Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session S63: Twisted 2D Heterostructures: Computational and Theoretical Studies I |
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Sponsoring Units: DCMP Chair: Nicolas Leconte Room: Mile High Ballroom 4D |
Thursday, March 5, 2020 11:15AM - 11:27AM |
S63.00001: Exploring van der Waals Heterostructures behavior: Stability vs. Twisting Andrea Silva, Victor P. E. Claerbout, Tomas Polcar, Denis Kramer, Paolo Nicolini While the outstanding properties of van der Waals 2D materials are widely known, recent efforts have been focusing on the physics emerging from the stacking and rotational degrees of freedom of these lamellar compounds. Notable examples are unconventional superconductivity in twisted bilayer graphene^{1} and frictionless dynamics obtained by switching from commensurate to incommensurate orientation in graphitic systems^{2}. |
Thursday, March 5, 2020 11:27AM - 11:39AM |
S63.00002: Structural and electronic properties of twisted bilayer transition metal dichalcogenides from linear-scaling DFT Valerio Vitale, Johannes Lischner, Arash A Mostofi Recently, it has been proposed that Moiré superlattices of twisted homobilayer transition metal dichalcogenides (TMDs) exhibit many interesting structural and electronic properties, including shear solitons, phasons, topological point defects and ultraflatbands close to the Fermi level [1,2]. For small twist angles the unit cells of these systems contain thousands of atoms rendering first-principles investigations of their structural and electronic properties numerically challenging. To overcome this obstacle, we employ the linear-scaling DFT code ONETEP [3] and present results for relaxed geometries and band structures for both homo- and heterobilayers of transition metal dichalcogenides as function of twist angle. |
Thursday, March 5, 2020 11:39AM - 11:51AM |
S63.00003: Moire quasicrystals in graphene encapsulated by hexagonal boron nitride Nicolas Leconte, Jeil Jung Interference of double moire patterns of graphene (G) encapsulated by hexagonal boron nitride (BN) can alter the electronic structure features near the primary/secondary Dirac points and the electron-hole symmetry introduced by a single G/BN moire pattern depending on the relative stacking arrangements of the top/bottom BN layers. For equal moire periods and commensurate patterns with Δφ = 60° angle differences, the patterns can add up constructively or cancel out destructively depending on their relative sliding, while moire quasicrystals are expected for Δφ = 30° differences. We present calculations on the double moire interference effects in the density of states and magnetic oscillations in nearly aligned BN/G/BN systems giving rise to moire quasicrystals and their evolution as a function of moire superlattice twist angles. |
Thursday, March 5, 2020 11:51AM - 12:03PM |
S63.00004: Modeling mechanical relaxation and electronic states of incommensurate trilayer van der Waals heterostructures Ziyan Zhu, Stephen Carr, Paul Cazeaux, Mitchell Luskin, Efthimios Kaxiras Incommensurate stacking provides an intriguing avenue for manipulating the physical properties of layered two-dimensional materials, but is a challenging problem from a theoretical perspective. Here, we present a multiscale model to obtain the mechanical relaxation pattern and electronic structure of twisted trilayer van der Waals heterostructures with two independent twist angles. This serves as a prototype system of a generally incommensurate system without a supercell description. |
Thursday, March 5, 2020 12:03PM - 12:15PM |
S63.00005: First-principles calculation of gate-tunable magnetism in twisted bilayer graphene under pressure Xiao Chen, Shuanglong Liu, Hai-ping Cheng Magic angle twisted bilayer graphene (MAtBLG) is believed to be a highly tunable platform for investigating strongly correlated phenomena such as high-T_{c} superconductors and quantum spin liquids, due to easy control of doping level through gating and sensitive dependence of the magic angle on hydrostatic pressure. Experimental observations of correlated insulating states, unconventional superconductivity and ferromagnetism in MAtBLG indicate that rich exotic phases exist in this system, which is also suggested by various theoretical works. In this work, with a combination of density functional theory and effective screening medium method, we systematically study the ground state of twisted bilayer graphene at magic twist angle 2.88° under pressure and simulate how the ground state evolves when doping level is gate-tuned. We find that at zero doping, a ferromagnetic solution showing half-metallic behavior with spin density localized at AA stacking sites is lower in energy than the trivial spin non-polarized solution. Interestingly, the magnetic moment per moiré unit cell of this ferromagnetic state decreases upon both electron and hole doping and vanishes at four electrons/holes doped per moiré unit cell. |
Thursday, March 5, 2020 12:15PM - 12:27PM |
S63.00006: Origin and Evolution of Ultraflatbands in Twisted Bilayer Transition Metal Dichalcogenides: Realization of Triangular Quantum Dots Mit H. Naik, Sudipta Kundu, Indrajit Maity, Manish Jain Using a multiscale computational approach, we probe the origin and evolution of ultraflatbands in moiré superlattices of twisted bilayer MoS_{2} (TBLM), a prototypical transition metal dichalcogenide. Unlike twisted bilayer graphene, we find no special magic angles in TBLM. Ultraflatbands, which form at the valence band edge for twist angles (θ) close to 0° and at both the valence and conduction band edges for θ close to 60°, have distinct origins. For θ close to 0°, inhomogeneous hybridization in the relaxed moiré superlattice is sufficient to explain the formation of flatbands. For θ close to 60°, apart from the inhomogeneous hybridisation, local strains cause the formation of modulating triangular potential wells such that electrons and holes are spatially separated. This leads to multiple energy-separated ultraflatbands closely resembling eigenfunctions of a quantum particle in an equilateral triangle well. |
Thursday, March 5, 2020 12:27PM - 12:39PM |
S63.00007: Correlated states in transition metal dichalcogenide heterobilayer moiré superlattices Naichao Hu, Ajesh Kumar, Allan MacDonald 2D van der Waals moiré superlattices have attracted much attention recently as highly tunable platforms to study strong correlation phenomena. In particular, transition metal dichalcogenide (TMD) heterobilayers at small twist angles develop isolated flat moiré bands that are accurately described [1] by triangular lattice (generalized) Hubbard models. Using a fully self-consistent mean-field-theory, we investigate the competing correlated phases at various fractional filling factors and twist angle, and identify metal-Mott insulator phase transition at 1/2 band filling and Wigner crystal phases at 1/6 band filling. We will discuss the relationship between our calculations and recent related experiments. |
Thursday, March 5, 2020 12:39PM - 12:51PM |
S63.00008: Spontaneous symmetry breaking and topology in twisted bilayer graphene: the nature of the correlated insulating states and the quantum anomalous Hall effect Jianpeng Liu, Xi Dai We theoretically study the nature of the correlated insulating states and the quantum anomalous Hall (QAH) effect in twisted bilayer graphene (TBG) at the magic angle. Using a generic Hartree-Fock theory applied to all the energy bands, both the experimentally observed correlated insulating states at +/-1/2 filling and the quantum anomalous Hall effect at 3/4 filling of the flat bands near magic angle can be successfully explained. Our results indicate that the correlated insulating states at +/-1/2 filling are states with co-existing valley coherent order and valley polarized order. When a hexagonal boron nitride (hBN) substrate is aligned with the TBG, the valley polarized states are energetically favored over the valley coherent states at +/-1/2 filling, giving rise to QAH insulating states with Chern numbers (C) +/-2 by virtue of the C2z symmetry breaking induced by the substrate. We propose that such valley polairzed QAH states would be strongly suppressed by weak in-plane magnetic fields. We further predict that, for the same valley polarization, the anomalous Hall conductivities are opposite for +/- p (p=1/2 or 3/4) fillings, which implies opposite hysteresis behaviors for the QAH states at the electron and hole fillings. |
Thursday, March 5, 2020 12:51PM - 1:03PM |
S63.00009: Quasiperiodicity induced eigenstate criticality and flat \mathbb{Z}_2 bands in a topological insulator Yixing Fu, Jed Pixley, Justin Wilson Flat topological bands have been sought after as a route to the fascinating phenomena of topology with strong correlations. The discovery of correlated insulator and superconductivity in various twisted multilayer graphene systems has shown that the superlattice pattern generated in van der Waals heterostructure, or more generally incommensurate effects, provides an ample playground for generating flat bands. We demonstrate that perturbing topological insulators with a quasiperiodic potential creates flat topological bands in a controllable fashion as well as induces non-trivial quantum phase transitions that are beyond the Landau-Ginzburg paradigm. Using a combination of analytic and numeric calculations, including a Convolutionary Neural Network classification of wavefunctions, we reveal the rich phase diagram induced by quasiperiodicity and topology. We show that quasiperiodicity can make a trivial insulator topological through a topological eigenstate phase transition that represent a unique universality class, and in the process creates essentially flat topological bands. |
Thursday, March 5, 2020 1:03PM - 1:15PM |
S63.00010: Modelling realistic Hamiltonians for moire lattices Arkadiy Davydov, Alexey Soluyanov The tight-binding (TB) approximation to the Hamiltonian of the Twisted Bilayer Graphene (TBG) is considered in the Slater-Koster (SK) form. Instead of taking the commonly-accepted parametrization, we generate a new one by fitting the data obtained in the ab-initio Density Functional Based (DFT) calculations. The resulting Hamiltonian is further projected to a smaller twelve-band tight-binding model, giving a systematic way to reduce the dimensionality of the problem. Finally, the localization of the Wannier orbitals and the symmetry of the final Hamiltonian is considered in this work |
Thursday, March 5, 2020 1:15PM - 1:27PM |
S63.00011: Electronic properties of twisted transition metal dichalcogenide bilayers Madeleine Phillips, C Stephen Hellberg The electronic properties of transition metal dichalcogenide (TMD) bilayers vary according to the local atomic structure [1]. In minimally twisted bilayers, there is significant atomic reconstruction away from the rigid moire pattern [2]. We present the results of density functional theory (DFT) calculations that model the electronic properties of MoSe_{2}/WSe_{2} bilayers with a small (~1 degree) twist away from commensurate 180-degree stacking. We consider how the local density of states and charge density vary with atomic reconstruction and compare our results to recent experiments. |
Thursday, March 5, 2020 1:27PM - 1:39PM |
S63.00012: Structure of twisted transition metal dichalcogenide bilayers C Stephen Hellberg, Madeleine Phillips We present density functional (DFT) calculations of the reconstruction in transition metal dichalcogenide (TMD) hetero-bilayers twisted a small angle from from commensurate 180-degree stacking [1]. In both MoS_{2}/WS_{2} and MoSe_{2}/WSe_{2}, the bilayer untwists and reconstructs to increase the area of the domains with the lowest energy stacking [2]. The overall twist is concentrated in the domain walls and vertices, where the energetic cost of twisting away from 180 degrees is reduced. |
Thursday, March 5, 2020 1:39PM - 1:51PM |
S63.00013: Surface States and Arcless Angles in Twisted Weyl Semimetals Herbert Fertig, Ganpathy Murthy, Efrat Shimshoni Fermi arc states are features of Weyl semimetal (WSM) surfaces which are robust due to the topological character of the bulk band structure. We demonstrate that Fermi arcs may undergo profound restructurings when surfaces of different systems with a well-defined twist angle are tunnel-coupled. The twisted WSM interface supports a moiré pattern which may be approximated as a periodic system with large real-space unit cell. States bound to the interface emerge, with interesting consequences for the magneto-oscillations expected when a magnetic field is applied perpendicular to the system surfaces. As the twist angle passes through special "arcless angles”, for which open Fermi arc states are absent at the interface, Fermi loops of states confined to the interface may break off, without connecting to bulk states of the WSM. We argue that such states have interesting resonance signatures in the optical conductivity of the system in a magnetic field perpendicular to the interface. |
Thursday, March 5, 2020 1:51PM - 2:03PM |
S63.00014: Open Momentum Space Method for Hofstadter Butterfly: Application in Moire Models Biao Lian, Fang Xie, Andrei Bernevig We develop a generic open momentum space method for calculating the Hofstadter butterfly of both continuum models (for Moire superlattices, etc) and tight-binding models, where the quasimomentum is directly substituted by the Landau level (LL) operators. By taking a LL cutoff (and a reciprocal lattice cutoff for continuum models), one obtains the Hofstadter butterfly with in-gap spectral flows. For continuum models such as the model for twisted bilayer graphene, our method gives a sparse Hamiltonian, making it much more efficient to calculate the spectrum. The spectral flows can be understood as edge states on a momentum space boundary, from which one can determine the two integers (tv, sv) of a gap v satisfying the Diophantine equation. The spectral flows can also be removed to plot a clear Hofstadter butterfly. While tv is known as the Chern number, our picture identifies sv as a dual Chern number in the momentum space, which corresponds to a quantized Lorentz susceptibility γxy = eBsv. |
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