Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session S35: 2D Materials - Metals, Semiconductors, and Correlated MaterialsFocus
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Sponsoring Units: DMP Chair: Humberto Gutierrez, University of South Florida Room: LACC 409B |
Thursday, March 8, 2018 11:15AM - 11:51AM |
S35.00001: Low-Dimensional Semiconductors beyond Graphene: An Insight from Theory Invited Speaker: David Tomanek If graphene had a band gap, it would probably be the optimum 2D system for electronics applications. Layered transition metal dichalcogenides (TMDs) with a robust intrinsic band gap appear to be the next-best alternative. But TMD applications in electronics suffer from two limitations: the difficulty to construct transparent contacts [1], and the adverse effect of atomic defects especially in systems grown by CVD. Defects such as chalcogen vacancies scatter charge carriers and quench photoluminescence. To remedy these deficiencies, optimum strategies have been devised to make low- resistance, ohmic contacts to TMDs [2] such as MoS2 using similar, epitaxial TMDs. Recent experimental and theoretical [3] studies suggest that exposure of defective MoS2 to sulfur-containing compounds may effectively heal S vacancy and adatom defects. There is rapid progress in 2D systems other than graphene and TMDs, including elemental group V and group VI 2D semiconductors. Alternative coil allotropes with unusual optical and transport properties appear to self-assemble inside cylindrical cavities with few nanometers in diameter [4,5]. Predictive ab initio calculations provide useful guidance to experimental studies in this case. |
Thursday, March 8, 2018 11:51AM - 12:03PM |
S35.00002: Shubnikov-de Haas Oscillations in bilayer MoSe2: effective mass and carrier density dependent quantum Hall states sequence Stefano Larentis, Hema Movva, Babak Fallahazad, Kyounghwan Kim, Armand Behroozi, Takashi Taniguchi, Kenji Watanabe, Sanjay Banerjee, Emanuel Tutuc We report magnetotransport studies of high-mobility electrons in dual-gated bilayer MoSe2 Hall-bars. Samples are fabricated using dry-transfer techniques, where an exfoliated MoSe2 bilayer is encapsulated in hexagonal boron-nitride dielectrics. The introduction of metal bottom-contacts coupled with electrostatic doping of the contact regions yields low-resistance n-type contacts down to low temperatures (0.3 K). The measured longitudinal resistance shows clear Shubnikov-de Haas (SdH) oscillations, and quantum Hall states (QHSs) developing at high magnetic fields. The temperature dependence of the SdH oscillations amplitude allows to extract an electron effective mass of 0.8me. The QHSs sequence shows transitions from even to odd filling factors as a function of electron density, which indicate the Zeeman to cyclotron energy ratio changes with density, likely because of electron-electron interaction. The QHSs are insensitive to an applied in-plane magnetic field, which suggest the electron spins are locked perpendicular to the MoSe2 plane because of strong spin-orbit interaction. |
Thursday, March 8, 2018 12:03PM - 12:15PM |
S35.00003: Valley-Dependent Longitudinal Magnetoconductivity in Two-Dimensional Semiconductors Akihiko Sekine, Allan MacDonald Recently the valley degrees of freedom of electrons in solids have attracted greater attention and revealing their roles in new electronic phenomena has become an important research theme. In this work, we use a quantum kinetic theory to study magnetotransport in two-dimensional semiconductors with valley degrees of freedom, such as monolayer transition-metal dichalcogenides. We find that, in a magnetic field applied perpendicular to the system, a longitudinal magnetoconductivity contribution that is odd in magnetic field and odd in valley index arises from the interplay between the momentum-space Berry curvature of Bloch electrons and the presence of a magnetic field. We take the effect of short-range disorder scattering into account using a Born approximation, which corresponds to a ladder-diagram approximation vertex correction, and find that the vertex correction enhances the linear magnetoconductivity. The valley-dependent longitudinal magnetoconductivity can be measured by studying the sensitivity of magnetoresistance to valley polarization, or by using Kerr microscopy to detect current-induced valley polarization near a gate-induced inhomogeneity. |
Thursday, March 8, 2018 12:15PM - 12:27PM |
S35.00004: Magnetotransport in Dual-gated MoS2 van der Waals Heterostructure En-Min Shih, Daniel Rhodes, Takashi Taniguchi, Kenji Watanabe, Cory Dean Semiconductor transition metal dichalcogenides (TMDs) have attracted significant interest because of their unique combination of two-dimensional nature, large-band gap and strong spin-orbit interaction. Recent advances in contact engineering have allowed Ohmic contact to n-type [1] and p-type [2] TMDs thus enabling study of quantum transport phenomenon at cryogenic temperature. Combining the n-type contact scheme and MoS2-Boron Nitride (BN) dual-gated heterostructure, we are able to tune the contact and carrier densities of monolayer MoS2 independently and measure magnetotransport in the density range of 2-10x1012 cm-2. The samples display well-defined Shubnikov-de Haas (SdH) oscillations in high magnetic fields. From the temperature dependence of the SdH oscillation amplitude we determine an electron effective mass of 0.46 me for monolayer MoS2. [1] Cui, X.; Shih, E.-M. et al. Nano Lett. 2017, 17, 4781–4786. [2] Movva, H. et al., ACS Nano 2015, 9, 10402–10. |
Thursday, March 8, 2018 12:27PM - 12:39PM |
S35.00005: Quantum Hall Effect and insulating behavior induced by magnetic field in ZrTe5 Fangdong Tang, Yafei Ren, Ruidan Zhong, Genda Gu, Zhenhua Qiao, Liyuan Zhang The discovery of Dirac Materials (DM) and Topological Insulator (TI) provided a fascinating field in exploring the quantum dynamics of relativistic field theory in condensed matter systems. With a strong magnetic field, the state of DM could convert into other exotic phases like as Weyl fermions, Charge Density Wave, Spin Density Wave, Axion insulator, and excitonic insulator. In this work, we report the experimental observation of a quantum Hall effect in 3D Zirconium Penta-Telluride(ZrTe5)within a small magnetic field B (<1.3T) regime. As further increasing magnetic field, the system has reached the quantum limit regime, beyond its lowest Landau Level. The magneto-resistance ρxx increases dramatically and display insulator behavior above certain critical magnetic field Bc. We suggest this metal-insulator transition (MIT) is more like relevant to the many-body interactions. |
Thursday, March 8, 2018 12:39PM - 12:51PM |
S35.00006: Strong Interaction Effects Revealed by Landau Levels in Bilayer MoS2 Jiangxiazi Lin, Tianyi Han, Benjamin Piot, Duncan Maude, Fan Zhang, Ning Wang Two-dimensional semiconducting transition metal dichalcogenides (TMDCs) have been a recent hot topic in physics research, for their novel optical/electronic properties and potential applications. While phenomena such as direct-to-indirect bandgap transition, spin-orbital coupling, and symmetry-controlled valleytronics have attracted wide studies, interaction effect was only reported recently in monolayer WSe2. |
Thursday, March 8, 2018 12:51PM - 1:03PM |
S35.00007: Realistic Description of Competing Interactions in Metallic TMDCs Gunnar Schönhoff, Malte Rösner, Ebad Kamil, Jan Berges, Tim Wehling Two-dimensional transition metal dichalcogenides constitute a prominent showplace for competing many-body instabilities such as superconductivity [Frindt: PRL 28, 299 (1972)], charge-density waves [Ugeda et al.: Nat. Phys. 12, 92 (2016)] and magnetism [Ma et al.: ACS Nano 6, 1695 (2012)]. In this study, we show that even though the observed phase diagrams are complex, the underlying mechanisms are captured by a compact unifying theoretical framework. We apply the constrained random-phase approximation (cRPA) [Aryasetiawan et al.: PRB 74, 125106 (2006)] and constrained density-functional perturbation theory (cDFPT) [Nomura, Arita: PRB 92, 245108 (2015)] to the metallic monolayers H-MX2 with M ∈ {V, Nb, Ta} and X ∈ {S, Se} and summarize the material specifics with a small number of representative Coulomb and electron-phonon interaction parameters. Both cRPA and cDFPT imply a separation of the electrons into a correlated subspace, here an isolated metallic band, and the rest. We find that all relevant physics emerges from interactions within this subspace. Beyond that, the materials can be well described by very similar tight-binding and mass-spring models. |
Thursday, March 8, 2018 1:03PM - 1:15PM |
S35.00008: Sign-problem-free Quantum Monte Carlo Study of Exciton Condensation in Electron-hole Doped Hubbard Bilayer Xuxin Huang, Martin Claassen, Brian Moritz, Thomas Devereaux The bilayer Hubbard model with electron-hole doping is an ideal platform to study exciton condensation due to spatial separation of electrons and holes which can significantly suppress recombination. However, suffering from the sign problem, a previous determinant quantum Monte Carlo(DQMC) study of the Hubbard model in this system could not arrive at an unequivocal conclusion regarding the presence of exciton condensation. Here, we develop a sign-problem-free DQMC algorithm for the bilayer Hubbard model with equal and opposite doping in the two layers. We study the competition between excitonic condensation, charge and spin order as a function of electron-hole doping and interactions away from SU(4) symmetry, the point where inter-layer and intra-layer interaction are the same. For finite electron-hole doping we find that the intra-layer Hubbard interaction enhances (π, π) exciton correlations. We also discuss the spectral function and the charge and spin excitation spectra. |
Thursday, March 8, 2018 1:15PM - 1:27PM |
S35.00009: Electronic wavefunctions and flux quantization in the pseudomagnetic quantum limit Hari Manoharan, Yang Liu, Yi-Ting Chen, Bitan Roy, Vladimir Juricic, Fakher Assaad Deliberate triaxial strain of a monolayer graphene flake realizes an uniform static axial magnetic gauge field for the embedded massless Dirac fermions. Such a field leads to the formation of a set of pseudo Landau levels, and in particular to a topologically protected Landau level located precisely at zero energy. Thanks to its extraordinary tunability, molecular graphene offers an unprecedented platform that can access fields at the moderate to ultra quantum limit where the magnetic length scale is comparable to the lattice constant. We experimentally demonstrate that such a pseudomagnetic field supports the zeroth Landau level at the band center up to 300 T when the Landau level wavefunctions are confined within a few unit cells. Nonetheless, via detailed mapping of the corresponding electronic wave functions, we show that the axial magnetic field displays flux quantization within the cyclotron orbit, even when its dimension is comparable to the lattice constant. |
Thursday, March 8, 2018 1:27PM - 1:39PM |
S35.00010: Electronic Ground State in Bilayer Graphene with Realistic Coulomb Interactions Jia Ning Leaw, Ho Kin Tang, Igor Herbut, Pinaki Sengupta, Fakher Assaad, Shaffique Adam In bilayer graphene at charge neutrality, previous theoretical studies have shown that even with infinitesimal long-range electron-electron interactions, the quadratic band structure is susceptible to several different symmetry breaking ground states. This is in stark contrast to a recent numerical quantum Monte Carlo study [Pujari et. al., PRL 117, 086404 (2016)] showing that the short-range Hubbard interaction spontaneously generates a linear band that stabilises the metallic phase over a finite range of weak interactions. In this theoretical work, using a combination of renormalization group, quantum Monte Carlo and lattice perturbation theory, we address the question of what happens for a realistic model of the Coulomb interaction that includes both short-range and long-range components. Surprisingly, we find that the metallic phase remains stable without the generation of linear bands. We discuss the implications of this finding on the interpretation of available experiments. |
Thursday, March 8, 2018 1:39PM - 1:51PM |
S35.00011: Out-of-bounds hydrodynamics in anisotropic Dirac fluids Julia Link, Boris Narozhny, Jõrg Schmalian We study the hydrodynamic behavior in two-dimensional, interacting electronic systems with merging Dirac points at charge neutrality. The dispersion along one crystallographic direction is Dirac-like, while it is Newtonian-like in the orthogonal direction. As a result, two diverging length scales emerge giving rise to highly anisotropic hydrodynamic transport coefficients. The electrical conductivity exhibits the metallic temperature dependence if the external electric field is applied in the direction of the Dirac-like linear dispersion and the insulating one in the orthogonal direction. The shear viscosity is a fourth-rank tensor with distinct elements exhibiting fundamentally different scaling with temperature, depending on the direction of the momentum flow. Most dramatically, we find a violation of the famous lower bound for the shear viscosity to entropy density ratio. Our results are of importance for oxide heterostructures and organic charge transfer salts. |
Thursday, March 8, 2018 1:51PM - 2:03PM |
S35.00012: Aperiodic Quantum Oscillations of Particle-hole Asymmetric Dirac Cones Emilie Tisserond
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Thursday, March 8, 2018 2:03PM - 2:15PM |
S35.00013: Magneto-transport in One-dimensional van der Waals Chiral Material Tellurium Gang Qiu, Yixiu Wang, Wenzhuo Wu, Peide (Peter) Ye Tellurium (Te) is a p-type narrow-bandgap high-mobility semiconductor with one dimensional van der Waals (vdW) structure. It has a unique chiral-chain crystal lattice in which individual helical chains of Te atoms are stacked together by vdW type bonds. With recently developed solvent-based growth method, we are able to probe the magneto-transport of Te in its 2D limit. Shubnikov-de Haas oscillations were observed in high-quality tellurium films which shed lights on intrinsic material properties such as its effective mass and carrier lifetime. Strong anisotropic magneto resistance indicates very different transport mechanisms along van der Waals bonds and covalence bonds. Our Te samples show evidence of strong spin-orbit coupling and Rashba-like splitting at H point of Brillouin zone which originates from its broken spatial-inversion-symmetry in the helical crystal structure. The spin-orbit coupling combined with the presence of large external magnetic fields gives rise to anomalous transport properties which is considered as strong evidence of chirality. Our results of the unique magneto- transport can be interpreted as a macroscopic manifestation of its 1D van der Waals nature and strong helicity. |
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