Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session J53: Scanning Tunneling Microscopy of 2D Materials and Hall Effect in 2D Materials |
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Sponsoring Units: DCMP Chair: Geng Li Room: Mile High Ballroom 1F |
Tuesday, March 3, 2020 2:30PM - 2:42PM |
J53.00001: Atomic-scale spectroscopy of 2D Transition metal dichalcogenide heterostructures Sara Shabani, Abhay Pasupathy Bilayers of 2H-WSe2 and 2H-MoSe2 form a type-II semiconductor heterostructure. A number of interesting optoelectronic phenomena have been observed in such heterostructures including charge transfer excitons, photovoltaic and photothermal effects, and Mott transitions at high exciton density. In this talk, we will describe complementary single-particle measurements of the electronic structure of these heterostructures using scanning tunneling microscopy (STM) and spectroscopy. Using a large-area exfoliation technique, coupled with pre-patterned electrodes to make ohmic contact to these bilayers, we have succeeded in fabricating devices that are compatible with surface-sensitive probes such as STM and angle resolved photoemission. We will describe the spatial structure of the bilayers including the effect of lattice mismatch on the electronic properties, as well as spectroscopic measurements performed under optical illumination both below and above the Mott threshold. |
Tuesday, March 3, 2020 2:42PM - 2:54PM |
J53.00002: Unraveling the Tunneling Spectrum of Bernal Stacked Bilayer Graphene John Davenport, Zhehao Ge, Frederic Joucken, Eberth A Quezada, Takashi Taniguchi, Kenji Watanabe, Jairo Velasco Jr. Scanning tunneling spectroscopy (STS) has recently provided important insight on the symmetry breaking effects and correlated states in magic angle twisted bilayer graphene (MATBLG). Conventional STS, however, can unintentionally change the dynamic band structure of MATBLG sheets due to the unavoidable and prominent electric field from the STS probe tip. Such an effect can obscure the intrinsic nature of correlated electron states present in MATBLG. We will discuss our latest experimental progress towards using a simpler tunneling system—planar tunneling spectroscopy nanodevices—to controllably probe the dynamic band structure of Bernal-stacked bilayer graphene. Our methods can be applied to MATBLG as well as other two-dimensional material systems that have a dynamical band structure. |
Tuesday, March 3, 2020 2:54PM - 3:06PM |
J53.00003: Proximity-induced superconducting gap in the quantum spin Hall edge state of monolayer WTe_{2} Felix Luepke, Dacen Waters, Sergio de la Barrera, Michael Widom, David Mandrus, Jiaqiang Yan, Randall M Feenstra, Benjamin Hunt Van der Waals heterostructures allow the combination of different material properties, e.g. non-trivial topology and superconductivity in order to create a topological superconducting state. In my talk, I demonstrate a novel dry-transfer flip technique which allows to place atomically-thin layers of WTe_{2}, a quantum spin hall system, on NbSe_{2}, a van der Waals superconductor. Using scanning tunneling microscopy and spectroscopy (STM/STS), we demonstrate atomically clean surfaces and interfaces and the presence of a proximity-induced superconducting gap in the WTe_{2} for thicknesses from a monolayer up to 7 crystalline layers. At the edge of the WTe_{2} monolayer, we show that the superconducting gap coexists with the characteristic spectroscopic signature of the QSH edge state [1]. Taken together, these observations provide conclusive evidence for proximity-induced superconductivity in the QSH edge state in WTe_{2}, a crucial step towards realizing 1D topological superconductivity and Majorana states in this van der Waals material platform. |
Tuesday, March 3, 2020 3:06PM - 3:18PM |
J53.00004: Observation of Deep In-gap States of Line and Point defects in Monolayer WS_{2} via Scanning Tunneling Microscopy and Spectroscopy Madisen Holbrook, Wei-Ting Hsu, Chih-Kang Shih Atomic defects in crystalline semiconductors strongly affect their electronic properties, such as electron transport and optical response. Defects in two dimensional (2D) semiconductors, such as transition metal dichalcogenides (TMD’s), have a more dramatic impact than bulk counterparts due to less screening and increased substrate interactions. In particular, different atomic vacancies have been reported, but their role in the electronic structure must be established. Deep in-gap states were predicted for different defect structures in 2D TMD’s, but direct experimental observation of defect structure and electronic properties is necessary to tailor TMD’s for device design. Here we report the creation of point and line defects in WS_{2} monolayers by vacuum annealing. The electronic properties and structure of these defects was probed using scanning tunneling microscopy and spectroscopy. We find these defects have a rich variety of deep in-gap states, shedding light on the role of defects in TMD electronic properties. |
Tuesday, March 3, 2020 3:18PM - 3:30PM |
J53.00005: Determination of the trigonal warping orientation in Bernal-stacked bilayer graphene via quasiparticle interference imaging Frederic Joucken, Zhehao Ge, Eberth A Quezada, Kenji Watanabe, Takashi Taniguchi, Jairo Velasco Jr. The existence of strong trigonal warping around the K point for the low energy electronic states in multilayer graphene films and graphite is well established. It is responsible for phenomena such as Lifshitz transitions and anisotropic ballistic transport. The absolute orientation of the trigonal warping with respect to the center of the Brillouin zone is however not agreed upon. Here, we use quasiparticle scattering experiments on a gated bilayer graphene/hexagonal boron nitride heterostructure to resolve this disagreement. We compare Fourier transforms of scattering interference maps acquired at various energies away from the charge neutrality point with joint density of states simulations. This comparison permits unambiguous determination of the trigonal warping orientation for bilayer graphene at low energy. Our experimental technique is promising for studying quasi-directly fine features of the band structure of two-dimensional materials such as band topologies and Lifshitz transitions. |
Tuesday, March 3, 2020 3:30PM - 3:42PM |
J53.00006: Charge/spin density waves at the edges and mirror twin boundaries of Transition Metal Di-Chalcogenides Sridevi Krishnamurthi Grain boundaries and edges of two-dimensional semiconducting TMDCs MX_{2} (M=Mo,W; X=S,Se,Te) exhibit metallicity, which is remarkably robust against variations in structure or stoichiometry. The origin of these metallic boundary states lies in the lattice polarization and the states are topologically protected by the D_{3h} symmetry of the TMDC lattice. From first-principles (DFT+U) calculations we show, however, that symmetry breaking at edges or boundaries leads to one-dimensional charge and spin density wave (C/SDW) instabilities. Such C/SDWs are commensurable with a period of three lattice sites, dictated by the lattice symmetry, and open up a band gap. The resulting electronic structure agrees well with experimental measurements on, for instance, the mirror twin boundary. |
Tuesday, March 3, 2020 3:42PM - 3:54PM |
J53.00007: Quantum spin Hall effect in twisted bilayer WTe_{2} measured with STM Dacen Waters, Felix Lukas Lüpke, Benjamin Hunt, Randall M Feenstra Tungsten telluride is a two-dimensional (2D) material that exhibits a range of properties, depending on the number of layers. Bilayers are reported to be a topologically trivial semimetal that exhibit a ferroelectric effect^{1}. In the monolayer (ML) limit, WTe_{2} has been shown to be a quantum spin Hall (QSH) insulator^{2}, resulting in topologically protected edge modes. In this work, we report scanning tunneling microscopy and spectroscopy (STM/STS) studies of twisted bilayers of WTe_{2} for a variety of configurations. We find that tunneling spectra of the twisted bilayers resemble that of decoupled monolayers for twist angles ≥5°, and observe edge states to exist at the boundaries of the twisted bilayer regions, in contrast to the case of the trivial bilayer. To study exfoliated WTe_{2} in STM, which is air sensitive, we utilize a novel transfer technique^{3} that enables stacking in an inert gas environment but maintains an atomically pristine surface available for surface probe measurements. |
Tuesday, March 3, 2020 3:54PM - 4:06PM |
J53.00008: Disorder-induced nonlinear Hall effect with time-reversal symmetry Zongzheng Du, Chunming Wang, Shuai Li, Haizhou Lu, Xincheng Xie The nonlinear Hall effect has opened the door towards deeper understanding of topological states of matter. It can be observed as the double-frequency Hall voltage response to an ac longitudinal current in the presence of time-reversal symmetry. Disorder plays indispensable roles in various linear Hall effects, such as the localization in the quantized Hall effects and the extrinsic mechanisms of the anomalous, spin, and valley Hall effects. Unlike in the linear Hall effects, disorder enters the nonlinear Hall effect even in the leading order. Here, we derive the formulas of the nonlinear Hall conductivity in the presence of disorder scattering. We apply the formulas to calculate the nonlinear Hall response of the tilted 2D Dirac model, which is the symmetry-allowed minimal model for the nonlinear Hall effect and can serve as a building block in realistic band structures. More importantly, we construct the general scaling law of the nonlinear Hall effect, which may help in experiments to distinguish disorder-induced contributions to the nonlinear Hall effect in the future. This work will be insightful for studying the disorder effects in the nonlinear Hall effect. |
Tuesday, March 3, 2020 4:06PM - 4:18PM |
J53.00009: Intrinsic orbital moment and prediction of a large orbital Hall effect in the 2D transition metal dichalcogenides Sayantika Bhowal, Sashi Sekhar Satpathy Orbital Hall effect (OHE) is the phenomenon of transverse flow of orbital angular momentum caused by an applied longitudinal electric field. From density-functional and tight-binding model studies, we predict the existence of a large OHE in the 2D transition metal dichalcogenides (TMDC) due to the intrinsic orbital moment induced by the broken inversion symmetry of the system. We show that monolayer TMDC, the prototypical example of broken inversion symmetric 2D material, have a hitherto-unknown intrinsic orbital moment in the momentum space, which in turn leads to a large OHE with possible applications in the newly emerging field of orbitronics. The orbital moment and the OHE appear even in absence of the spin-orbit coupling (SOC) and give rise to “valley dependent spin splitting” and spin Hall effect in presence of SOC. We show that the OHE can also be tuned by a transverse electric field due to the orbital Rashba coupling, that produces an additional orbital texture which in turn modifies the OHE. |
Tuesday, March 3, 2020 4:18PM - 4:30PM |
J53.00010: Magnus Hall effect Michal Papaj, Liang Fu A new type of a linear response Hall effect is predicted in time-reversal-invariant systems with built-in electric field at zero magnetic field. The Hall response results from a quantum Magnus effect where a self-rotating Bloch electron wavepacket moving under electric field develops an anomalous velocity in the transverse direction. We show that in the ballistic limit the Magnus Hall conductance measures the distribution of the Berry curvature on the Fermi surface. |
Tuesday, March 3, 2020 4:30PM - 4:42PM |
J53.00011: The Curious Magnetic Properties of Orbital Chern Insulators Jihang Zhu, Allan Macdonald Chern insulator ferromagnets are characterized by a quantized anomalous Hall effect, and have so far been identified experimentally in magnetically-doped topological insulator (MTI) thin films and in bilayer graphene moiré superlattices. We classify Chern insulator ferromagnets as either spin or orbital, depending on whether the orbital magnetization results from spontaneous spin-polarization combined with spin-orbit interactions, as in the MTI case, or directly from spontaneous orbital currents, as in the moiré superlattice case. We argue that in a given magnetic state, characterized for example by the |
Tuesday, March 3, 2020 4:42PM - 4:54PM |
J53.00012: Hall Conductivity of Two-Dimensional Electrons in an Abrikosov Magnetic Flux Lattice Jonathan Schirmer, Ravi Khandelwal, Kenji Watanabe, Takashi Taniguchi, Pratap Raychaudhuri, Jainendra Jain, Chaoxing Liu, Anindya Das We study, both theoretically and experimentally, the Hall transport of two-dimensional electrons exposed to a magnetic field produced by an Abrikosov flux lattice. Our experiments on monolayer graphene-hBN-NbSe2 heterostructure demonstrate a substantial reduction in the Hall conductivity as the temperature is reduced from above the superconducting critical temperature of NbSe2, when the magnetic field is uniform, to below, where the magnetic field bunches into an Abrikosov flux lattice. We provide a quantitative explanation of this phenomenon by evaluating the cumulative Berry curvature of occupied states using a lattice model, while also taking account of disorder, and extrapolating to the continuum limit. A fundamental role is played by a correlation between the Berry curvature and the energy within Landau levels broadened by the non-uniform magnetic field. |
Tuesday, March 3, 2020 4:54PM - 5:06PM |
J53.00013: Energy gaps of low filling factor quantum Hall states in two-dimensional Indium Selenide. Dmitry Shcherbakov, Petr Stepanov, Jiawei Yang, Shahriar Memaran, Wenkai Zheng, Kenji Watanabe, Takashi Taniguchi, Luis Balicas, Chun Ning Lau Indium selenide is a promising two-dimensional semiconductor with thickness-dependent bandgap and high electron mobility. We have fabricated high mobility few-layer InSe devices and studied them in quantum Hall regime. Landau levels at filling factors between 1 and 10 are resolved. Energy gaps at different magnetic fields, gate voltage and temperatures will be discussed. |
Tuesday, March 3, 2020 5:06PM - 5:18PM |
J53.00014: Non-perturbative valley Hall effect in graphenes Wei-Yuan Tu, Ci Li, Hongyi Yu, Wang Yao Valley Hall currents can be induced in graphene by breaking its version symmetry that opens a gap which is usually very small. A finite electric field can easily exceed the adiabatic limit on which the standard semiclassical theory for the valley Hall effect is based. Here we provide a non-perturbative formulation for valley Hall effect. We show that the non-perturbation effects of the electric field directly lead to an oscillating anomalous velocity in the coherent evolution of an electron, in contrast to the stationary one inherited to the adiabatic limit. By properly taking into account the decoherence effect, we obtain a steady-state valley Hall current that is in all orders of the electric field. It clearly shows nonlinearity in the Hall conductivity to high electric-field values and still retains the intrinsic role of the Berry curvature. Through the inspection of the non-perturbative effect of the electric field, it becomes obvious that the valley Hall conduction in gapped graphene is mediated by the interference between the electric field induced electron-hole excitations and the fermi sea. |
Tuesday, March 3, 2020 5:18PM - 5:30PM |
J53.00015: Edge channels in graphene with proximity-induced Spin-Orbit Interaction Taro Wakamura, Sophie Gueron, NianJheng Wu, Meydi Ferrier, Hélène Bouchiat, Cecilia Mattevi, Pawel Palczynski, Mauro Och, Kenji Watanabe, Takashi Taniguchi Enhancing spin-orbit interaction (SOI) in graphene may lead to new properties, including a possible Quantum Spin Hall state, characterized by counterpropagating, spin-polarized channels at the edges of graphene. |
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