Publications

2024
Lujan, D. ; Choe, J. ; Chaudhary, S. ; Ye, G. ; Nnokwe, C. ; Rodriguez-Vega, M. ; He, J. ; Gao, F. Y. ; Nunley, T. N. ; Baldini, E. ; et al. Spin–orbit exciton–induced phonon chirality in a quantum magnet. Proceedings of the National Academy of SciencesProceedings of the National Academy of Sciences 121, e2304360121. Publisher's VersionAbstract
The interplay of charge, spin, lattice, and orbital degrees of freedom in correlated materials often leads to rich and exotic properties. Recent studies have brought new perspectives to bosonic collective excitations in correlated materials. For example, inelastic neutron scattering experiments revealed non-trivial band topology for magnons and spin?orbit excitons (SOEs) in a quantum magnet CoTiO3 (CTO). Here, we report phonon properties resulting from a combination of strong spin?orbit coupling, large crystal field splitting, and trigonal distortion in CTO. Specifically, the interaction between SOEs and phonons endows chirality to two Eg phonon modes and leads to large phonon magnetic moments observed in magneto-Raman spectra. The remarkably strong magneto-phononic effect originates from the hybridization of SOEs and phonons due to their close energy proximity. While chiral phonons have been associated with electronic topology in some materials, our work suggests opportunities may arise by exploring chiral phonons coupled to topological bosons.The interplay of charge, spin, lattice, and orbital degrees of freedom in correlated materials often leads to rich and exotic properties. Recent studies have brought new perspectives to bosonic collective excitations in correlated materials. For example, inelastic neutron scattering experiments revealed non-trivial band topology for magnons and spin?orbit excitons (SOEs) in a quantum magnet CoTiO3 (CTO). Here, we report phonon properties resulting from a combination of strong spin?orbit coupling, large crystal field splitting, and trigonal distortion in CTO. Specifically, the interaction between SOEs and phonons endows chirality to two Eg phonon modes and leads to large phonon magnetic moments observed in magneto-Raman spectra. The remarkably strong magneto-phononic effect originates from the hybridization of SOEs and phonons due to their close energy proximity. While chiral phonons have been associated with electronic topology in some materials, our work suggests opportunities may arise by exploring chiral phonons coupled to topological bosons.
De Palma, A. C. ; Peng, X. ; Arash, S. ; Gao, F. Y. ; Baldini, E. ; Li, X. ; Yu, E. T. Elucidating Piezoelectricity and Strain in Monolayer MoS2 at the Nanoscale Using Kelvin Probe Force Microscopy. Nano Letters 24, 1835 - 1842. Publisher's VersionAbstract
Strain engineering modifies the optical and electronic properties of atomically thin transition metal dichalcogenides. Highly inhomogeneous strain distributions in two-dimensional materials can be easily realized, enabling control of properties on the nanoscale; however, methods for probing strain on the nanoscale remain challenging. In this work, we characterize inhomogeneously strained monolayer MoS2 via Kelvin probe force microscopy and electrostatic gating, isolating the contributions of strain from other electrostatic effects and enabling the measurement of all components of the two-dimensional strain tensor on length scales less than 100 nm. The combination of these methods is used to calculate the spatial distribution of the electrostatic potential resulting from piezoelectricity, presenting a powerful way to characterize inhomogeneous strain and piezoelectricity that can be extended toward a variety of 2D materials.Strain engineering modifies the optical and electronic properties of atomically thin transition metal dichalcogenides. Highly inhomogeneous strain distributions in two-dimensional materials can be easily realized, enabling control of properties on the nanoscale; however, methods for probing strain on the nanoscale remain challenging. In this work, we characterize inhomogeneously strained monolayer MoS2 via Kelvin probe force microscopy and electrostatic gating, isolating the contributions of strain from other electrostatic effects and enabling the measurement of all components of the two-dimensional strain tensor on length scales less than 100 nm. The combination of these methods is used to calculate the spatial distribution of the electrostatic potential resulting from piezoelectricity, presenting a powerful way to characterize inhomogeneous strain and piezoelectricity that can be extended toward a variety of 2D materials.
Li, Y. ; Zhang, F. ; Ha, V. - A. ; Lin, Y. - C. ; Dong, C. ; Gao, Q. ; Liu, Z. ; Liu, X. ; Ryu, S. H. ; Kim, H. ; et al. Tuning commensurability in twisted van der Waals bilayers. 625, 494 - 499. Publisher's VersionAbstract
Moiré superlattices based on van der Waals bilayers1–4 created at small twist angles lead to a long wavelength pattern with approximate translational symmetry. At large twist angles (θt), moiré patterns are, in general, incommensurate except for a few discrete angles. Here we show that large-angle twisted bilayers offer distinctly different platforms. More specifically, by using twisted tungsten diselenide bilayers, we create the incommensurate dodecagon quasicrystals at θt = 30° and the commensurate moiré crystals at θt = 21.8° and 38.2°. Valley-resolved scanning tunnelling spectroscopy shows disparate behaviours between moiré crystals (with translational symmetry) and quasicrystals (with broken translational symmetry). In particular, the K valley shows rich electronic structures exemplified by the formation of mini-gaps near the valence band maximum. These discoveries demonstrate that bilayers with large twist angles offer a design platform to explore moiré physics beyond those formed with small twist angles.
2023
Quan, J. ; Chen, G. ; Linhart, L. ; Liu, Z. ; Taniguchi, T. ; Watanabe, K. ; Libisch, F. ; Huang, R. ; Li, X. Quantifying Strain in Moiré Superlattice. Nano Letters 23, 11510 - 11516. Publisher's VersionAbstract
In twisted van der Waals (vdW) bilayers, intrinsic strain associated with the moiré superlattice and unintentionally introduced uniaxial strain may be present simultaneously. Both strains are able to lift the degeneracy of the E2g phonon modes in Raman spectra. Because of the different rotation symmetry of the two types of strain, the corresponding Raman intensity exhibits a distinct polarization dependence. We compare a 2.5° twisted MoS2 bilayer, in which the maximal intrinsic moiré strain is anticipated, and a natural MoS2 bilayer with an intentionally introduced uniaxial strain. By analyzing the frequency shift of the E2g doublet and their polarization dependence, we can not only determine the direction of unintentional uniaxial strain in the twisted bilayer but also quantify both strain components. This simple strain characterization method based on far-field Raman spectra will facilitate the studies of electronic properties of moiré superlattices under the influence of combined intrinsic and external strains.In twisted van der Waals (vdW) bilayers, intrinsic strain associated with the moiré superlattice and unintentionally introduced uniaxial strain may be present simultaneously. Both strains are able to lift the degeneracy of the E2g phonon modes in Raman spectra. Because of the different rotation symmetry of the two types of strain, the corresponding Raman intensity exhibits a distinct polarization dependence. We compare a 2.5° twisted MoS2 bilayer, in which the maximal intrinsic moiré strain is anticipated, and a natural MoS2 bilayer with an intentionally introduced uniaxial strain. By analyzing the frequency shift of the E2g doublet and their polarization dependence, we can not only determine the direction of unintentional uniaxial strain in the twisted bilayer but also quantify both strain components. This simple strain characterization method based on far-field Raman spectra will facilitate the studies of electronic properties of moiré superlattices under the influence of combined intrinsic and external strains.
Electrostatic moiré potential from twisted hexagonal boron nitride layers
Kim, D. S. ; Dominguez, R. C. ; Mayorga-Luna, R. ; Ye, D. ; Embley, J. ; Tan, T. ; Ni, Y. ; Liu, Z. ; Ford, M. ; Gao, F. Y. ; et al. Electrostatic moiré potential from twisted hexagonal boron nitride layers. Nature Materials 1–6.
Kim, D. S. ; Huang, D. ; Guo, C. ; Li, K. ; Rocca, D. ; Gao, F. Y. ; Choe, J. ; Lujan, D. ; Wu, T. - H. ; Lin, K. - H. ; et al. Anisotropic Excitons Reveal Local Spin Chain Directions in a van der Waals Antiferromagnet. Advanced Materials 2206585.
Choi, J. ; Embley, J. ; Blach, D. D. ; Perea-Causín, R. ; Erkensten, D. ; Kim, D. S. ; Yuan, L. ; Yoon, W. Y. ; Taniguchi, T. ; Watanabe, K. ; et al. Fermi Pressure and Coulomb Repulsion Driven Rapid Hot Plasma Expansion in a van der Waals Heterostructure. Nano Letters.
Low resistance electrical contacts to few-layered MoS2 by local pressurization
Manzanares-Negro, Y. ; Quan, J. ; Rassekh, M. ; Moaied, M. ; Li, X. ; Ares, P. ; Palacios, J. J. ; Gomez-Herrero, J. ; Gomez-Navarro, C. Low resistance electrical contacts to few-layered MoS2 by local pressurization. 2D Materials 10, 021003.
Quantum Dynamics of Attractive and Repulsive Polarons in a Doped MoSe 2 Monolayer
Huang, D. ; Sampson, K. ; Ni, Y. ; Liu, Z. ; Liang, D. ; Watanabe, K. ; Taniguchi, T. ; Li, H. ; Martin, E. ; Levinsen, J. ; et al. Quantum Dynamics of Attractive and Repulsive Polarons in a Doped MoSe 2 Monolayer. Physical Review X 13, 011029.
2022
Lee, W. ; Wang, Y. ; Qin, W. ; Kim, H. ; Liu, M. ; Nunley, N. T. ; Fang, B. ; Maniyara, R. ; Dong, C. ; Robinson, J. A. ; et al. Confined Monolayer Ag As a Large Gap 2D Semiconductor and Its Momentum Resolved Excited States. Nano letters 22, 7841–7847.
Wang, J. ; Ahmadi, Z. ; Lujan, D. ; Choe, J. ; Taniguchi, T. ; Watanabe, K. ; Li, X. ; Shield, J. E. ; Hong, X. Physical Vapor Transport Growth of Antiferromagnetic CrCl3 Flakes Down to Monolayer Thickness. Advanced Science 2203548.
Kim, S. ; Moon, D. ; Jeon, B. R. ; Yeon, J. ; Li, X. ; Kim, S. Accurate Atomic-Scale Imaging of Two-Dimensional Lattices Using Atomic Force Microscopy in Ambient Conditions. Nanomaterials 12, 1542.
Li, X. ; Liu, Z. ; Liu, Y. ; Karki, S. ; Li, X. ; Akinwande, D. ; Incorvia, J. A. C. All Electrical Control and Temperature Dependence of the Spin and Valley Hall Effect in Monolayer WSe2 Transistors. ACS Appl. Electron. Mater. 4 3930–3937.
Zhu, L. ; Zhu, L. ; Ma, X. ; Li, X. ; Buhrman, R. A. Critical role of orbital hybridization in the Dzyaloshinskii-Moriya interaction of magnetic interfaces. Communications Physics 5 1–7.
Huang, D. ; Choi, J. ; Shih, C. - K. ; Li, X. Excitons in semiconductor moiré superlattices. Nature Nanotechnology 17, 227.
Lujan, D. ; Choe, J. ; Rodriguez-Vega, M. ; Ye, Z. ; Leonardo, A. ; Nunley, N. T. ; Chang, L. - J. ; Lee, S. - F. ; Yan, J. ; Fiete, G. A. ; et al. Magnons and magnetic fluctuations in atomically thin MnBi2Te4. Nature Communications 13, 1–7.
Nunley, N. T. ; Guo, S. ; Chang, L. - J. ; Lujan, D. ; Choe, J. ; Lee, S. - F. ; Yang, F. ; Li, X. Quantifying spin Hall topological Hall effect in ultrathin Tm 3 Fe 5 O 12/Pt bilayers. Physical Review B 106, 014415.
2021
Fang, J. ; Wang, M. ; Yao, K. ; Zhang, T. ; Krasnok, A. ; Jiang, T. ; Choi, J. ; Kahn, E. ; Korgel, B. A. ; Terrones, M. ; et al. Directional modulation of exciton emission using single dielectric nanospheres. Advanced Materials 33, 2007236.
Helmrich, S. ; Sampson, K. ; Huang, D. ; Selig, M. ; Hao, K. ; Tran, K. ; Achstein, A. ; Young, C. ; Knorr, A. ; Malic, E. ; et al. Phonon-Assisted Intervalley Scattering Determines Ultrafast Exciton Dynamics in MoSe 2 Bilayers. Physical Review Letters 127, 157403.
Ma, X. ; Zhang, F. ; Chu, Z. ; Hao, J. ; Chen, X. ; Quan, J. ; Huang, Z. ; Wang, X. ; Li, X. ; Yan, Y. ; et al. Superior photo-carrier diffusion dynamics in organic-inorganic hybrid perovskites revealed by spatiotemporal conductivity imaging. Nature communications 12, 1–7.

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