Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session L12: 2D Materials (Metals, Superconductors, and Correlated Materials) -- TMDC TheoryFocus
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Sponsoring Units: DMP DCOMP Chair: Ting Cao, University of California, Berkeley Room: BCEC 153A |
Wednesday, March 6, 2019 11:15AM - 11:51AM |
L12.00001: Transport, Interactions, and Flavortronics in 2D Transition Metal Dichalcogenides Invited Speaker: Fan Zhang Transition metal dichalcogenides (TMDs) provide a unique 2D material platform for discovering novel physics. First, I will briefly discuss our original predictions on the unconventional quantum Hall effect and “topological” valley Hall effect of massive Dirac fermions in monolayer TMDs, which have been confirmed by optical spectroscopy, quantum transport, and local compressibility measurements. Next, I will introduce our recent theories and experiments that have discovered the G-valley holes and Q-valley electrons in the quantum transport of few-layer TMDs. Particularly, the G-valley holes have an extremely large effective mass that produces an odd-integer predominated quantum Hall effect with giant spin susceptibility and extreme density sensitivity, prerequisites for the Wigner crystal phase. Finally, I will elucidate that the TMD Q valleys offer an unprecedented opportunity to realize the solid-state version of SU(3) flavor symmetry that is rare in electron systems. In the quantum-Hall regime, we have predicted that the spontaneous flavor symmetry breaking yields ferroelectric valley nematics tunable by an in-plane electric field. In the quantum-dot geometry, we have predicted flavor enforced irrational Coulomb peaks and fractional Kondo peaks. These effects lead to a new concept–flavortronics. |
Wednesday, March 6, 2019 11:51AM - 12:03PM |
L12.00002: WITHDRAWN ABSTRACT
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Wednesday, March 6, 2019 12:03PM - 12:15PM |
L12.00003: Giant spin Hall effect in two-dimensional monochalcogenides Jagoda Slawinska, Frank T Cerasoli, Haihang Wang, Sara Postorino, Andrew Supka, Stefano Curtarolo, Marco Fornari, Marco Buongiorno Nardelli One of the most exciting properties of two dimensional materials is their sensitivity to external tuning of the electronic properties, for example via electric field or strain. Recently discovered analogues of phosphorene, group-IV monochalcogenides (MX with M = Ge, Sn and X = S, Se, Te), display several interesting phenomena related to the in-plane strain, such as giant piezoelectricity and multiferroicity, which combines ferroelastic and ferroelectric properties. Here, using calculations from first principles, we predict for the first time giant spin Hall effect (SHE) in these materials which suggests their high potential for 2D spintronics. We reveal that the spin Hall conductivity is tunable via combinations of external strain and doping. In some configurations, the strain-induced semiconductor to metal transition enables the logic functionality of switch on/off control of spin currents indicating a new route for the design of multi-tunable spintronics devices. |
Wednesday, March 6, 2019 12:15PM - 12:27PM |
L12.00004: Evolution of Weyl fermions along the polarity reversal paths in MoTe2 Sobhit Singh, Jinwoong Kim, Karin Rabe, David Vanderbilt MoTe2 is a layered material with rich physics arising from structural and electronic phase competition. The three observed crystal phases - 2H, 1T′, and Td - have distinct electronic properties. The non-centrosymmetric Td -MoTe2 is particularly fascinating due to the presence of type-I and type-II Weyl nodes in this system. In this study, we investigate the energetics of the possible structural phase transitions in MoTe2, and explore the possibility of controlling the dynamics of Weyl fermions in MoTe2. Our first-principles calculations reveal that one can systematically tune the location and chirality of Weyl nodes by exploiting the connection between polar distortions and spin-orbit coupling effects in Td - MoTe2. By restoring the broken inversion symmetry, we locate a highly symmetric saddle point structure (Td0) on the energy landscape of MoTe2, establishing a polarity reversal path connecting up and down variants of Td. The Td0 phase resembles a paraelectric phase of a ferroelectric compound, and it is dynamically and elastically unstable leading to structural phase transitions into the Td and 1T′ phases. We study the evolution of Weyl nodes as a function of structural distortions in the vicinity of Td0. |
Wednesday, March 6, 2019 12:27PM - 12:39PM |
L12.00005: Statistical mechanics of anisotropic 2D sheets Mohamed El Hedi Bahri, Andrej Kosmrlj Atomically thin 2D sheets are now routinely produced and are used in a variety of electronic applications as well as in self-folding origami structures. The majority of 2D sheets, such as graphene, hexagonal boron nitride and the H phase of Transition Metal Dichalcogenides (TMDs), are isotropic. However, there also exist anisotropic 2D sheets, such as the T’ phase of TMDs. Note that in equilibrium most are in the isotropic H phase, however the T’ phase is stable for WTe2. This motivated us to investigate the statistical mechanics of freely suspended anisotropic 2D sheets. Similar to isotropic sheets, thermal fluctuations effectively renormalize elastic constants for anisotropic sheets and make them scale dependent. Thermal fluctuations effectively increase flexural rigidities, while the in-plane elastic constants are reduced. We found 3 different universality classes (isotropic, orthorombic and monoclinic) that are characterized with different power-law exponents of the renormalized elastic constants. The T’ phase of TMDs falls in the orthorhombic universality class. Therefore, the elastic constants for WTe2 are expected to scale differently than for other isotropic 2D sheets. |
Wednesday, March 6, 2019 12:39PM - 12:51PM |
L12.00006: Dynamical synchronization transition in interacting electron systems Tanay Nag Synchronization processes are a ubiquitous phenomenon in nature. We propose a new perspective centered around the topic of a novelly introduced dynamical synchronization transition (DST) in interacting electron systems. In particular, using graphene irradiated by an intense bi-circular pulse laser as a prototypical and experimental viable example, we theoretically investigate how to selectively generate a coherent |
Wednesday, March 6, 2019 12:51PM - 1:03PM |
L12.00007: Symmetry, spin-texture, and tunable quantum geometry in WTe2 monolayer Li-kun Shi, Justin Song The spin orientation of electronic wavefunctions in crystals is an internal degree of freedom, typically insensitive to electrical knobs. We argue from a general symmetry analysis and a k.p perspective, that monolayer 1T'-WTe2 possesses an electrically tunable bulk band quantum geometry arising from a gate-activated canted spin texture. In particular, we find that due to its out-of-plane asymmetry, an applied out-of-plane electric field breaks inversion symmetry to induce both in-plane and out-of-plane electric dipoles. These in-turn generate spin-orbit coupling to lift the spin degeneracy and enable a bulk band Berry curvature and magnetic moment distribution to develop. Further, due to its low symmetry, Berry curvature and magnetic moment in 1T'-WTe2 possess a dipolar distribution in momentum space, and can lead to unconventional effects such as a current induced magnetization and quantum non-linear anomalous Hall effect. These render 1T'-WTe2 a rich two-dimensional platform for all-electrical control over quantum geometric effects. |
Wednesday, March 6, 2019 1:03PM - 1:15PM |
L12.00008: Edge photocurrent response of type-II Weyl semimetal WTe2 Qinsheng Wang, Jin Cao, Jingchuan Zheng, Yuan He, Dong Sun, Yugui Yao Photodetectors based on new materials or new structures are of great potentials to promote the performance limits of existing photo detection devices. As topological nontrivial materials, Weyl semimetals were reported to have novel optoelectronic properties and potential applications arising from their gapless linear dispersion near Weyl nodes, berry curvature divergence of Weyl nodes, internal broken of inversion or time reversal symmetry, robust fermi-arc type surface states. These unique properties lead to extraordinary optoelectronic response of Weyl semimetals. Here, using scanning photocurrent microscopy, we demonstrate that robust photocurrent will generate on edges with certain crystal direction in type-II Weyl semimetal WTe2, this photocurrent persists under a wide excitation photon energy range from near Weyl point region to high above Weyl point region. We show that the direction of photocurrent did not change with the polarization of excitation photon and a photon energy range from 0.12 eV to 1.96 eV, this edge photocurrent was determined by the symmetry of edges and may come from the nontrivial Fermi-arc type edge state of Weyl semimetal, and may result in a new type of photodetector based on it. |
Wednesday, March 6, 2019 1:15PM - 1:27PM |
L12.00009: Quantum Mechanical Calculations of the Dielectric Properties of Metallic 2D Transition Metal Dichalcogenides Xiao Shen 2D materials are widely studied for their optical properties due to their reduced dimensionality. The key optical properties are reflectivity and absorption coefficients, which can be determined from the complex dielectric function obtained from first-principles quantum mechanical calculations based on time-dependent current-density functional theory (TDCDFT) [1]. Here we focus on the metallic phases of 2D transition metal dichalcogenides (TMD), including MoS2, MoSe2, and WSe2. We reveal their unique optical properties and discuss how the optical properties change upon the change of the chemical composition. |
Wednesday, March 6, 2019 1:27PM - 1:39PM |
L12.00010: First-Principles Prediction of Stable Transition Metal Dichalcogenide Alloys John Cavin, Sung Beom Cho, Rohan Mishra Quasibinary alloying among pairs of 2-dimensional (2D) transition metal dichalcogenides (TMDCs) is an attractive method for tuning properties for applications such as optoelectronics and catalysis. Of the many possible combinations of TMDCs, the small subset of semiconducting alloys have garnered widespread attention. Outside this limited subset, the synthesizability of alloys remains largely unknown. In order to the guide synthesis of such alloys, we present ab initio calculations of equilibrium phase diagrams with regions of stability for 27 TMDC alloys: M1-x M'xX2 and MX2(1-x) X'2x (M,M'= V, Nb, Ta, Mo, or W; X,X'= S or Se) and two heterostructural alloys, Mo1-x WxTe2 and WS2(1-x)Te2x. We predict four new alloys that are miscible at all temperatures: Nb1-xTaxS2, Nb1-xTaxSe2, VS2(1-x) Se2x, and TaS2(1-x) Se2x. For the rest of the alloys, we present a qualitative analysis of synthesizability based on miscibility temperatures. We relate TMDC size mismatch with a mixing asymmetry that indicates the synthesizability of low concentration alloys. Our results open new compositional spaces that can be used to design and synthesize TMDC alloys for various applications. |
Wednesday, March 6, 2019 1:39PM - 1:51PM |
L12.00011: Unified microscopic description of the origin and interplay between CDW and superconducting order in TiSe2 Vitor Pereira, Chuan Chen, Lei Su, Bahadur Singh, Antonio Helio Castro Neto, Hsin Lin We theoretically address the striking interplay between charge density wave (CDW) order and superconductivity (SC) in 1T-TiSe2, whose microscopic details are under active scrutiny. Starting with undoped TiSe2, the commensurate CDW instability is captured microscopically by the mechanism of excitonic condensation involving interactions among the overlapping electron and hole pockets characteristic of this material. Fixing the only parameter in the theory to the undoped state, the predicted reduction of Tcdw with doping follows the experimental curve without further fitting. A Ginzburg-Landau theory further shows that the transition to the incommensurate CDW phase as a function of doping takes place via an intervening near-commensurate regime, characterized by the proliferation of discommensurations. As these are simply fluctuations of the CDW, we investigated the spectrum of fluctuations predicted by the microscopic model and found they induce s-wave pairing among the quasiparticles of the excitonic condensate. The predicted SC transition temperature as a function of doping is dome-shapped, precisely as in the experiments, and our estimated Tsc is in satisfactory agreement. We are thus able to capture the full CDW and SC phase diagram within a single microscopic picture. |
Wednesday, March 6, 2019 1:51PM - 2:03PM |
L12.00012: Nonreciprocal electrical transport in van der Waals crystals Toshiya Ideue, Yoshihiro Iwasa Nonreciprocal electrical transport, which is the second order nonlinear electrical transport in noncentrosymmetric crystals, represents the rectification effect originating from lattice symmetry breaking. Recently, nonreciprocal electrical transport has been observed in a variety of van der Waals materials without special inversion symmetry[1-3], including electric-field-induced superconductors. In this presentation, we review the nonreciprocal electrical transport in van der Waals crystals. Characteristic behavior and microscopic mechanism of nonreciprocal electrical transport will be also discussed. |
Wednesday, March 6, 2019 2:03PM - 2:15PM |
L12.00013: The shear sound of 2D metals Jun Khoo, Inti Sodemann Classical liquids have only longitudinal sound waves, in contrast to classical solids which also have transverse sound. Fermi liquids, however, can differ dramatically from classical liquids by developing a sharp transverse sound mode outside of the particle-hole continuum for sufficiently strong interactions. This shear sound mode is unaltered by the presence of Coulomb interactions and might be present in a variety of strongly interacting metals, in particular two-dimensional metals in which the quasiparticle mass is renormalised to be about twice the bare mass. We study this mode within the bosonization description of Landau Fermi liquids by solving for the entire spectrum of coherent and incoherent excitations of a Fermi liquid with non-zero F_0 and F_1 Landau parameters. This is accomplished by mapping the kinetic equation into a 1D non-hermitian tight binding model. We will also discuss potential routes to excite and detect this mode in experiments. |
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