Kyle Seyler

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• Li, X., Esin, I., Han, Y., Liu, Y., Zhao, H., Ning, H., Barrett, C., Shan, J., Seyler, K., Cao, G., Refael, G., & Hsieh, D. (2025). Time-hidden magnetic order in a multi-orbital Mott insulator. Nature Physics, 1-8.
• Seyler, K., Ron, A., Van Beveren, D., Rotundu, C., Lee, Y., & Hsieh, D. (2022). Direct visualization and control of antiferromagnetic domains and spin reorientation in a parent cuprate. Physical Review B, 106(14). doi:10.1103/PhysRevB.106.L140403
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  We report magnetic optical second-harmonic generation (SHG) polarimetry and imaging on Sr2Cu3O4Cl2, which allows direct visualization of the mesoscopic antiferromagnetic (AFM) structure of a parent cuprate. Temperature- and magnetic-field-dependent SHG reveals large domains with 90∘ relative orientations that are stabilized by a combination of uniaxial magnetic anisotropy and the Earth's magnetic field. Below a temperature TR ∼ 97 K, we observe an unusual 90∘ spin-reorientation transition, possibly driven by competing magnetic anisotropies of the two copper sublattices, which swaps the AFM domain states while preserving the domain structure. This allows deterministic switching of the AFM states by thermal or laser heating. Near TR, the domain walls become exceptionally responsive to an applied magnetic field, with the Earth's field sufficient to completely expel them from the crystal. Our findings unlock opportunities to study the mesoscopic AFM behavior of parent cuprates and explore their potential for AFM technologies.
• de la Torre, A., Seyler, K., Buchhold, M., Baum, Y., Zhang, G., Laurita, N., Harter, J., Zhao, L., Phinney, I., Chen, X., Wilson, S., Cao, G., Averitt, R., Refael, G., & Hsieh, D. (2022). Decoupling of static and dynamic criticality in a driven Mott insulator. Communications Physics, 5(1). doi:10.1038/s42005-022-00813-6
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  Strongly driven antiferromagnetic Mott insulators have the potential to exhibit exotic transient phenomena that are forbidden in thermal equilibrium. However, such far-from-equilibrium regimes, where conventional time-dependent Ginzburg-Landau descriptions fail, are experimentally challenging to prepare and to probe especially in solid state systems. Here we use a combination of time-resolved second harmonic optical polarimetry and coherent magnon spectroscopy to interrogate n-type photo-doping induced ultrafast magnetic order parameter dynamics in the antiferromagnetic Mott insulator Sr2IrO4. We find signatures of an unusual far-from-equilibrium critical regime in which the divergences of the magnetic correlation length and relaxation time are decoupled. This violation of conventional thermal critical behavior arises from the interplay of photo-doping and non-thermal magnon population induced demagnetization effects. Our findings, embodied in a non-equilibrium phase diagram, provide a blueprint for engineering the out-of-equilibrium properties of quantum matter, with potential applications to terahertz spintronics technologies.
• Song, T., Anderson, E., Tu, M., Seyler, K., Taniguchi, T., Watanabe, K., McGuire, M., Li, X., Cao, T., Xiao, D., Yao, W., & Xu, X. (2021). Spin photovoltaic effect in magnetic van der Waals heterostructures. Science Advances, 7(36). doi:10.1126/sciadv.abg8094
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  The development of van der Waals (vdW) crystals and their heterostructures has created a fascinating platform for exploring optoelectronic properties in the two-dimensional (2D) limit. With the recent discovery of 2D magnets, the control of the spin degree of freedom can be integrated to realize 2D spin-optoelectronics. Here, we report spin photovoltaic effects in vdW heterostructures of 2D magnet chromium triiodide (CrI3) sandwiched by graphene contacts. The photocurrent displays a distinct dependence on light helicity, which can be tuned by varying the magnetic states and photon energy. Circular polarization-resolved absorption measurements reveal that these observations originate from magnetic order-coupled and, thus, helicity-dependent charge-transfer excitons. The photocurrent displays multiple plateaus as the magnetic field is swept, associated with different CrI3 spin configurations. Giant photo-magnetocurrent is observed, which tends to infinity for a small applied bias. Our results pave the way to explore emergent photospintronics by engineering magnetic vdW heterostructures.
• Torre, A., Seyler, K., Zhao, L., Matteo, S., Scheurer, M., Li, Y., Yu, B., Greven, M., Sachdev, S., Norman, M., & Hsieh, D. (2021). Mirror symmetry breaking in a model insulating cuprate. Nature Physics, 17(7). doi:10.1038/s41567-021-01210-6
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  Among the most actively studied issues in the cuprates are the natures of the pseudogap and strange metal states and their relationship to superconductivity1. There is general agreement that the low-energy physics of the Mott-insulating parent state is well captured by a two-dimensional spin S = 1/2 antiferromagnetic Heisenberg model2. However, recent observations of a large thermal Hall conductivity in several parent cuprates appear to defy this simple model and suggest proximity to a magneto-chiral state that breaks all mirror planes that are perpendicular to the CuO2 layers3–6. Here we use optical second harmonic generation to directly resolve the point group symmetries of the model parent cuprate Sr2CuO2Cl2. We report evidence of an order parameter that breaks all perpendicular mirror planes and is consistent with a magneto-chiral state in zero magnetic field. Although this order is clearly coupled to the antiferromagnetism, we are unable to realize its time-reversed partner by thermal cycling through the antiferromagnetic transition temperature or by sampling different spatial locations. This suggests that the order onsets above the Néel temperature and may be relevant to the mechanism of pseudogap formation.
• Wang, X., Zhu, J., Seyler, K., Rivera, P., Zheng, H., Wang, Y., He, M., Taniguchi, T., Watanabe, K., Yan, J., Mandrus, D., Gamelin, D., Yao, W., & Xu, X. (2021). Moiré trions in MoSe2/WSe2 heterobilayers. Nature Nanotechnology, 16(11). doi:10.1038/s41565-021-00969-2
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  Transition metal dichalcogenide moiré bilayers with spatially periodic potentials have emerged as a highly tunable platform for studying both electronic1–6 and excitonic4,7–13 phenomena. The power of these systems lies in the combination of strong Coulomb interactions with the capability of controlling the charge number in a moiré potential trap. Electronically, exotic charge orders at both integer and fractional fillings have been discovered2,5. However, the impact of charging effects on excitons trapped in moiré potentials is poorly understood. Here, we report the observation of moiré trions and their doping-dependent photoluminescence polarization in H-stacked MoSe2/WSe2 heterobilayers. We find that as moiré traps are filled with either electrons or holes, new sets of interlayer exciton photoluminescence peaks with narrow linewidths emerge about 7 meV below the energy of the neutral moiré excitons. Circularly polarized photoluminescence reveals switching from co-circular to cross-circular polarizations as moiré excitons go from being negatively charged and neutral to positively charged. This switching results from the competition between valley-flip and spin-flip energy relaxation pathways of photo-excited electrons during interlayer trion formation. Our results offer a starting point for engineering both bosonic and fermionic many-body effects based on moiré excitons14.
• Seyler, K., De La Torre, A., Porter, Z., Zoghlin, E., Polski, R., Nguyen, M., Nadj-Perge, S., Wilson, S., & Hsieh, D. (2020). Spin-orbit-enhanced magnetic surface second-harmonic generation in Sr2Ir O4. Physical Review B, 102(20). doi:10.1103/PhysRevB.102.201113
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  An anomalous optical second-harmonic generation (SHG) signal was previously reported in Sr2IrO4 and attributed to a hidden odd-parity bulk magnetic state. Here we investigate the origin of this SHG signal using a combination of bulk magnetic susceptibility, magnetic-field-dependent SHG rotational anisotropy, and overlapping wide-field SHG imaging and atomic force microscopy measurements. We find that the anomalous SHG signal exhibits a twofold rotational symmetry as a function of in-plane magnetic field orientation that is associated with a crystallographic distortion. We also show a change in SHG signal across step edges that tracks the bulk antiferromagnetic stacking pattern. While we do not rule out the existence of hidden order in Sr2IrO4, our results altogether show that the anomalous SHG signal in parent Sr2IrO4 originates instead from a surface-magnetization-induced electric-dipole process that is enhanced by strong spin-orbit coupling.
• Zhong, D., Seyler, K., Linpeng, X., Wilson, N., Taniguchi, T., Watanabe, K., McGuire, M., Fu, K., Xiao, D., Yao, W., & Xu, X. (2020). Layer-resolved magnetic proximity effect in van der Waals heterostructures. Nature Nanotechnology, 15(3). doi:10.1038/s41565-019-0629-1
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  Magnetic proximity effects are integral to manipulating spintronic1,2, superconducting3,4, excitonic5 and topological phenomena6–8 in heterostructures. These effects are highly sensitive to the interfacial electronic properties, such as electron wavefunction overlap and band alignment. The recent emergence of magnetic two-dimensional materials opens new possibilities for exploring proximity effects in van der Waals heterostructures9–12. In particular, atomically thin CrI3 exhibits layered antiferromagnetism, in which adjacent ferromagnetic monolayers are antiferromagnetically coupled9. Here we report a layer-resolved magnetic proximity effect in heterostructures formed by monolayer WSe2 and bi/trilayer CrI3. By controlling the individual layer magnetization in CrI3 with a magnetic field, we show that the spin-dependent charge transfer between WSe2 and CrI3 is dominated by the interfacial CrI3 layer, while the proximity exchange field is highly sensitive to the layered magnetic structure as a whole. In combination with reflective magnetic circular dichroism measurements, these properties allow the use of monolayer WSe2 as a spatially sensitive magnetic sensor to map out layered antiferromagnetic domain structures at zero magnetic field as well as antiferromagnetic/ferromagnetic domains at finite magnetic fields. Our work reveals a way to control proximity effects and probe interfacial magnetic order via van der Waals engineering13.
• Seyler, K., Rivera, P., Yu, H., Wilson, N., Ray, E., Mandrus, D., Yan, J., Yao, W., & Xu, X. (2019). Signatures of moiré-trapped valley excitons in MoSe2/WSe2 heterobilayers. Nature, 567(7746). doi:10.1038/s41586-019-0957-1
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  The formation of moiré patterns in crystalline solids can be used to manipulate their electronic properties, which are fundamentally influenced by periodic potential landscapes. In two-dimensional materials, a moiré pattern with a superlattice potential can be formed by vertically stacking two layered materials with a twist and/or a difference in lattice constant. This approach has led to electronic phenomena including the fractal quantum Hall effect1–3, tunable Mott insulators4,5 and unconventional superconductivity6. In addition, theory predicts that notable effects on optical excitations could result from a moiré potential in two-dimensional valley semiconductors7–9, but these signatures have not been detected experimentally. Here we report experimental evidence of interlayer valley excitons trapped in a moiré potential in molybdenum diselenide (MoSe2)/tungsten diselenide (WSe2) heterobilayers. At low temperatures, we observe photoluminescence close to the free interlayer exciton energy but with linewidths over one hundred times narrower (around 100 microelectronvolts). The emitter g-factors are homogeneous across the same sample and take only two values, −15.9 and 6.7, in samples with approximate twist angles of 60 degrees and 0 degrees, respectively. The g-factors match those of the free interlayer exciton, which is determined by one of two possible valley-pairing configurations. At twist angles of approximately 20 degrees the emitters become two orders of magnitude dimmer; however, they possess the same g-factor as the heterobilayer at a twist angle of approximately 60 degrees. This is consistent with the umklapp recombination of interlayer excitons near the commensurate 21.8-degree twist angle7. The emitters exhibit strong circular polarization of the same helicity for a given twist angle, which suggests that the trapping potential retains three-fold rotational symmetry. Together with a characteristic dependence on power and excitation energy, these results suggest that the origin of the observed effects is interlayer excitons trapped in a smooth moiré potential with inherited valley-contrasting physics. This work presents opportunities to control two-dimensional moiré optics through variation of the twist angle.
• Tung, I., Krishnamoorthy, A., Sadasivam, S., Zhou, H., Zhang, Q., Seyler, K., Clark, G., Mannebach, E., Nyby, C., Ernst, F., Zhu, D., Glownia, J., Kozina, M., Song, S., Nelson, S., Kumazoe, H., Shimojo, F., Kalia, R., Vashishta, P., , Darancet, P., et al. (2019). Anisotropic structural dynamics of monolayer crystals revealed by femtosecond surface X-ray scattering. Nature Photonics, 13(6). doi:10.1038/s41566-019-0387-5
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  Ultrafast X-ray scattering is one of the primary tools to track intrinsic crystallographic evolution with atomic accuracy in real time. However, its application to study nonequilibrium structural properties at the two-dimensional limit remains a long-standing challenge due to a significant reduction of diffraction volume and complexity of data analysis. Here, we report femtosecond surface X-ray diffraction in combination with crystallographic model-refinement calculations to quantify the ultrafast structural dynamics of monolayer WSe2 crystals supported on a substrate. We found the absorbed optical photon energy is preferably coupled to the in-plane lattice vibrations within one picosecond whereas the out-of-plane lattice vibration amplitude remains unchanged during the first ten picoseconds. The model-assisted fitting suggests an asymmetric intralayer spacing change upon excitation. The observed nonequilibrium anisotropic structural dynamics agrees with first-principles modelling in both real and momentum space, marking the distinct structural dynamics of monolayer crystals from their bulk counterparts.
• De La Barrera, S., Sinko, M., Gopalan, D., Sivadas, N., Seyler, K., Watanabe, K., Taniguchi, T., Tsen, A., Xu, X., Xiao, D., & Hunt, B. (2018). Tuning Ising superconductivity with layer and spin-orbit coupling in two-dimensional transition-metal dichalcogenides. Nature Communications, 9(1). doi:10.1038/s41467-018-03888-4
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  Systems simultaneously exhibiting superconductivity and spin-orbit coupling are predicted to provide a route toward topological superconductivity and unconventional electron pairing, driving significant contemporary interest in these materials. Monolayer transition-metal dichalcogenide (TMD) superconductors in particular lack inversion symmetry, yielding an antisymmetric form of spin-orbit coupling that admits both spin-singlet and spin-triplet components of the superconducting wavefunction. Here, we present an experimental and theoretical study of two intrinsic TMD superconductors with large spin-orbit coupling in the atomic layer limit, metallic 2H-TaS2 and 2H-NbSe2. We investigate the superconducting properties as the material is reduced to monolayer thickness and show that high-field measurements point to the largest upper critical field thus reported for an intrinsic TMD superconductor. In few-layer samples, we find the enhancement of the upper critical field is sustained by the dominance of spin-orbit coupling over weak interlayer coupling, providing additional candidate systems for supporting unconventional superconducting states in two dimensions.
• Huang, B., Clark, G., Klein, D., MacNeill, D., Navarro-Moratalla, E., Seyler, K., Wilson, N., McGuire, M., Cobden, D., Xiao, D., Yao, W., Jarillo-Herrero, P., & Xu, X. (2018). Electrical control of 2D magnetism in bilayer CrI3. Nature Nanotechnology, 13(7). doi:10.1038/s41565-018-0121-3
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  Controlling magnetism via electric fields addresses fundamental questions of magnetic phenomena and phase transitions 1-3, and enables the development of electrically coupled spintronic devices, such as voltage-controlled magnetic memories with low operation energy 4-6 . Previous studies on dilute magnetic semiconductors such as (Ga,Mn)As and (In,Mn)Sb have demonstrated large modulations of the Curie temperatures and coercive fields by altering the magnetic anisotropy and exchange interaction 2,4,7-9 . Owing to their unique magnetic properties 10-14, the recently reported two-dimensional magnets provide a new system for studying these features 15-19 . For instance, a bilayer of chromium triiodide (CrI3) behaves as a layered antiferromagnet with a magnetic field-driven metamagnetic transition 15,16 . Here, we demonstrate electrostatic gate control of magnetism in CrI3 bilayers, probed by magneto-optical Kerr effect (MOKE) microscopy. At fixed magnetic fields near the metamagnetic transition, we realize voltage-controlled switching between antiferromagnetic and ferromagnetic states. At zero magnetic field, we demonstrate a time-reversal pair of layered antiferromagnetic states that exhibit spin-layer locking, leading to a linear dependence of their MOKE signals on gate voltage with opposite slopes. Our results allow for the exploration of new magnetoelectric phenomena and van der Waals spintronics based on 2D materials.
• Rivera, P., Yu, H., Seyler, K. L., Wilson, N. P., Yao, W., & Xu, X. (2018). Interlayer valley excitons in heterobilayers of transition metal dichalcogenides. Nature nanotechnology, 13(11), 1004-1015.
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  Stacking different two-dimensional crystals into van der Waals heterostructures provides an exciting approach to designing quantum materials that can harness and extend the already fascinating properties of the constituents. Heterobilayers of transition metal dichalcogenides are particularly attractive for low-dimensional semiconductor optics because they host interlayer excitons-with electrons and holes localized in different layers-which inherit valley-contrasting physics from the monolayers and thereby possess various novel and appealing properties compared to other solid-state nanostructures. This Review presents the contemporary experimental and theoretical understanding of these interlayer excitons. We discuss their unique optical properties arising from the underlying valley physics, the strong many-body interactions and electrical control resulting from the electric dipole moment, and the unique effects of a moiré superlattice on the interlayer exciton potential landscape and optical properties.
• Seyler, K., Zhong, D., Huang, B., Linpeng, X., Wilson, N., Taniguchi, T., Watanabe, K., Yao, W., Xiao, D., McGuire, M., Fu, K., & Xu, X. (2018). Valley Manipulation by Optically Tuning the Magnetic Proximity Effect in WSe2/CrI3 Heterostructures. Nano Letters, 18(6). doi:10.1021/acs.nanolett.8b01105
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  Monolayer valley semiconductors, such as tungsten diselenide (WSe2), possess valley pseudospin degrees of freedom that are optically addressable but degenerate in energy. Lifting the energy degeneracy by breaking time-reversal symmetry is vital for valley manipulation. This has been realized by directly applying magnetic fields or via pseudomagnetic fields generated by intense circularly polarized optical pulses. However, sweeping large magnetic fields is impractical for devices, and the pseudomagnetic fields are only effective in the presence of ultrafast laser pulses. The recent rise of two-dimensional (2D) magnets unlocks new approaches to controlling valley physics via van der Waals heterostructure engineering. Here, we demonstrate the wide continuous tuning of the valley polarization and valley Zeeman splitting with small changes in the laser-excitation power in heterostructures formed by monolayer WSe2 and 2D magnetic chromium triiodide (CrI3). The valley manipulation is realized via the optical control of the CrI3 magnetization, which tunes the magnetic exchange field over a range of 20 T. Our results reveal a convenient new path toward the optical control of valley pseudospins and van der Waals magnetic heterostructures.
• Seyler, K., Zhong, D., Klein, D., Gao, S., Zhang, X., Huang, B., Navarro-Moratalla, E., Yang, L., Cobden, D., McGuire, M., Yao, W., Xiao, D., Jarillo-Herrero, P., & Xu, X. (2018). Ligand-field helical luminescence in a 2D ferromagnetic insulator. Nature Physics, 14(3). doi:10.1038/s41567-017-0006-7
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  Bulk chromium tri-iodide (CrI 3 ) has long been known as a layered van der Waals ferromagnet 1 . However, its monolayer form was only recently isolated and confirmed to be a truly two-dimensional (2D) ferromagnet 2 , providing a new platform for investigating light-matter interactions and magneto-optical phenomena in the atomically thin limit. Here, we report spontaneous circularly polarized photoluminescence in monolayer CrI 3 under linearly polarized excitation, with helicity determined by the monolayer magnetization direction. In contrast, the bilayer CrI 3 photoluminescence exhibits vanishing circular polarization, supporting the recently uncovered anomalous antiferromagnetic interlayer coupling in CrI 3 bilayers 2 . Distinct from the Wannier-Mott excitons that dominate the optical response in well-known 2D van der Waals semiconductors 3 , our absorption and layer-dependent photoluminescence measurements reveal the importance of ligand-field and charge-transfer transitions to the optoelectronic response of atomically thin CrI 3 . We attribute the photoluminescence to a parity-forbidden d-d transition characteristic of Cr 3+ complexes, which displays broad linewidth due to strong vibronic coupling and thickness-independent peak energy due to its localized molecular orbital nature.
• Song, T., Cai, X., Tu, M., Zhang, X., Huang, B., Wilson, N., Seyler, K., Zhu, L., Taniguchi, T., Watanabe, K., McGuire, M., Cobden, D., Xiao, D., Yao, W., & Xu, X. (2018). Giant tunneling magnetoresistance in spin-filter van der Waals heterostructures. Science, 360(6394). doi:10.1126/science.aar4851
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  Magnetic multilayer devices that exploit magnetoresistance are the backbone of magnetic sensing and data storage technologies. Here, we report multiple-spin-filter magnetic tunnel junctions (sf-MTJs) based on van der Waals (vdW) heterostructures in which atomically thin chromium triiodide (CrI 3 ) acts as a spin-filter tunnel barrier sandwiched between graphene contacts. We demonstrate tunneling magnetoresistance that is drastically enhanced with increasing CrI 3 layer thickness, reaching a record 19,000% for magnetic multilayer structures using four-layer sf-MTJs at low temperatures. Using magnetic circular dichroism measurements, we attribute these effects to the intrinsic layer-by-layer antiferromagnetic ordering of the atomically thin CrI 3 . Our work reveals the possibility to push magnetic information storage to the atomically thin limit and highlights CrI 3 as a superlative magnetic tunnel barrier for vdW heterostructure spintronic devices.
• Fryett, T., Seyler, K., Zheng, J., Liu, C., Xu, X., & Majumdar, A. (2017). Silicon photonic crystal cavity enhanced second-harmonic generation from monolayer WSe2. 2D Materials, 4(1). doi:10.1088/2053-1583/4/1/015031
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  Nano-resonators integrated with two-dimensional materials (e.g. transition metal dichalcogenides) have recently emerged as a promising nano-optoelectronic platform. Here we demonstrate resonator-enhanced second-harmonic generation (SHG) in tungsten diselenide using a silicon photonic crystal cavity. By pumping the device with ultrafast laser pulses near the cavity mode at the telecommunication wavelength, we observe a near visible SHG with a narrow linewidth and near unity linear polarization, originated from the coupling of the pump photon to the cavity mode. The observed SHG is enhanced by factor of ∼200 compared to a bare monolayer on silicon. Our results imply the efficacy of cavity integrated monolayer materials for nonlinear optics and the potential of building a silicon-compatible second-order nonlinear integrated photonic platform.
• Huang, B., Clark, G., Navarro-Moratalla, E., Klein, D., Cheng, R., Seyler, K., Zhong, D., Schmidgall, E., McGuire, M., Cobden, D., Yao, W., Xiao, D., Jarillo-Herrero, P., & Xu, X. (2017). Layer-dependent ferromagnetism in a van der Waals crystal down to the monolayer limit. Nature, 546(7657). doi:10.1038/nature22391
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  Since the discovery of graphene, the family of two-dimensional materials has grown, displaying a broad range of electronic properties. Recent additions include semiconductors with spin-valley coupling, Ising superconductors that can be tuned into a quantum metal, possible Mott insulators with tunable charge-density waves, and topological semimetals with edge transport. However, no two-dimensional crystal with intrinsic magnetism has yet been discovered; such a crystal would be useful in many technologies from sensing to data storage. Theoretically, magnetic order is prohibited in the two-dimensional isotropic Heisenberg model at finite temperatures by the Mermin-Wagner theorem. Magnetic anisotropy removes this restriction, however, and enables, for instance, the occurrence of two-dimensional Ising ferromagnetism. Here we use magneto-optical Kerr effect microscopy to demonstrate that monolayer chromium triiodide (CrI 3) is an Ising ferromagnet with out-of-plane spin orientation. Its Curie temperature of 45 kelvin is only slightly lower than that of the bulk crystal, 61 kelvin, which is consistent with a weak interlayer coupling. Moreover, our studies suggest a layer-dependent magnetic phase, highlighting thickness-dependent physical properties typical of van der Waals crystals. Remarkably, bilayer CrI 3 displays suppressed magnetization with a metamagnetic effect, whereas in trilayer CrI 3 the interlayer ferromagnetism observed in the bulk crystal is restored. This work creates opportunities for studying magnetism by harnessing the unusual features of atomically thin materials, such as electrical control for realizing magnetoelectronics, and van der Waals engineering to produce interface phenomena.
• Mannebach, E., Nyby, C., Ernst, F., Zhou, Y., Tolsma, J., Li, Y., Sher, M., Tung, I., Zhou, H., Zhang, Q., Seyler, K., Clark, G., Yu, Y., Zhu, D., Glownia, J., Kozina, M., Song, S., Nelson, S., Mehta, A., , Pant, A., et al. (2017). Dynamic Optical Tuning of Interlayer Interactions in the Transition Metal Dichalcogenides. Nano Letters, 17(12). doi:10.1021/acs.nanolett.7b03955
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  Modulation of weak interlayer interactions between quasi-two-dimensional atomic planes in the transition metal dichalcogenides (TMDCs) provides avenues for tuning their functional properties. Here we show that above-gap optical excitation in the TMDCs leads to an unexpected large-amplitude, ultrafast compressive force between the two-dimensional layers, as probed by in situ measurements of the atomic layer spacing at femtosecond time resolution. We show that this compressive response arises from a dynamic modulation of the interlayer van der Waals interaction and that this represents the dominant light-induced stress at low excitation densities. A simple analytic model predicts the magnitude and carrier density dependence of the measured strains. This work establishes a new method for dynamic, nonequilibrium tuning of correlation-driven dispersive interactions and of the optomechanical functionality of TMDC quasi-two-dimensional materials.
• Wilson, N., Nguyen, P., Seyler, K., Rivera, P., Marsden, A., Laker, Z., Constantinescu, G., Kandyba, V., Barinov, A., Hine, N., Xu, X., & Cobden, D. (2017). Determination of band offsets, hybridization, and exciton binding in 2D semiconductor heterostructures. Science Advances, 3(2). doi:10.1126/sciadv.1601832
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  Combining monolayers of different two-dimensional semiconductors into heterostructures creates new phenomena and device possibilities. Understanding and exploiting these phenomena hinge on knowing the electronic structure and the properties of interlayer excitations. We determine the key unknown parameters in MoSe2/WSe2 heterobilayers by using rational device design and submicrometer angle-resolved photoemission spectroscopy (m-ARPES) in combination with photoluminescence. We find that the bands in the K-point valleys are weakly hybridized, with a valence band offset of 300 meV, implying type II band alignment. We deduce that the binding energy of interlayer excitons is more than 200 meV, an order of magnitude higher than that in analogous GaAs structures. Hybridization strongly modifies the bands at G, but the valence band edge remains at the K points. We also find that the spectrum of a rotationally aligned heterobilayer reflects a mixture of commensurate and incommensurate domains. These results directly answer many outstanding questions about the electronic nature of MoSe2/WSe2 heterobilayers and demonstrate a practical approach for high spectral resolution in ARPES of device-scale structures.
• Zhong, D., Seyler, K., Linpeng, X., Cheng, R., Sivadas, N., Huang, B., Schmidgall, E., Taniguchi, T., Watanabe, K., McGuire, M., Yao, W., Xiao, D., Fu, K., & Xu, X. (2017). Van der Waals engineering of ferromagnetic semiconductor heterostructures for spin and valleytronics. Science Advances, 3(5). doi:10.1126/sciadv.1603113
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  The integration of magnetic material with semiconductors has been fertile ground for fundamental science as well as of great practical interest toward the seamless integration of information processing and storage.We create van der Waals heterostructures formed by an ultrathin ferromagnetic semiconductor CrI3 and a monolayer of WSe2. We observe unprecedented control of the spin and valley pseudospin in WSe2, where we detect a large magnetic exchange field of nearly 13 T and rapid switching of the WSe2 valley splitting and polarization via flipping of the CrI3 magnetization. The WSe2 photoluminescence intensity strongly depends on the relative alignment between photoexcited spins in WSe2 and the CrI3 magnetization, because of ultrafast spin-dependent charge hopping across the heterostructure interface. The photoluminescence detection of valley pseudospin provides a simple and sensitive method to probe the intriguing domain dynamics in the ultrathin magnet, as well as the rich spin interactions within the heterostructure.
• Dhall, R., Seyler, K., Li, Z., Wickramaratne, D., Neupane, M., Chatzakis, I., Kosmowska, E., Lake, R., Xu, X., & Cronin, S. (2016). Strong Circularly Polarized Photoluminescence from Multilayer MoS2 Through Plasma Driven Direct-Gap Transition. ACS Photonics, 3(3). doi:10.1021/acsphotonics.5b00593
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  We report circularly polarized photoluminescence spectra taken from few layer MoS2 after treatment with a remotely generated oxygen plasma. Here, the oxygen plasma decouples the individual layers in MoS2 by perturbing the weak interlayer van der Waals forces without damaging the lattice structure. This decoupling causes a transition from an indirect to a direct band gap material, which causes a strong enhancement of the photoluminescence intensity. Furthermore, up to 80% circularly polarized photoluminescence is observed after plasma treatment of few layer MoS2 flakes, consistent with high spin polarization of the optically excited carriers. A strong degree of polarization continues up to room temperature, further indicating that the quality of the crystal does not suffer degradation due to the oxygen plasma exposure. Our results show that the oxygen plasma treatment not only engineers the van der Waals separation in these TMDC multilayers for enhanced PL quantum yields, but also produces high quality multilayer samples for strong circularly polarized emission, which offers the benefit of layer index as an additional degree of freedom, absent in monolayer MoS2.
• Liu, F., You, L., Seyler, K., Li, X., Yu, P., Lin, J., Wang, X., Zhou, J., Wang, H., He, H., Pantelides, S., Zhou, W., Sharma, P., Xu, X., Ajayan, P., Wang, J., & Liu, Z. (2016). Room-temperature ferroelectricity in CuInP 2 S 6 ultrathin flakes. Nature Communications, 7. doi:10.1038/ncomms12357
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  Two-dimensional (2D) materials have emerged as promising candidates for various optoelectronic applications based on their diverse electronic properties, ranging from insulating to superconducting. However, cooperative phenomena such as ferroelectricity in the 2D limit have not been well explored. Here, we report room-temperature ferroelectricity in 2D CuInP2 S6 (CIPS) with a transition temperature of-1/4320 K. Switchable polarization is observed in thin CIPS of-1/44 nm. To demonstrate the potential of this 2D ferroelectric material, we prepare a van der Waals (vdW) ferroelectric diode formed by CIPS/Si heterostructure, which shows good memory behaviour with on/off ratio of-1/4100. The addition of ferroelectricity to the 2D family opens up possibilities for numerous novel applications, including sensors, actuators, non-volatile memory devices, and various vdW heterostructures based on 2D ferroelectricity.
• Rivera, P., Seyler, K., Yu, H., Schaibley, J., Yan, J., Mandrus, D., Yao, W., & Xu, X. (2016). Valley-polarized exciton dynamics in a 2D semiconductor heterostructure. Science, 351(6274). doi:10.1126/science.aac7820
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  Heterostructures comprising different monolayer semiconductors provide an attractive setting for fundamental science and device technologies, such as in the emerging field of valleytronics. We realized valley-specific interlayer excitons in monolayer WSe2-MoSe2 vertical heterostructures. We created interlayer exciton spin-valley polarization by means of circularly polarized optical pumping and determined a valley lifetime of 40 nanoseconds. This long-lived polarization enables the visualization of the expansion of a valley-polarized exciton cloud over several micrometers. The spatial pattern of the polarization evolves into a ring with increasing exciton density, a manifestation of valley exciton exchange interactions. Our work introduces van der Waals heterostructures as a promising platform from which to study valley exciton physics.
• Schaibley, J. R., Yu, H., Clark, G., Rivera, P., Ross, J. S., Seyler, K. L., Yao, W., & Xu, X. (2016). Valleytronics in 2D materials. Nat Rev Mater, 1(11), 1-15.
• Schaibley, J., Rivera, P., Yu, H., Seyler, K., Yan, J., Mandrus, D., Taniguchi, T., Watanabe, K., Yao, W., & Xu, X. (2016). Directional interlayer spin-valley transfer in two-dimensional heterostructures. Nature Communications, 7. doi:10.1038/ncomms13747
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  Van der Waals heterostructures formed by two different monolayer semiconductors have emerged as a promising platform for new optoelectronic and spin/valleytronic applications. In addition to its atomically thin nature, a two-dimensional semiconductor heterostructure is distinct from its three-dimensional counterparts due to the unique coupled spin-valley physics of its constituent monolayers. Here, we report the direct observation that an optically generated spin-valley polarization in one monolayer can be transferred between layers of a two-dimensional MoSe2-WSe2 heterostructure. Using non-degenerate optical circular dichroism spectroscopy, we show that charge transfer between two monolayers conserves spin-valley polarization and is only weakly dependent on the twist angle between layers. Our work points to a new spin-valley pumping scheme in nanoscale devices, provides a fundamental understanding of spin-valley transfer across the two-dimensional interface, and shows the potential use of two-dimensional semiconductors as a spin-valley generator in two-dimensional spin/valleytronic devices for storing and processing information.
• Rivera, P., Schaibley, J., Jones, A., Ross, J., Wu, S., Aivazian, G., Klement, P., Seyler, K., Clark, G., Ghimire, N., Yan, J., Mandrus, D., Yao, W., & Xu, X. (2015). Observation of long-lived interlayer excitons in monolayer MoSe 2-WSe 2 heterostructures. Nature Communications, 6. doi:10.1038/ncomms7242
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  Van der Waals bound heterostructures constructed with two-dimensional materials, such as graphene, boron nitride and transition metal dichalcogenides, have sparked wide interest in device physics and technologies at the two-dimensional limit. One highly coveted heterostructure is that of differing monolayer transition metal dichalcogenides with type-II band alignment, with bound electrons and holes localized in individual monolayers, that is, interlayer excitons. Here, we report the observation of interlayer excitons in monolayer MoSe 2-WSe 2 heterostructures by photoluminescence and photoluminescence excitation spectroscopy. We find that their energy and luminescence intensity are highly tunable by an applied vertical gate voltage. Moreover, we measure an interlayer exciton lifetime of ∼1.8‰ns, an order of magnitude longer than intralayer excitons in monolayers. Our work demonstrates optical pumping of interlayer electric polarization, which may provoke further exploration of interlayer exciton condensation, as well as new applications in two-dimensional lasers, light-emitting diodes and photovoltaic devices.
• Seyler, K., Schaibley, J., Gong, P., Rivera, P., Jones, A., Wu, S., Yan, J., Mandrus, D., Yao, W., & Xu, X. (2015). Electrical control of second-harmonic generation in a WSe2 monolayer transistor. Nature Nanotechnology, 10(5). doi:10.1038/nnano.2015.73
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  Nonlinear optical frequency conversion, in which optical fields interact with a nonlinear medium to produce new field frequencies1, is ubiquitous in modern photonic systems. However, the nonlinear electric susceptibilities that give rise to such phenomena are often challenging to tune in a given material and, so far, dynamical control of optical nonlinearities remains confined to research laboratories as a spectroscopic tool2. Here, we report a mechanism to electrically control second-order optical nonlinearities in monolayer WSe2, an atomically thin semiconductor. We show that the intensity of second-harmonic generation at the A-exciton resonance is tunable by over an order of magnitude at low temperature and nearly a factor of four at room temperature through electrostatic doping in a field-effect transistor. Such tunability arises from the strong exciton charging effects in monolayer semiconductors3,4, which allow for exceptional control over the oscillator strengths at the exciton and trion resonances. The exciton-enhanced second-harmonic generation is counter-circularly polarized to the excitation laser due to the combination of the two-photon and one-photon valley selection rules5-8, which have opposite helicity in the monolayer. Our study paves the way towards a new platform for chip-scale, electrically tunable nonlinear optical devices based on two-dimensional semiconductors.
• Wang, X., Jones, A., Seyler, K., Tran, V., Jia, Y., Zhao, H., Wang, H., Yang, L., Xu, X., & Xia, F. (2015). Highly anisotropic and robust excitons in monolayer black phosphorus. Nature Nanotechnology, 10(6). doi:10.1038/nnano.2015.71
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  Semi-metallic graphene and semiconducting monolayer transition-metal dichalcogenides are the most intensively studied two-dimensional materials of recent years. Lately, black phosphorus has emerged as a promising new two-dimensional material due to its widely tunable and direct bandgap, high carrier mobility and remarkable in-plane anisotropic electrical, optical and phonon properties. However, current progress is primarily limited to its thin-film form. Here, we reveal highly anisotropic and strongly bound excitons in monolayer black phosphorus using polarization-resolved photoluminescence measurements at room temperature. We show that, regardless of the excitation laser polarization, the emitted light from the monolayer is linearly polarized along the light effective mass direction and centres around 1.3 eV, a clear signature of emission from highly anisotropic bright excitons. Moreover, photoluminescence excitation spectroscopy suggests a quasiparticle bandgap of 2.2eV, from which we estimate an exciton binding energy of ∼ 0.9eV, consistent with theoretical results based on first principles. The experimental observation of highly anisotropic, bright excitons with large binding energy not only opens avenues for the future explorations of many-electron physics in this unusual two-dimensional material, but also suggests its promising future in optoelectronic devices.
• Hyun, B., Choi, J., Seyler, K., Hanrath, T., & Wise, F. (2013). Heterojunction pbs nanocrystal solar cells with oxide charge-transport layers. ACS Nano, 7(12). doi:10.1021/nn404457c
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  Oxides are commonly employed as electron-transport layers in optoelectronic devices based on semiconductor nanocrystals, but are relatively rare as hole-transport layers. We report studies of NiO hole-transport layers in PbS nanocrystal photovoltaic structures. Transient fluorescence experiments are used to verify the relevant energy levels for hole transfer. On the basis of these results, planar heterojunction devices with ZnO as the photoanode and NiO as the photocathode were fabricated and characterized. Solution-processed devices were used to systematically study the dependence on nanocrystal size and achieve conversion efficiency as high as 2.5%. Optical modeling indicates that optimum performance should be obtained with thinner oxide layers than can be produced reliably by solution casting. Roomerature sputtering allows deposition of oxide layers as thin as 10 nm, which enables optimization of device performance with respect to the thickness of the charge-transport layers. The best devices achieve an open-circuit voltage of 0.72 V and efficiency of 5.3% while eliminating most organic material from the structure and being compatible with tandem structures. © 2013 American Chemical Society.