoretically and experimentally the effects of (2,5) chemical substitution on the optical absorption in the phenylenebased conjugated polymers. Theoretically, substitution destroys both the chargeconjugation symmetry and spatial symmetry that characterize the unsubstituted materials. Within Coulombcorrelated theoretical models, the effect of broken chargeconjugation symmetry alone on the underlying electronic structure and on the absorption spectrum is rather weak. When both broken spatial symmetry and broken chargeconjugation 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 opticalabsorption spectrum is weak. This surprising result is a consequence of the subtle nature of the configuration interaction in the substituted polyphenylenes within Coulombcorrelated 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(paraphenylene vinylene) (PPV) derivatives. Although substitution can make a previously forbidden transition weakly allowed, the latter acquires strength from a "finitesize band" at about the same energy, and not from a higherenergy band at 4.7 eV, as has been previously claimed. It is further predicted that the 3.7eV band is polarized predominantly along the polymerchain axis. We have measured the polarization dependence of the optical absorption in an orientedsubstituted PPV film. We found that the two lowestenergy 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(13), 10011006.More infoAbstract: 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 twophoton states can be broadly classified into triplettriplet (TT), chargetransfer (CT) and singletsinglet (SS) excitations. In the above CT refers to chargetransfer from one unit to another, and TT and SS are two electrontwo 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 twoexciton 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(12), 8590.More infoAbstract: Exact finite chain calculations within a diagrammatic exciton basis give pictorial descriptions of all excited states of πconjugated polymers. In linear chain polymers the 1Bu is an exciton, and the fundamental twophoton states can be broadly classified into triplettriplet, chargetransfer and singletsinglet excitations. The mAg, a twophoton state that plays a strong role in nonlinear optics, is a correlated chargetransfer state. The singletsinglet states occur higher in energy and split into bound biexcitons and a continuum of free twoexciton states.
 Gallagher, F. B., & Mazumdar, S. (1997). Excitons and optical absorption in onedimensional extended Hubbard models with short and longrange interactions. Physical Review B  Condensed Matter and Materials Physics, 56(23), 1502515039.More infoAbstract: We show that charge excitations within onedimensional extended Hubbard models in the limit of large onsite Coulomb interaction can be mapped onto pairs of spin reversals from the ferromagnetic IsingHeisenberg spin Hamiltonian. The exciton in the extended Hubbard models is equivalent to the bound twomagnon 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 electronhole continuum only for the intersite interaction larger than half the oneelectron 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 onsite 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 electronhole continuum become optically visible in the dimerized case. For longrange 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 nearestneighbor interaction. The binding energy of the lowest optical exciton again depends weakly on the dimerization for longrange interaction. Our theoretical results can explain the occurrence of two chargetransfer absorption bands in halffilled band segregated stack charge transfer solids below the spinPeierls dimerization temperature. Our results also suggest that the lowest optical state of πconjugated 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(13), 10271028.More infoAbstract: 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 PariserParrPople 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 triplettriplet transition lies below the socalled m1Ag chargetransfer exciton due to correlation effects higher than single configurationinteraction. 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(pphenylene vinylene).
 Mazumdar, S., & Guo, F. (1996). Theory of biexcitons in mixed and segregated stack chargetransfer solids, molecular crystals and πconjugated polymers. Synthetic Metals, 78(3), 187193.More infoAbstract: 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 onephoton allowed exciton to the biexciton and to the twoexciton 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 strongcoupling limits of models describing mixed and segregated stack chargetransfer solids, while studies by other investigators have shown that the criterion also remains valid within the Frenkel exciton model with moderate excitonexciton binding. Based on the same criterion of biexciton stability, we present theoretical evidence for stable biexcitons within the Coulombcorrelated model for πconjugated polymers. Highenergy picosecond photoinduced absorption in πconjugated polymers is explained as a transition from the optical exciton to the biexciton.
 Mazumdar, S., Guo, F., Meissner, K., Fluegel, B., Peyghambarian, N., KuwataGonokami, 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), 92839291.More infoAbstract: Optical excitation in a strongly neutral quasionedimensional mixedstack chargetransfer 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 oneelectron hopping. We present a joint theoreticalexperimental 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 chargetransfer 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 onedimensional semiconductors with long range Coulomb interactions, the occurrence of stable 2exciton string (biexciton) necessarily implies stable higher multiexcitons. Experimentally, evidence for the multiexciton strings is demonstrated by femtosecond pumpprobe spectroscopy of anthracene pyromellitic acid dianhydride. Excellent qualitative agreement is found between the calculated and the measured differential transmission spectra. Photoinduced absorptions to the 2exciton string at low pump intensity and to the 3exciton string at high pump intensity are observed, in agreement with the theory of excited state absorption. The 2exciton string is confirmed also by a direct twophoton absorption measurement. The binding energies of the 2exciton and the 3exciton strings are obtained from the experimental data. The larger binding energy of the 3exciton is in agreement with theory. © 1996 American Institute of Physics.
 Mazumdar, S., Guo, F., Meissner, K., Fluegel, B., Peyghambarian, N., KuwataGonokami, M., Sato, Y., Ema, K., Shimano, R., Tokihiro, T., Ezaki, H., & Hanamura, E. (1996). Excitontobiexciton transition in quasionedimensional organics. Journal of Chemical Physics, 104(23), 92929296.More infoAbstract: In the previous paper we demonstrated novel multiexcitons in a neutral mixedstack chargetransfer solid. The lowest multiexciton, the biexciton, has recently been of interest also in the context of quasionedimensional organic materials that are different from the mixedstack solids. The nature and strength of the optical transition from the exciton to the twoexciton states is of importance in understanding photoinduced absorption as well as twophoton 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 twoexciton 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(paraphenylenevinylene). Synthetic Metals, 69(13), 625628.More infoAbstract: The Hückel model fails to describe optical absorption in PPV and its derivatives, and explicit inclusion of moderate electronelectron 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 πconjugated 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), 20862089.More infoAbstract: Theoretical evidence for stable biexcitons are found within the Coulombcorrelated model for πconjugated polymers, for realistic Coulomb interaction parameters. High energy picosecond photoinduced absorption, not anticipated within oneelectron theories of πconjugated polymers, is explained as a transition from the optical exciton to the biexciton.
 Mazumdar, S., & Chandross, M. (1995). Theory of photoexcitations in phenylenebased polymers. Proceedings of SPIE  The International Society for Optical Engineering, 2528, 6273.More infoAbstract: We show that optical absorption spectra of polyphenylenes can be explained only within theoretical models that explicitly include the Coulomb interaction among the πelectrons. 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(paraphenylenevinylene) is about 0.9 eV. Such a large binding energy would be in agreement with nonlinear spectroscopic measurements and pumpprobe 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(paraphenylenevinylene). Physical Review B, 50(19), 1470214705.More infoAbstract: We show that oneelectron band theory fails to describe optical absorption in PPV and the absorption spectrum can be described only within a Coulombcorrelated 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[2methoxy,5(2'ethylhexyloxy)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). Fourlevel `essential states' model of an optical nonlinearity in πconjugated polymers. Molecular Crystals and Liquid Crystals Science and Technology Section B: Nonlinear Optics, 6(34), 337350.More infoAbstract: It is shown that the theoretical description of optical nonlinearity in πconjugated polymers must necessarily include both the onsite and intersite electron correlations. In spite of the infinite number of states that, in principle, contribute to the thirdorder 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, thirdharmonic generation experiments should find two threephoton 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 threephoton resonance and a twophoton resonance. Twophoton resonances other than the one due to the essential twophoton 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, 787792.More infoAbstract: 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 twophoton absorptions to lowlying evenparity states in linearchain conjugated polymers. Physical Review B, 49(15), 1010210112.More infoAbstract: Interest in the lowest evenparity twophoton states in linearchain πconjugated polymers has been largely limited to their energies relative to the lowest oddparity onephoton state. We show that the intensities of the twophoton absorptions to evenparity states that occur below the lowest onephoton states should be tiny in the infinite chain, because of cancellations. This has important implications for the interpretation of thirdharmonic generation spectra of polydiacetylenes. A resonance seen in thirdharmonic generation of polydiacetylenes that has commonly been assigned as a twophoton resonance is interpreted here as a threephoton resonance. © 1994 The American Physical Society.
 KuwataGonokami, 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 chargetransfer crystal. Nature, 367(6458), 4748.More infoAbstract: COLLECTIVE excitations resulting from manybody Coulomb interactions have been studied extensively in the solid state: for example, the exchange interaction between the electrons in two excitons (bound electronhole 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 electronhole 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 chargetransfer solid anthracene(pyromellitic acid dianhydride). Coulomb interactions along the onedimensional stacks of this material can stabilize trains of several chargetransfer 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(pphenylenevinylene). Physical Review Letters, 72(1), 156159.More infoAbstract: We have studied electronic excited states in films of poly(pphenylenevinylene) 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 twoelectron (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 fourlevel essential states model. The Journal of Chemical Physics, 100(2), 16651672.More infoAbstract: Much of the theoretical effort to understand the mechanism of optical nonlinearity in πconjugated polymers has been focused towards explaining the early observation of only two resonances in the third harmonic generation spectrum of transpolyacetylene. 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 πconjugated polymers. Very recent experimental results obtained with cispolyacetylene, as well as earlier experiments on two blue forms of polydiacetylenes, are in complete agreement with this prediction. An isolated single strand of cispolyacetylene should exhibit exciton behavior. We briefly discuss the latest experimental results for transpolyacetylene. © 1994 American Institute of Physics.
 Peyghambarian, N., Fluegel, B., Meissner, K., Guo, F., Mazumdar, S., KuwataGonokami, M., Sato, Y., Ema, K., Shimano, R., Tokihiro, T., Ezaki, H., & Hanamura, E. (1994). Observation of excitonic strings in a quasione dimensional charge transfer crystal. Proceedings of the International Quantum Electronics Conference (IQEC'94), 2930.More infoAbstract: This paper demonstrates both experimental and theoretical analysis of the Coulomb attraction of CT excitons, assembling nstring excitons in a quasionedimensional charge transfer (CT) organic crystal. The excitonic strings were experimentally investigated by applying an ultrafast pumpprobe study on high caliber anthracinePMDA 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., KuwataGonokami, M., Sato, Y., Ema, K., Shimano, R., Tokihiro, T., Ezaki, H., & Hanamura, E. (1994). Observation of excitonic nstrings in a quasionedimensional chargetransfer crystal. Materials Research Society Symposium Proceedings, 328, 683688.More infoAbstract: Collective excitations, such as plasmons, rotons, electronhole 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 chargetransfer (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 quasionedimensional CT crystal of anthracene PMDA. High density excitation by femtosecond light pulses generates multiexciton chains, which we refer to as excitonic nstrings 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 (triexciton) are observed. This report provides evidence for an n>2 exciton chain in this system. The stability of the nstring exciton is supported by our theoretical calculations based on the extended Hubbard Hamiltonian in one dimension.
 Ezaki, H., Tokihiro, T., KuwataGonokami, M., Shimano, R., Ema, K., Hanamura, E., Fluegel, B., Meissner, K., Mazumdar, S., & Peyghambarian, N. (1993). Excitonic nstring in linear chains. Solid State Communications, 88(3), 211216.More infoAbstract: The bound states of n Frenkel excitons (n = 2,3,...), which are called excitonic nstrings, are theoretically shown to exist in contrast with a system of Wannier excitons. The electronic structure and nonlinear optical responses of these excitonic nstrings are clarified. The first evidence for the excitonic 2 and 3strings is provided from comparison of the calculated and observed differential transmission spectrum of an AnthracenePMDA 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, twophoton absorption, and thirdharmonic generation in polydiacetylenes. Physical Review B, 48(3), 14331459.More infoAbstract: We report experimental and theoretical investigations of electroabsorption in a polydiacetylene, and determine the complete mechanism of thirdharmonic generation (THG) and twophoton absorption (TPA) in linearchain 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 highenergy 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 onedimensional Coulomb correlated models. We show that the highenergy oscillatory feature in the electroabsorption spectrum originates from the conductionband threshold, which is separated from the exciton in polydiacetylenes. We also demonstrate that evenparity twophoton states that occur below the onephoton exciton are not observed in electroabsorption due to a cancellation effect. However, a dominant twophoton state that is predicted to occur in between the lowest optical exciton and the conductionband threshold should be observable. The cancellation, which is only partial for the dominant twophoton state, can, however, reduce the intensity of the resonance due to the state. We show that the conductionband threshold state, which is an oddparity onephoton state, also plays an important role in other nonlinear optical processes such as thirdharmonic generation and twophoton absorption. Thirdorder optical nonlinearity in linearcorrelated chains is dominated by four essential states: the ground state, the lowest optical exciton and the conductionband threshold states, and the twophoton state that lies in between the two excited oddparity states. The two most important predictions of our theoretical work are (a) thirdharmonicgeneration experiments on ideal isolated strands should find two, not merely one, threephoton resonances, originating from the exciton and the conductionband threshold states, and (b) only one dominant twophoton resonance in the infinitechain 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 fourlevel "essential states" model of third order optical nonlinearity in πconjugated polymers. Synthetic Metals, 57(1), 38813888.More infoAbstract: Theoretical description of optical nonlinearity in πconjugated polymers must necessarily include both the onsite 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 threephoton resonances, and not merely one, as has been generally assumed. Twophoton resonances other than the one due to the essential twophonon 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 nonhalffilled bands. Solid State Communications, 85(11), 917920.More infoAbstract: Commensurate 1 4 and 1 3filled bands in which the electrons are coupled to both intersite phonons and intramolecular vibrations are investigated. Unlike in the 1 2filled band, the bond order wave and the charge density wave coexist and interact cooperatively for all values of the two electronphonon 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 nonhalffilled bands to the metalinsulator transitions in segregated stack chargetransfer solids is discussed. © 1993.
 Ung, K., Mazumdar, S., Campbell, D. K., & Lin, H. Q. (1993). Metalinsulator transitions and electronphonon interactions in organic conductors. Synthetic Metals, 57(2 3 pt 6), 46604665.More infoAbstract: Commensurate 1/4 and 1/3filled bands in which the electrons are coupled to both intersite phonons and intramolecular vibrations are investigated. Unlike in the 1/2filled band, the bond order wave and the charge density wave coexist and interact cooperatively for all values of the two electronphonon 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 nonhalffilled bands to the metalinsulator transitions in segregated stack chargetransfer solids is discussed.
 Ung, K., Mazumdar, S., Campbell, D. K., & Lin, H. Q. (1993). Metalinsulator transitions and electronphonon interactions in organic conductors. Synthetic Metals, 57(23), 46604665.More infoAbstract: Commensurate 1/4 and 1/3filled bands in which the electrons are coupled to both intersite phonons and intramolecular vibrations are investigated. Unlike in the 1/2filled band, the bond order wave and the charge density wave coxexist and interact cooperatively for all values of the two electronphonon 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 nonhalffilled bands to the metalinsulator transitions in segregated stack chargetransfer 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), 21702171.
 Guo, D., & Mazumdar, S. (1992). On the tightbinding theory of C_{60}. Synthetic Metals, 49(13), 175179.More infoAbstract: Optical absorption in a C60 molecule is calculated within a oneelectron tightbinding model. It is seen that it is necessary to include nonzero secondneighbor hopping in order to explain the experimental absorption spectrum. Thus, the secondneighbor hopping plays a stronger role in C60 than in linear conjugated polymers. © 1992.
 Guo, D., Mazumdar, S., & Dixit, S. N. (1992). Microscopic theory of thirdharmonic generation and electroabsorption in conjugated polymers. Synthetic Metals, 49(13), 111.More infoAbstract: We develop a physical, intuitive description of optical nonlinearity in conjugated polymers within a completely general microscopic theoretical model. It is shown that thirdharmonic generation and electroabsorption give complimentary information on the electronic structures of πconjugated systems. © 1992.
 Kawabe, Y., Jarka, F., Peyghambarian, N., Guo, D., Mazumdar, S., Dixit, S. N., & Kajzar, F. (1992). Electroabsorption in a polydiacetylene. Synthetic Metals, 50(1 3 pt 2), 517523.More infoAbstract: The electroabsorption in a DCHpolydiacetylene 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 PeierlsHubbard model), the roles played by the dipoleallowed onephoton states and the dipoleforbidden twophoton states are studied in detail. It is found that the highenergy signals in the difference spectrum are due to the strong fieldinduced mixing of the onephoton state at the edge of a continuum structure, or 'conduction band' in the language of band theory, with its neighboring twophoton states. Contributions to the electroabsorption from the one and twophoton states cancel deep within the band. Threephoton resonance due to the band edge is expected in thirdharmonic 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). Electroabsorption in a polydiacetylene. Synthetic Metals, 50(13), 517523.More infoAbstract: The electroabsorption in a DCHpolydiacetylene 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 PeierlsHubbard model), the roles played by the dipoleallowed onephoton states and the dipoleforbidden twophoton states are studied in detail. It is found that the highenergy signals in the difference spectrum are due to the strong fieldinduced mixing of the onephoton state at the edge of a continuum structure, or 'conduction band' in the language of band theory, with its neighboring twophoton states. Contributions to the electroabsorption from the one and twophoton states cancel deep within the band. Threephoton resonance due to the band edge is expected in thirdharmonic generation (THG) and has been seen in both a polydiacetylene and in polysilanes. © 1992.
 Mazumdar, S., Guo, D., & Dixit, S. N. (1992). High energy twophoton states in finite versus infinite polyenes. The Journal of Chemical Physics, 96(9), 68626867.More infoAbstract: Optical nonlinearity in πconjugated polyenes depends crucially on a single twophoton state that is located above the onephoton 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 twophoton 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 twophoton state in question, we predict it to be extremely close to the onephoton 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). Essentialstates mechanism of optical nonlinearity in conjugated polymers. Physical Review B, 43(8), 67816784.More infoAbstract: 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 welldefined channels, even though an infinite number of channels are possible in principle. We are able to explain the complete frequency dependence of the thirdharmonic generation in both transpolyacetylene 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 peierlshubbard models describing finite polyenes and polyacetylene. Synthetic Metals, 43(12), 34713475.More infoAbstract: We study the extent to which finite polyenes and polyacetylene can be described by the extended PeierlsHubbard 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 transpolyacetylene, (CH)x. We search for sets of parameters describing the electronphonon (ep) and electronelectron (ee) 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 ep coupling is substantially weaker than that suggested by purely ep models and that the ee 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 twophoton transitions in the electroabsorption of a polydiacetylene. Physical Review B, 44(12), 65306533.More infoAbstract: 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 highenergy feature is seen in EA, whose origin has been the subject of controversy. Based on exact theoretical calculation of the EA within an extended PeierlsHubbard model, we ascribe the highenergy signal to transitions to a continuum threshold. We briefly discuss the contributions of these band threshold states to the thirdorder optical nonlinearity, (3). © 1991 The American Physical Society.
 Lin, H., Campbell, D., & Mazumdar, S. (1991). Normal state broken symmetries in quasitwodimensional, quarterfilled chargetransfer solids. Synthetic Metals, 43(3), 3613.
 Mazumdar, S., Campbell, D. K., & Lin, H. . (1991). A novel valencetransition model for the pressureinduced metaltoinsulator transition in the organic conductor (2,5DM  DCNQI)_{2}Cu. Synthetic Metals, 42(12), 1859.
 Mazumdar, S., Campbell, D. K., & Lin, H. . (1991). Novel valencetransition model for the pressureinduced metaltoinsulator transition in the organic conductor (2,5DMDCNQI)_{2}Cu. Synthetic Metals, 42(12), 1859.More infoAbstract: Although metaltoinsulator transitions in organic chargetransfer 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 highpressure 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 neutraltoionic transition in donoracceptor chargetransfer solids, such as TTFChloranil. The ionicity of a narrow band donoracceptor 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(12), 3189.
 Mazumdar, S., Guo, D., & Dixit, S. N. (1991). Microscopic mechanism of optical nonlinearity in conjugated polymers and other quasionedimensional systems. Proceedings of SPIE  The International Society for Optical Engineering, 1436, 136149.More infoAbstract: We present a microscopic mechanism of optical nonlinearity in quasionedimensional semiconductors within the context of rigid band Peierlsextended 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), 40474050.More infoAbstract: 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., GarcíaVázquez, 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), 47974800.More infoAbstract: 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 singleCuO2layer hole superconductor La1.85Sr0.15CuO4. Theoretical implications are discussed briefly. © 1990 The American Physical Society.
 Mazumdar, S. (1989). Comment on "peierls instability in the twodimensional halffilled Hubbard model". Physical Review B, 39(16), 1232412326.More infoAbstract: The analyses of the twodimensional PeierlsHubbard 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 longrange 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 electrondoped hightemperature superconductors. Physica C: Superconductivity and its applications, 161(3), 423430.More infoAbstract: The discovery of ntype hightemperature superconductors implies an electronhole symmetry that is missing in twoband descriptions of the copper oxides. At the same time, several peculiar differences between the new electroncarriers and the older holecarriers cannot be explained within the Cubased singleband Hubbard model. It is shown that these differences are expected within a dopantinduced valence transition mechanism proposed earlier, in which a firstorder valence transition accompanies the antiferromagneticsuperconductor 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), D463D468.More infoAbstract: We reexamine the subgap neutral and charged soliton absorptions in polyacetylene and the 21 Ag state in finite polyenes within the context of a simple PeierlsHubbard model. Our motivation is to determine the origin of the optical gap in transpolyacetylene and to determine the magnitude of the effective onsite Coulomb repulsion. We conclude that this term lies between 2.25t0 and 2.75t0, where t0 is the nearest neighbor oneelectron hopping integral. © 1989.
 Mazumdar, S. (1988). Coulomb correlations, broken symmetries and dimensionality effects. Synthetic Metals, 27(12), A127A132.More infoAbstract: 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 PeierlsHubbard 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 spinPeierls 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 spinPeierls phase and the appearance of an antiferromagnetic phase in TMTTFsalts under pressure can be explained within the present theory. © 1988.
 Mazumdar, S., & Ramasesha, S. (1988). Organic superconductivity: An experiment based theory. Synthetic Metals, 27(12), A105A113.More infoAbstract: 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 chargetransfer solids, are quasitwo dimensionality (as opposed to the more common one dimensionality) and quarterfilled 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 nonBCS superconductivity. Numerical results supporting the present model are presented. © 1988.
 Campbell, D. K., Baeriswyl, D., & Mazumdar, S. (1987). Electronelectron interaction effects in quasionedimensional conducting polymers and related systems. Synthetic Metals, 17(13), 197202.More infoAbstract: We review recent results on the role of electronelectron (ee) interactions in quasionedimensional conducting polymers and related systems. Within the PeierlsHubbard model, the effects of both shortrange (onsite U and nearest neighbor V1) and longrange (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 chargetransfer solids. Synthetic Metals, 19(13), 9398.More infoAbstract: Theoretical description of the complete family of quasionedimensional segregated stack organic chargetransfer 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 xray scattering. We do not attempt to explain the superconductivity in the TMTSF and ETsalts, 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 quasionedimensional conductors. Physica B+C, 143(13), 533537.More infoAbstract: We review recent results on the role of electronelectron (ee) interactions  "correlation effects"  in quasionedimensional conductors. Within the PeierlsHubbard model, we examine the consequences of short range (onsite U and nearest neighbor V) ee 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 electronelectron interaction effects in conducting polymers. Journal of Statistical Physics, 43(56), 803814.More infoAbstract: 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 electronelectron interactions into the "standard" electronphonon 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 longrange Coulomb interactions. Synthetic Metals, 13(13), 163172.More infoAbstract: We investigate the effects of longrange Coulomb interactions on bond and site dimerizations in a onedimensional halffilled 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 bonddimerized and sitedimerized phases. Application of these inequalities to the PariserParrPople model for linear polyenes shows that the infinite polyene has enhanced bond alternation for both Ohno and MatagaNishimoto 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 segregatedstack organic chargetransfer solids: Magnetic properties. Physical Review B, 34(6), 36833699.More infoAbstract: A theoretical description of the complete family of quasionedimensional segregatedstack chargetransfer 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 narrowband 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 electronelectron or electronphonon 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 hightemperature 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 chargetransfer 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 chargetransfer 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, 497501.More infoAbstract: A purely pi electron description via the PariserParrPople (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 lowenergy absorptions with comparable intensity, rather than a single absorption as in polyacetylene.
 Soos, Z. G., Mazumdar, S., & Kuwajima, S. (1986). Extended PPP model: 2^{1}A and 1^{3}B states of polydiacetylene fragments. Physica B+C, 143(13), 538541.More infoAbstract: 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 dipoleallowed 1B excitations, ππ′ 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, 519521.More infoAbstract: 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 OneDimensional HalfFilled Band with Arbitrarily LongRange Coulomb Interactions. Physical Review Letters, 55(19), 20672070.More infoAbstract: The nature of the groundstate broken symmetry in a onedimensional halffilled band with arbitrarily longrange Coulomb interactions between electrons is shown to be precisely given by two simple inequalities involving the Coulomb parameters. It is further shown that non2kF 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 PeierlsHubbard models. Physical Review Letters, 52(19), 17171720.More infoAbstract: We study the effect of electronelectron correlations on the energetics of solitons in electronphonon models of quasi onedimensional materials. In these combined PeierlsHubbard 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 neutralsoliton creation energies; and (3) prove that "soliton doping" persists in the presence of correlations. We discuss the relevance of our results for transpolyacetylene. © 1984 The American Physical Society.
 Dixit, S. N., & Mazumdar, S. (1984). Electronelectron interaction effects on Peierls dimerization in a halffilled band. Physical Review B, 29(4), 18241839.More infoAbstract: A novel realspace approach to dimerization in a halffilled band is developed to investigate effects of electronelectron 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 mirrorplane symmetry passing through the longest diagonal of the infinite ring, and the kinetic and potentialenergy contributions to the barrier to resonance are identified separately. The effects of including the onsite, nearestneighbor, and nextnearestneighbor 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 tensite ring, and finitesize effects are shown to be small. The principal results that are obtained are the following: (i) The onsite correlation U strongly enhances the dimerization, the enhancement being strongest for U4t0, where 4t0 is the bandwidth of the uniform chain; (ii) the nearestneighbor interaction V1 further enhances the dimerization until V112U, while V1>12U favors a uniform chain with a different brokensymmetry ground state, an onsite chargedensity wave; (iii) for V112U, the secondneighbor interaction V2 reduces the dimerization slightly, although the dimerization is still stronger than that with an effective nearestneighbor interaction V1V2; (iv) for V1>12U, V2 destroys the onsite chargedensity wave and the ground state is strongly dimerized again. The complete ParisierParrPople (PPP) Hamiltonian is discussed, and it is pointed out that the above results, together with the excitedstate orderings in the PPP Hamiltonian, strongly indicate that the ground state of the PPP Hamiltonian is the dimerized state. The excitedstate orderings in finite polyenes, spindensity 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). Electronelectron interactions and broken symmetry in one dimension. Synthetic Metals, 9(2), 275281.More infoAbstract: A novel realspace approach to dimerization in a halffilled band is developed to investigate effects of electronelectron 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 chargeordered states in a correlated diatomic polymer. Physical Review B, 29(4), 23172320.More infoAbstract: Using physical arguments based on a realspace 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 chargedensity waves in narrowband onedimensional conductors. Physical Review B, 30(8), 48424845.More infoAbstract: We show quantitatively that the conditions for charge ordering in onedimensional metals depend strongly on the band filling and the relative magnitudes of intermolecular electronic correlations. We thereby explain the irregular appearance of 4kF chargedensity waves in conducting organic charge transfer salts. For band fillings incrementally larger than onefourth, we predict a new kind of Coulombinduced defect. © 1984 The American Physical Society.
 Mazumdar, S., & Bloch, A. N. (1983). Systematic trends in shortrange coulomb effects among nearly onedimensional organic conductors. Physical Review Letters, 50(3), 207211.More infoAbstract: A resolution to the controversy over the magnitude of the effective shortrange 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 onedimensional peierls instability: The peierlshubbard model. Physical Review Letters, 51(4), 292295.More infoAbstract: A novel approach to dimerization is developed, based on a manybody valencebond approach, that predicts enhancement of the dimerization on inclusion of the onsite electron repulsion. Exact numerical calculations are presented to substantiate this conclusion. © 1983 The American Physical Society.
 Mazumdar, S., & Soos, Z. G. (1981). Valencebond analysis of extended Hubbard models: Chargetransfer excitations of molecular conductors. Physical Review B, 23(6), 28102823.More infoAbstract: The lowenergy charge transfer (CT) excitation characteristic of both molecular conductors and complexionradical salts is interpreted as a nearestneighbor Coulomb interaction V that is comparable to the bandwidth, 4t. Partly filled segregated regular stacks in organic conductors are represented by extended Hubbard models, whose exact CT energies and intensities are obtained by diagrammatic valencebond (VB) methods for four electrons on finite rings and chains, together with an approximate treatment of V in partly filled infinite stacks for infinite onsite correlations U. Finite V4t yields an intense lowlying CT band, containing V and U2V 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 UV, weak ones around U, and much weaker bands around U+V and U+2V. The CT spectra of mixedvalence tetrathiofulvalene (TTF) salts are modeled with V0.4 eV, U1.4 eV, and t0.100.13 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 dipoleallowed optical transitions. © 1981 The American Physical Society.
 Mazumdar, S., & Soos, Z. G. (1979). Valence bond theory of narrowband conductors. Synthetic Metals, 1(1), 7794.More infoAbstract: A diagrammatic valence bond (VB) theory is introduced for solids with one valence state per site, Coulomb intersite interactions, and nearestneighbor electron transfers leading to a bandwidth 4 t. The VB method is applied to partlyfilled regular segregated stacks of acceptors (A) and donors (D), which occur in all πmolecular 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 manyelectron 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 onsite repulsion, U. The VB method extends the computation of the susceptibility χ(T) of partlyfilled Hubbard models to the previously inaccessible regime U ∼ 4t which, however, is suggested by experiment. Comparison with absolute χ(T) data for TTFTCNQ is slightly improved for U ∼ 4t = 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), 331334.More infoAbstract: A model hamiltonian with only onsite 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), 713716.More infoAbstract: 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 donoracceptor stacks. Chemical Physics Letters, 56(3), 515518.More infoAbstract: The ground state charge transfer γ(z) and magnetic gap ΔEm(z) of organic donoracceptor complexes with one dimensional⋯D+γAγD+γA+γ⋯ stacks are computed for arbitrary ratios of z = δ/2 1 2twhere t is the chargetransfer integral and 2δ is the excitation energy for a D+A electronhole pair. Valence bond techniques give exact results for rings of N = 4, 6, 8, and 10 sites, which suffice to show that the neutralionic 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). Neutralionic interface in organic chargetransfer salts. Physical Review B, 18(4), 19912003.More infoAbstract: Charge transfer (CT) stabilization in linear stacks D+AD+A of electron donors (D) and acceptors (A) involve spindependent configuration interactions that are treated exactly in rings of N=4, 6, 8, 10 sites, and extrapolated to N, by adapting valencebond techniques to electronhole excitations. The ground state CT (z) and the magnetic gap Em(z)2t to the lowest triplet are computed for arbitrary z=2t, where 2 is the energy for DAD+A, t=D+AHDA 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 t0, finite overlap gives a continuous (z) and (zc)=0.680.01 at the neutralionic 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 longrange threedimensional Coulomb interactions are included in a Hartree approximation and produce a firstorder transition, with discontinuous (z), when the critical value m2t=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 CHARGETRANSFER SALTS.. Mol Cryst Liq Cryst, 52(14), 93102.More infoAbstract: A valence bond (VB) description is introduced for organic chargetransfer (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″ TMPDTCNQ complex indicates incomplete CT, in agreement with the resonant Raman result of gamma similar 0. 7. The extension of diagrammatic VB techniques to partlyfilled segregated ionradical 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). ValenceBond theory of antiferromagnetism, chargeordering and correlatedelectron superconductivity in organic chargetransfer solids. International Symposium on Crystalline Organic Materials. Tomar, Portugal: Instituto Superior Técnico, Portugal.
 Khan, S. (2019, July). THE ROLE OF THE QUANTUMENTANGLED TRIPLETTRIPLET 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 QuantumEntangled TripletTriplet 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 DonorAcceptor Copolymers. Excited State Processes 2016. Santa Fe, NM: Los Alamos National Laboratory.
 Mazumdar, S. (2016, JuneJuly). Theory of Primary Photoexcitations in Low Bandgap DonorAcceptor 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 DonorAcceptor 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 quarterfilled 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 QUARTERFILLED BAND. International Symposium on Crystalline Organic Materials. Bad Gögging, Germany: ISCOM.
 Dutta, T. (2014, July). Unified theoretical approach to superconductivity in organic chargetransfer 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 metalintercalated phenacenes?. International Conference on Science and Technology of Synthetic Metals 2014. Turku, Finland.
 Dutta, T. (2014, November). Unified theoretical approach to superconductivity in organic chargetransfer solids, phenacenes and fullerides.. Ushimado International Workshop on Physics and Chemistry of Novel Superconductors and Related Materials. Ushimado, Okayama Prefecture, Japan.: Okayama Univ. and EUJapan 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..