Bulletin of the American Physical Society
APS March Meeting 2021
Volume 66, Number 1
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session M57: 2D Materials: Metals, Superconductors, and Correlated Materials - 2Focus Live
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Sponsoring Units: DMP Chair: Juan-Jose Lietor-Santos, APS |
Wednesday, March 17, 2021 11:30AM - 12:06PM Live |
M57.00001: THz Emission Spectroscopy of Surface Photogalvanic Effects in the Chiral Weyl Semimetal RhSi Invited Speaker: Darius Torchinsky One of the signature characteristics of Weyl semimetals is the topologically protected open Fermi arc surface states that connect surface projections of pairs of opposite parity bulk Weyl points. Although photoemission has confirmed the existence of these exotic states, investigation of their electronic and optical properties is complicated by the overwhelming response from the bulk. This provides a unique opportunity to second order nonlinear optical spectroscopies, such as the linear and circular photogalvanic effects, that can selectively couple to surface responses when the bulk response is forbidden by lattice symmetry. In this talk, we will describe our measurements of these laser driven currents on the Fermi arc surface states of the chiral Weyl semimetal RhSi, which we measure through the THz radiation these impulsively driven currents emit. While our data broadly match the theoretical prediction for the amplitude of the current as a function of photon energy, we will discuss our observations of a surprising departure from theory on the direction that this current flows, instead being reminiscent of a higher degree of symmetry than permitted by the surface. |
Wednesday, March 17, 2021 12:06PM - 12:18PM Live |
M57.00002: Giant non-linear anomalous Hall effect in three-dimensional Td -MoTe2 Archana Tiwari, Fangchu Chen, Shazhou Zhong, Elizabeth Drueke, Jahyun Koo, Austin R Kaczmarek, Cong Xiao, Jingjing Gao, Xuan Luo, Qian Niu, Yuping Sun, Binghai Yan, Liuyan Zhao, Adam W Tsen While the anomalous Hall effect can manifest even without an external magnetic field, time reversal symmetry is nonetheless still broken by the internal magnetization of the sample. Recently, it has been shown that certain materials without an inversion center allow for a nonlinear type of anomalous Hall effect whilst retaining time reversal symmetry. Here, the application of a harmonic longitudinal current or voltage generates a second harmonic component in the transverse direction. The effect may arise from either Berry curvature properties intrinsic to the material’s band structure or through various asymmetric scattering mechanisms. In this talk, I will be presenting our observation of an extremely large c-axis nonlinear anomalous Hall effect in the non-centrosymmetric Td phase of MoTe2 without intrinsic magnetic order, whose strength obeys a universal scaling with sample conductivity. |
Wednesday, March 17, 2021 12:18PM - 12:30PM Live |
M57.00003: Microscopic Theory of Plasmons in Substrate-supported Borophene Anubhab Haldar, Cristian Cortes, Pierre Darancet, Sahar Sharifzadeh Two-dimensional boron, or borophene, is a metallic monolayer material that hosts low-loss, high-confinement, and visible light plasmons. In contrast with graphene, borophene cannot be exfoliated and has been synthesized on a variety of metallic substrates. Here, we present first-principles density functional theory calculations of the dielectric and plasmonic properties of borophene grown on Ag(111). We systematically investigate the linear response and the momentum-dependent polarizability of borophene as a function of its proximity to the metallic substrate. Our calculations indicate that the plasmons in borophene are quenched by the substrate, which we explain via a simple electrodynamic model of coupled polarizabilities between the monolayer and substrate. Using this model, we predict that the substrate plasmon frequency can be tuned to minimize the quenching of the monolayer plasmons. |
Wednesday, March 17, 2021 12:30PM - 12:42PM Live |
M57.00004: Phase-Dependent Band Gap Engineering in Alloys of Metal-Semiconductor Transition Metal Dichalcogenides Shuxi Wang, John Cavin, Zahra Hemmat, Khagesh Kumar, Alexander Ruckel, Leily Majidi, Hamed Gholivand, Radwa Dawood, Jordi Cabana, Nathan P Guisinger, Robert F Klie, Fatemeh Khalili-Araghi, Rohan Mishra, Amin Salehi-Khojin Bandgap engineering plays a critical role in optimizing the electrical and optical properties of semiconductors. Alloying can be used to tune band gaps by combining isovalent semiconductors. Here, we present a novel form of bandgap engineering involving alloying non-isovalent cations in a two-dimensional transition metal dichalcogenide [1]. By alloying semiconducting MoSe2 with metallic NbSe2, two structural phases of Mo0.5Nb0.5Se2, 1T and 2H, are produced, each with emergent electronic structure. At room temperature, the 1T and 2H phases are semiconducting and metallic, respectively. Electron diffraction patterns of the 1T structure show the presence of a nearly commensurate charge density wave (NCCDW). Density-functional theory calculations confirm that local distortions open a band gap in 1T-Mo0.5Nb0.5Se2 by facilitating charge transfer. In 2H-Mo0.5Nb0.5Se2, electrical transport measurements show a low-temperature transition to a commensurate CDW state with a bandgap. Our work expands the boundaries of alloy-based band gap engineering by using alloying to access CDW phases. |
Wednesday, March 17, 2021 12:42PM - 12:54PM Live |
M57.00005: Fermiology and Electron-Phonon Coupling in the 2H and 3R polytypes of NbS2 Zakariae El Youbi, Sungwon Jung, Cephise Cacho, Matthew D Watson We revisit the low-energy electronic structure of 2H-NbS2 and 3R-NbS2 combining Angle-Resolved Photoemission Spectroscopy (ARPES) and density functional theory (DFT), demonstrating the metallic ground states of both phases. We focus particularly on the Fermi surfaces, which reveal a considerably smaller size in the 3R phase. We attribute this difference to the additional Nb interstitials, without which this polytype cannot be stabilized, which yet act as electron donors, filling thus more Nb orbitals and resulting a n doped electronic structure. We estimate the bands to be shifted downwards by 250 meV into high binding energies compared to the stoichiometric system and the off-soichiometry to be 3R-Nb(1+x)S2 with x = 0.13. This off-stoichiometry is believed to be responsible for the absence of any instabilities in this phase. The different origin of the band splitting in each phase is carefully explained. Finally, our high-resolution data on the 2H phase reveals that the electron-phonon coupling is highly dependent on the orbital character of the bands, and this variation in electron-phonon coupling naturally links to the two-gap superconductivity. |
Wednesday, March 17, 2021 12:54PM - 1:06PM Live |
M57.00006: Ultrafast dynamics in the high-symmetry and in the charge density wave phase of 2H-NbSe2 Goran Karapetrov, A. A. Anikin, Richard D Schaller, Gary P Wiederrecht, Elena R Margine, Igor Mazin We investigate carrier and collective mode dynamics in 2H-NbSe2 using time-resolved optical pump-probe spectroscopy and compare the results with first-principle calculations. Broadband ultrafast reflectivity studies of 2H-NbSe2 in a wide temperature interval covering the normal, charge density wave (CDW) and superconducting phase were performed. Spectral features observed in the transient reflectivity experiment were associated with specific optical transitions obtained from band structure calculations. Displacive excitation of coherent phonons show CDW-associated coherent oscillations of the soft phonon mode across the whole spectral range. Temperature evolution of this coherent phonon mode in the low-excitation linear regime shows softening of the mode down to the TCDW with subsequent hardening below TCDW. From first principle calculations of electron-phonon coupling we associate the few picosecond electron-phonon relaxation time τ2 with a specific group of phonons with frequencies around 20 meV. On the other hand, the anomalously long relaxation time of τ3~100 ps is associated with anharmonicity-driven phonon-phonon scattering. All time dependent relaxation processes result from anomalies near the second order CDW phase transition. |
Wednesday, March 17, 2021 1:06PM - 1:18PM Live |
M57.00007: Electronically driven anharmonicities in charge-density-wave materials Arne Schobert, Jan Berges, Erik van Loon, Tim Wehling Charge-density waves (CDWs) occupy an important position in the phase diagram of low-dimensional systems such as the transition metal dichalcogenide monolayers. Although a CDW can often be identified already from the undistorted structure in linear response, anharmonic effects are eventually responsible for the stabilization of the distorted phase and its precise properties. To study the mechanisms responsible for these anharmonicities, we calculate Born-Oppenheimer potential energy surfaces for lattice distortions in 1T-TaS2 ( √13 × √13), 1T-VS2 (7 × √3), and 2H-NbSe2 (3 × 3), and we establish a connection to the electronic structure of these materials. |
Wednesday, March 17, 2021 1:18PM - 1:30PM Live |
M57.00008: Ultrafast Electron Diffraction in Charge Density Wave State of TiSe2 Paul Xhori, Anton Andreevich Anikin, Jacob Bolduc, Shalin Patel, Maher Harb, Goran Karapetrov We probe the ultrafast laser response in thin TiSe2 single crystals below and above the CDW transition temperature using ultrafast electron diffraction. The electron-phonon dynamics is initiated by a 150-fs photon pulse centered at 400 nm that induces a strain pulse and inter-layer |
Wednesday, March 17, 2021 1:30PM - 1:42PM Live |
M57.00009: Band hybridisation at the charge density wave transition in monolayer TiTe2 Tommaso Antonelli, Matthew D Watson, Akhil Rajan, Philip King In TiTe2 a charge density wave (CDW) phase emerges at low temperature only in the monolayer (ML) configuration but not in the bulk1. This is in contrast with its sister compound TiSe2 in which a similar lattice instability occurs, largely independent of material thickness2,3. The origin of the CDW transition that is stabilised in monolayer TiTe2 remains unclear, while its semimetallic nature raises questions that challenge the standard pictures of CDW formation in this family. Tracking the electronic structure evolution in ML-TiTe2 across the phase transition using angle-resolved photoemission spectroscopy (ARPES), we have identified spectroscopic signatures of a band hybridisation between the back-folded conduction and valence bands occurring below the critical temperature, which in turn leads to an electronic energy gain for the CDW transition to occur. |
Wednesday, March 17, 2021 1:42PM - 1:54PM Live |
M57.00010: An investigation of the optical properties of the misfit layer compound (PbSe)1+δ(NbSe2)n Paul Respicio, Maureen Reedyk Misfit layer compounds (MLCs) consist of two sublattices that have at least one differing lattice parameter. The incommensurate structure and composite nature enables the simultaneous presence of contradictory properties such as low thermal conductivity alongside high electrical conductivity, making them ideal candidates for thermoelectic materials [1]. The general structure of a MLCs is (MX)1+δ(TX2)n, where MX is a rock salt (M = Sn, Pb, Bi, rare earth; X = S, Se) and TX2 is a transition metal dichalcogenide (T = Ti, V, Nb, Ta, Cr; X = S, Se). The misfit parameter, δ, describes the ratio between the lattice parameters along the crystallographic a axis, while n describes the number of TX2 layers in between MX layers [2]. (PbSe)1+δ(NbSe2)n is a MLC composed of semiconductor (PbSe) and superconducting transition metal dichalcogenide (NbSe2). Single crystal samples were synthesized and characterized by x-ray diffraction and ac-resistivity. The optical reflectance of (PbSe)1+δ(NbSe2)n will be presented with the aim of investigating how the optical properties change as NbSe2 layers are added. |
Wednesday, March 17, 2021 1:54PM - 2:06PM Live |
M57.00011: Nonlinear Anomalous Hall Effect and Nonlinear Photocurrent Responses in 2D Topological Materials Hua Wang, Xiaofeng Qian Noninvasive detection and efficient control of electric and magnetic orders in 2D quantum materials is crucial for the development of 2D quantum electronics. We present our recent theoretical work on nonlinear response and sensing of 2D topological materials [1,2,3]. Few-layer WTe2 holds out-of-plane polarization and can be switched via interlayer sliding under electric field. Ferroicity-driven switching of nonreciprocal nonlinear photocurrent [4] such as ferroelectric nonlinear anomalous Hall effect can be achieved in WTe2 by utilizing intrinsic coupling among susceptibility, symmetry, and quantum geometry of electronic states [1], paving theoretical foundation for nonlinear memory such as Berry curvature memory recently demonstrated in trilayer WTe2 [2]. We discuss nonlinear photocurrent in PT-symmetry magnetic topological quantum materials [3]. Magnetic photocurrent can be magnetically and electrically switched in bilayer AFM MnBi2Te4, highly tunable down to a few THz regime with rich THz and magneto-optoelectronic applications. References: [1] npj Comput. Mater. 5, 119 (2019). [2] Nat. Phys. 16, 1028-1034 (2020). [3] arXiv:2006.13573 (2020). [4] Sci. Adv. 5, eaav9743 (2019). |
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