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 J42: Theory of Twisted BilayerLive
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Sponsoring Units: DCMP Chair: Michael Zaletel |
Tuesday, March 16, 2021 3:00PM - 3:12PM Live |
J42.00001: Ab initio four-band Wannier tight-binding model for generic twisted graphene systems Jin Cao, Maoyuan Wang, Cheng-Cheng Liu, Yugui Yao The newly realized twisted graphene systems such as twisted bilayer graphene (TBG), twisted double bilayer graphene (TDBG), and twisted trilayer graphene (TTG) have attracted widespread theoretical attention. Therefore, a simple and accurate model of the systems is of vital importance for the further study. Here, we construct the symmetry-adapted localized Wannier orbitals and the corresponding ab initio minimal two-valley four-band effective tight-binding models for generic twisted graphene systems with small twist angle. Such two-valley model evades the Wannier obstruction caused by the fragile topology in one-valley model. The real space valley operator is introduced to explicitly describe the valley Uv(1) symmetry. Each symmetry-adapted Wannier orbital shows a peculiar three-peak form with its maximum at AA spots and its center at AB or BA spots. An extended Hubbard model is also obtained and the related parameters are presented explicitly. We provide an approach to systematically build the Wannier tight-binding model for generic twisted graphene systems. Our model provides a firm basis for further study of the many-body effects in these systems. |
Tuesday, March 16, 2021 3:12PM - 3:24PM Live |
J42.00002: Flat band carrier confinement in magic angle twisted bilayer Graphene Nikhil Tilak, Xinyuan Lai, Shuang Wu, zhenyuan zhang, Mingyu Xu, RAQUEL DE ALMEIDA RIBEIRO, Paul C Canfield, Eva Andrei Magic angle twisted bilayer graphene has emerged as a powerful platform for studying strongly correlated electron physics, owing to its almost dispersionless low-energy, flat, bands and the ability to tune the band filling by electrostatic gating. Techniques to control the twist angle between graphene layers have led to rapid experimental progress but improving sample quality is essential for separating the delicate correlation physics from disorder effects. However, owing to the 2D nature of the system and the relatively low carrier density, the samples are highly susceptible to doping inhomogeneity which can drastically modify the local potential landscape. This potential disorder is distinct from the twist-angle variation which has been studied elsewhere. Using low temperature scanning tunneling spectroscopy we demonstrated that the flat bands in magic angle twisted bilayer graphene can substantially amplify even negligibly small doping inhomogeneity. As a result the charge carriers become confined, obscuring the correlation effects associated with the intrinsic physics of magic-angle twisted bilayer graphene |
Tuesday, March 16, 2021 3:24PM - 3:36PM Live |
J42.00003: Flat bands and gaps in twisted double bilayer graphene Francisco Culchac, Rafael Del Grande, Rodrigo Capaz, Leonor Chico, Eric Suarez Morell We present electronic structure calculations of twisted double bilayer graphene (TDBG): a tetralayer graphene structure composed of two AB-stacked graphene bilayers with a relative rotation angle between them. Using first-principles calculations, we find that TDBG is semiconducting with a band gap that depends on the twist angle, that can be tuned by an external electric field. The gap is consistent with TDBG symmetry and its magnitude is related to surface effects, driving electron transfer from outer to inner layers. The surface effect competes with an energy upshift of localized states at inner layers, giving rise to the peculiar angle dependence of the band gap, which reduces at low angles. For these low twist angles, the TDBG develops flat bands, in which electrons in the inner layers are localized at the AA regions, as in twisted bilayer graphene. |
Tuesday, March 16, 2021 3:36PM - 3:48PM Live |
J42.00004: General Theory of Flat Bands and Magic Angles in Twisted Dirac Materials Aaron Dunbrack, Jennifer Cano Twisted bilayer graphene has been a center of recent theoretical and experimental interest due to the emergence of magic angles and the interaction-driven insulating and superconducting phases that result from nearly-flat bands. The Bistritzer-MacDonald model provides a framework to understand the origin of the flat bands. Using perturbation theory, we generalize this analysis to any interface between two twisted 2D Dirac materials, one with N Dirac cones and one with M Dirac cones (where N and M may be odd, as surface states of topological insulators), under assumptions of time-reversal and rotational symmetry. We then use this framework to consider two special cases: a TI-on-TI interface and a graphene-on-TI interface. We find that spin-flipping interactions serve a vital role in forming magic angles in both systems. |
Tuesday, March 16, 2021 3:48PM - 4:00PM Live |
J42.00005: Higher Order Magic in Twisted Four Layer Graphene Adam Eaton, Herb Fertig, Yantao Li, Babak Seradjeh
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Tuesday, March 16, 2021 4:00PM - 4:12PM Live |
J42.00006: One-dimensional flat bands in twisted bilayer germanium selenide Dante Kennes, Lede Xian, Martin Claassen, Angel Rubio Experimental advances in the fabrication and characterization of few-layer materials stacked at a relative twist of small angle have recently shown the emergence of flat energy bands. As a consequence electron interactions become relevant, providing inroads into the physics of strongly correlated two-dimensional systems. Here, we demonstrate by combining large scale ab initio simulations with numerically exact strong correlation approaches that an effective one-dimensional system emerges upon stacking two twisted sheets of GeSe, in marked contrast to all moiré systems studied so far. This not only allows to study the necessarily collective nature of excitations in one dimension, but can also serve as a promising platform to scrutinize the crossover from two to one dimension in a controlled setup by varying the twist angle, which provides an intriguing benchmark with respect to theory. We thus establish twisted bilayer GeSe as an intriguing inroad into the strongly correlated physics of lowdimensional systems. |
Tuesday, March 16, 2021 4:12PM - 4:24PM Live |
J42.00007: Relaxation Effects in Twisted Bilayer Graphene: a Multi-Scale Approach Nicolas Leconte, Srivani Javvaji, Jiaqi An, Appalakondaiah Samudrala, Jeil Jung We present a multi-scale approach to capture the interdependent atomic and electronic structures of suspended and hexagonal boron nitride (hBN) supported twisted bilayer graphene (tBG) by using rescaled two center interlayer hopping terms to calibrate the flat band magic angle to ∼ 1.08ο, as a way to resolve the indeterminacy of atomic/electronic structure models whose magic angles are bracketed between 0.9ο −1.3ο. We use electronic structrure hopping parameters extracted from density functional local density approximation except for an enhanced Fermi velocity of υF ≈ 106 m/s and atomic force fields informed by stacking and interlayer distance-dependent density functional theory total energies, including systematically improved exact exchange and random phase approximation (EXX+RPA), to calculate the high-resolution spectral functions that can be compared with experimental angle-resolved photoemission spectra (ARPES). |
Tuesday, March 16, 2021 4:24PM - 4:36PM Live |
J42.00008: Theory of moiré nematic state in twisted double-bilayer graphene Mathias Scheurer, Rhine Samajdar, Simon Turkel, Carmen Rubio Verdú, Larry Song, Lennart Klebl, Kenji Watanabe, Takashi Taniguchi, Hector Ochoa, Lede Xian, Dante Kennes, Angel Rubio, Jörn W Venderbos, Abhay Narayan, Rafael Fernandes Graphene-based moiré superlattice systems, such as twisted bilayer graphene, have attracted considerable interest in the last few years, as they display a remarkable variety of correlated phenomena. In particular, besides correlated insulating phases in the vicinity of integer fillings, separated by superconducting domes, there is evidence of other ordering tendencies at non-integer fillings; these, however, remain less explored. A notable example is provided by scanning tunneling microscopy measurements on twisted double-bilayer graphene, which reveal the presence of spontaneous three-fold symmetry breaking for partially filled flat bands, i.e. nematic order. Here we discuss how these experimental results allow us to determine the underlying microscopic form of the nematic order parameter. Through a detailed comparison of theory and experiment, we show that the dominant contribution to the observed nematic pattern arises from states at the moiré scale rather than at the microscopic scale of the individual graphene lattices. We will also discuss theoretically that twisted double-bilayer graphene allows for an unprecedented tunability of the orientation of the nematic director via the displacement field. |
Tuesday, March 16, 2021 4:36PM - 4:48PM Live |
J42.00009: Atomic-scale structure and electronic properties of twisted double bilayer graphene: topological edge states and broken symmetries Carmen Rubio Verdú, Alexander Kerelsky, Simon Turkel, Larry Song, Lennart Klebl, Rhine Samajdar, Mathias Scheurer, Jorn W. F. Venderbos, Lei Wang, Dorri Halbertal, Nathan R Finney, Kenji Watanabe, Takashi Taniguchi, Hector Ochoa, Lede Xian, Dante Kennes, James Hone, Cory Dean, Dmitri Basov, Rafael Fernandes, Angel Rubio, Abhay Narayan Van der Waals materials stacked with an interlayer twist are an excellent platform towards achieving gate-tunable correlated phenomena linked to the formation of flat bands. We demonstrate the formation of correlated phases in twisted double bilayer graphene (tDBG) in two regimes of twist angle: minimally twisted (<0.1°) and 1.1°. Tiny-angle tDBG host large regions of uniform rhombohedral graphene where scanning tunneling spectroscopy reveals a sharp flat band of 3-5 meV half-width. We demonstrate that, when this flat band straddles the Fermi level, a correlated many-body gap emerges. Moreover, we show that ABCA graphene hosts surface topological edge states at natural interfaces with ABAB graphene. Scanning tunneling microscopy on tDBG at higher twist (~1.1°) reveals the presence of van Hove singularities on all inequivalent moiré sites. Tuning carrier density and displacement field reveals long-range broken C3 symmetry that emerge when the Fermi level is at the electronic flat bands. We demonstrate that the C3 symmetry breaking is a manifestation of an interaction-driven electronic nematic phase, which emerges even away from integer fillings. The nematic instability is an emergent phenomenon at the scale of the moiré lattice, pointing to its universal character. |
Tuesday, March 16, 2021 4:48PM - 5:00PM Live |
J42.00010: Electronic nematicity in a transition metal dichalcogenide heterobilayer Moire system Michael Matty, Steven Allan Kivelson, Eun-Ah Kim Moiré systems have recently garnered tremendous attention due to their highly tunable electronic properties, which allow them to realize a rich set of electronic states. Recent experiments on a WS2/WSe2 hetero-bilayer system reveal a series of charge-ordered states at various commensurate Moiré lattice fillings. These experiments also find evidence for rotational symmetry breaking in the entire filling region between 1/3 and 2/3. Specifically, we use Monte Carlo simulations to explore the phase diagram for charge density wave and nematic phases at non-zero temperatures in the vicinity of and away from various commensurate densities. We also study the properties of topological defects involved in the melting of the charge density waves. |
Tuesday, March 16, 2021 5:00PM - 5:12PM Live |
J42.00011: Nematic order in twisted bilayer graphene by valley + spin fluctuation interference mechanism Seiichiro Onari, Hiroshi Kontani In the magic angle twisted bilayer graphene (TBG), the nearly flat band emerges as a good platform of the strongly correlated electron systems. Especially, the C3-symmetry-breaking nematic state observed in the TBG near the van Hove singularity (VHS) filling attracts increasing attention [1]. Here, we analyze the nematic state by focusing on the quantum interference mechanism, which was developed in the field of Fe-based superconductors [2]. We identify the nematic state in the TBG as the bond order [3], which is consistent with experiments. This nematic state originates from the interferences among the valley + spin fluctuations, due to the presence of the valley degrees of freedom and absence of the on-site Hund's coupling in the TBG. |
Tuesday, March 16, 2021 5:12PM - 5:24PM Live |
J42.00012: Flat bands in twisted transition metal dichalcogenide bilayers C Stephen Hellberg, Madeleine Phillips 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 moiré pattern [2]. We present the results of density functional theory (DFT) calculations that model the electronic properties of MoSe2/WSe2 bilayers with a small twist away from commensurate 180-degree stacking. We present band structures and local densities of states that exhibit flat bands in the gap near the band edges. |
Tuesday, March 16, 2021 5:24PM - 5:36PM Live |
J42.00013: Twisted bilayer WSe2 (II): Quantum phase diagram of a Moiré-Hubbard model Fengcheng Wu, Haining Pan We theoretically study a generalized Hubbard model on moiré superlattices of twisted bilayers, and find very rich filling-factor-dependent quantum phase diagrams tuned by interaction strength and twist angle. Strong long-range Coulomb interaction in the moiré-Hubbard model induces Wigner crystals at a series of fractional filling factors. The effective lattice of the Wigner crystal is controlled by the filling factor, and can be triangle, rectangle, honeycomb, kagome, etc., providing a single platform to realize many different spin models on various lattices by simply tuning carrier density. In addition to Wigner crystals that are topologically trivial, interaction-induced Chern insulators emerge in the phase diagram. This finding paves a way for engineering interaction-induced quantum anomalous Hall effect in moiré-Hubbard systems where the corresponding single-particle moiré band is topologically trivial. |
Tuesday, March 16, 2021 5:36PM - 5:48PM Live |
J42.00014: Twisted bilayer physics in the square lattice Fan Cui, Qiang Zhang, Congcong Le, Ching-Kai Chiu, Jiangping Hu Twisted bilayer graphene brings the flatness of the Moire energy bands serving a new playground for strong correlations and topological phases. Beyond the hexagonal lattice, a similar twisted platform might emerge in other crystal structures. We explore twisted bilayer physics in different types of lattices. In particular, we focus on the twisted physics in the square lattice and propose material realizations. |
Tuesday, March 16, 2021 5:48PM - 6:00PM Live |
J42.00015: Accurate ab-initio tight-binding model for twisted transition metal dichalcogenide bilayers Kemal Atalar, Valerio Vitale, Johannes Lischner, Arash A Mostofi The discovery of correlated and superconducting states in magic-angle twisted bilayer graphene has generated interest in twisted heterostructures composed of other 2D materials. For example, signatures of superconductivity [1-2] and exotic optical behaviour [3] have been observed recently in twisted bilayers of transition metal dichalcogenides (TMDs). |
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