Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session B51: Graphene: Electronic Structure and Interactions Interactions: Moire, Correlations, and Topology 
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Sponsoring Units: DCMP DMP Chair: Onder Gul, Harvard University Room: Mile High Ballroom 1D 
Monday, March 2, 2020 11:15AM  11:27AM 
B51.00001: Ground State of Magic Angle Twisted Bilayer Graphene at Charge Neutrality II Shang Liu, Nick Bultinck, Eslam Khalaf, Shubhayu Chatterjee, Ashvin Vishwanath, Michael Zaletel In magic angle twisted bilayer graphene (MATBG), interactions dominate over the kinetic energy, resulting in correlated insulating states and superconductivity whose origin remains a mystery. Here, using a combination of analytical arguments and numerical calculations we identify a family of candidate insulating ground states at charge neutrality and discuss how external perturbations such as the influence of the substrate or strain can alter the phase diagram. 
Monday, March 2, 2020 11:27AM  11:39AM 
B51.00002: Marginal Fermi liquid and dynamical symmetry breaking from Coulomb interaction in twisted bilayer graphene Jose Gonzalez, Tobias Stauber We investigate the effects of the strong Coulomb interaction near the magic angle of twisted bilayer graphene, focusing on the charge neutrality point and near halffilling of the highest valence band. In this latter instance, we predict the emergence of a marginal Fermi liquid, which can be traced back to the proximity to an extended van Hove singularity and the development of straight segments in the Fermi line. This leads to the linear scaling with energy of particlehole excitations across the Fermi line, implying in turn the linear temperature dependence of the resistivity, logarithmic corrections to the heat capacity, and the consequent modification of the WiedemannFranz law. At the charge neutrality point, we show that the Coulomb interaction may be responsible for the opening of a gap through the condensation of particlehole excitations about the Dirac nodes. We find in this case a direct competition between the dynamical breakdown of chiral symmetry (which prevails in the strong coupling regime) and the breakdown of timereversal invariance (which has instead a stronger onset and leads to a Chern insulator phase at intermediate coupling). 
Monday, March 2, 2020 11:39AM  11:51AM 
B51.00003: Electrostatic gatecontrolled fermi level dependent electronic band structure of graphene Ryan Muzzio, Alfred Jones, Roland Koch, Davide Curcio, Deepnarayan biswas, Jill Miwa, Philip Hofmann, Simranjeet Singh, Chris Jozwiak, Eli Rotenberg, Aaron Bostwick, Søren Ulstrup, Jyoti Katoch The observation of electronic band structure in 2D material devices as they operate opens limitless opportunities to explore fundamental physics. The most robust technique to probe electronic structure is through angleresolved photoemission spectroscopy (ARPES) and can now be used to investigate mesoscopic sized 2D materials and their heterostructures. Only being sensitive to filled states, ARPES measurements are restricted to states below the Fermi energy of the sample. Historically, chemical doping has been the preferred technique to manipulate the Fermi energy, but is difficult to control, introduces electronic states that are not intrinsic to the sample of interest, and can only be reversed through hightemperature annealing. Electrostatic doping, having none of these drawbacks, allows us to reversibly investigate the electron energy and momentum states of graphene far above the Dirac point. We will present our results on the electrostatic gate controlled electronic band structure of graphene using nanoARPES. 
Monday, March 2, 2020 11:51AM  12:03PM 
B51.00004: Correlated states in magic angle twisted bilayer graphene under the optical conductivity scrutiny Elena Bascones, María José Calderón Moiré systems displaying flat bands have emerged as novel platforms to study correlated electron physics. There is evidence of correlation induced effects at the charge neutrality point (CNP) of twisted bilayer graphene which could originate from spontaneous symmetry breaking. We will show how optical conductivity measurements allow to distinguish different symmetry breaking states and to follow the evolution of the band distortions with doping. In the specific case of a nematic order, which breaks the discrete rotational symmetry of the lattice, we find that the Dirac cones tend to be displaced, not only in momentum space but also in energy, inducing a finite density of states and Drude weight at the CNP. The nematic order is revealed in a dc conductivity anisotropy with its sign depending on the degree of relaxation, the doping and the nature of the symmetry breaking 
Monday, March 2, 2020 12:03PM  12:15PM 
B51.00005: Correlated states and tunable topological bands in twisted monolayerbilayer graphene heterostructures Hryhoriy Polshyn, Jihang Zhu, Manish Kumar, Takashi Taniguchi, Kenji Watanabe, Allan MacDonald, Andrea Young We experimentally investigate twisted van der Waals heterostructures of monolayer graphene rotated with respect to a bernal stacked graphene bilayer. We report transport measurements for devices with twist angles between 0.9 and 1.4°. The electric field allows efficient tuning of the width, isolation and the topology of the moiré bands in this system. By comparing magnetoresistance measurements to numerical simulations, we develop an understanding of the band structure. Finally, we observe correlated states at half and quarterfillings, which arise when narrow moire sublattice band is isolated by energy gaps from dispersive bands. We investigate the effects of inplane and outofplane magnetic field on these states and discuss the implication for their spin and valley polarization. 
Monday, March 2, 2020 12:15PM  12:27PM 
B51.00006: Hartree Fock study of insulating states in twisted bilayer graphene using hybrid Wannier basis Kasra Hejazi, Xiao Chen, Leon Balents In this work, the insulating behavior of twisted bilayer graphene and in particular the quantum anomalous Hall state has been studied; the effects of electronelectron interaction are taken into account by performing a HartreeFock analysis which is the standard approach for finding symmetry broken states. The model is truncated to the active relevant bands and the study is done using the basis of maximally localized Hybrid Wannier functions; this basis on the one hand respects important symmetries of the system, and on the other hand makes the interaction local at least in one direction. It also implies that the geometry of the system is that of a cylinder. Furthermore, by tuning the form/strength of the interaction, we are able to predict some other interesting phases which might potentially be found in the future. 
Monday, March 2, 2020 12:27PM  12:39PM 
B51.00007: Local electronic compressibility of magic angle twisted bilayer graphene Uri Zondiner, Asaf Rozen, Daniel Rodan, Yuan Cao, Raquel Queiroz, Takashi Taniguchi, Kenji Watanabe, Yuval Oreg, Felix von Oppen, Ady Stern, Erez Berg, Pablo JarilloHerrero, Shahal Ilani Twisted bilayer graphene near the magic angle (MATBG) exhibits remarkably rich electron correlation physics, displaying insulating, magnetic, and superconducting phases. In this talk, we probe the local electronic compressibility of MATBG using a scanning single electron transistor. We find that when carriers are added into this system, they repeatedly refill the same bands, leading to a Diraclike compressibility cascade near integer moiré fillings. Additionally, we present a clear compressibility asymmetry around the center of the conduction and valence bands, reflecting the evolution from low DOS at charge neutrality to high DOS at the band edges. A meanfield theory that includes this asymmetry reproduces the experimental results, and explains previous observations that were not yet fully understood. This cascade of Dirac revivals should thus serve as the starting point for understanding the unusual behavior in this system. 
Monday, March 2, 2020 12:39PM  12:51PM 
B51.00008: Towards correlated electron states in trilayergraphene Moiré band KanTing Tsai, Xi Zhang, Ziyan Zhu, Stephen Carr, Mitchell Luskin, Philip Kim, Efthimios Kaxiras, Ke Wang The recent discovery of Moiré superconductivity in twistedbilayer graphene (tBLG) and twisteddouble bilayergraphene (tDBG) systems [1][2][3] has generated a great amount of interest in the condensed matter community. Beyond this model system, this work focuses on studying a new Moiré platform based on the twisted trilayergraphene. Preliminary magnetotransport measurements will be presented. Characterization of interlayer screening, band evolution with displacement field and Moiré periodicity will also be discussed. 
Monday, March 2, 2020 12:51PM  1:03PM 
B51.00009: Band flattening in slightly twisted bilayers of Bravais networks Toshikaze Kariyado, Ashvin Vishwanath Discovery of superconductivity in twisted bilayer graphene ignited the current intensive studies on stacked 2D materials with moire patterns, especially focusing on flat bands and associated correlated physics. Here, we demonstrate band flattening in twisted bilayers of generic but high symmetric 2D lattices, i.e., 2D Bravais networks [1]. We first show how symmetry of each network gives constraints on the effective potential governing lowenergy physics in twisted bilayers. We further numerically demonstrate the band flattening due to the constrained potential using tightbinding models. From the generic theory, we can find an interesting possibility of anisotropic band flattening, in which quasi 1D band dispersion is generated from relatively isotropic original band dispersion. Rich physics is expected with the anisotropic band flattening, ranging from the valley dependent transport to the spinorbital intertwined model in the strongly correlated limit. 
Monday, March 2, 2020 1:03PM  1:15PM 
B51.00010: Twist disorder in twisted bilayer graphene Justin Wilson, Yixing Fu, Sankar Das Sarma, Jed Pixley Recent experiments in twisted bilayer graphene have set off a flurry of work due to the observation of purportedly correlated phases at the socalled ``magicangle.'' The nearly pristine samples of graphene used in the experiment mainly have one significant source of disorder: the twist angle is not uniform throughout the sample. Current models in the literature are inadequate to fully capture this effect, so we introduce and study a new microscopic model in which the twist angle enters as a free parameter in real space. After benchmarking the results of the model with the continuum model, we simulate the effects of twistangle disorder by constructing “patches” of uniform twist angle. We find that while the minibandwidth and gap are renormalized substantially, the Fermi velocity is not significantly altered. We discuss the implications of this for existing and ongoing experiments on twisted bilayer graphene. 
Monday, March 2, 2020 1:15PM  1:27PM 
B51.00011: HigherOrder Topological Insulator in Twisted Bilayer Graphene Moon Jip Park, Youngkuk Kim, Gil Young Cho, SungBin Lee Higherorder topological insulators are newly proposed topological phases of matter, whose bulk topology manifests as localized modes at twoor higherdimensional lower boundaries. In this work, we propose the twisted bilayer graphenes with large angles as higherorder topological insulators, hosting topological corner charges. At large commensurate angles, the intervalley scattering opens up the bulk gap and the corner states occur at half filling. Based on both firstprinciples calculations and analytic analysis, we show the striking results that the emergence of the corner states do not depend on the choice of the specific angles as long as the underlying symmetries are intact. Our results show that the twisted bilayer graphene can serve as a robust candidate material of twodimensional higherorder topological insulator. 
Monday, March 2, 2020 1:27PM  1:39PM 
B51.00012: Topological flat bands without magic angles in massive twisted bilayer graphenes Srivani Javvaji, JinHua Sun, Jeil Jung We show that in massive twisted bilayer graphenes, isolated nearly flat bands below a threshold bandwidth W_{c} are expected for continuous small twist angles up to a critical θ_{c} depending on the flatness of the original bands and the interlayer coupling strength. The phase diagram of finite valley Chern numbers of the isolated moire bands expands with increasing difference between the sublattice selective interlayer tunneling parameters. The valley contrasting circular dichroism for interband optical transitions is constructive near 0° and destructive near 60° alignments and can be tuned through electric field and gate driven polarization of the minivalleys. Combining massive Dirac materials with various intrinsic gaps, Fermi velocities, interlayer tunneling strengths suggest optimistic prospects of increasing θ_{c} and achieving correlated states with large U/W effective interaction versus bandwidth ratios. 
Monday, March 2, 2020 1:39PM  1:51PM 
B51.00013: Attractive electronelectron interactions from internal screening in magic angle twisted bilayer graphene Zachary Goodwin, Arash A Mostofi, Johannes Lischner Twisted bilayer graphene (tBLG) has emerged as a new platform for studying electron correlations. tBLG exhibits correlated insulating and superconducting states at twist angles close to the largest magic angle, with the phase diagram resembling that of cuprates, which suggests an unconventional mechanism for superconductivity. We have studied the effect of internal screening on electronelectron interactions and Hubbard parameters in undoped tBLG, within the random phase approximation (RPA) and constrained RPA (cRPA). Near the magic angle, the flattening of bands drastically increases the RPA dielectric constant of tBLG and the abrupt change in the band velocity as function of the band energy gives rise to attractive regions in the RPA screened interaction in real space, which could be intimately connected to the reported correlatedinsulator and superconducting phases. The cRPA polarizability was used to parametrize a twistangledependent Keldysh model which exhibits a dramatic enhancement with decreasing twist angle, and to calculate the extended Hubbard parameters of a downfolded Hamiltonian. We find that the extended Hubbard parameters depend sensitively on the twist angle and the onsite Hubbard U is a not simply a linear function of twist angle. 
Monday, March 2, 2020 1:51PM  2:03PM 
B51.00014: Twistronics in bilayer graphene and other systems Sujay Ray The discovery of twisted bilayer graphene initiated the idea that a novel platform with unprecedented tunability can be garnered with a simple relative twist between two layers. For graphene systems, this technology has generated a versatile system where flat band, superconductivity and magnetism are intertwined. We present our work on the theory of Wannier pair in twisted bilayer graphene. For comparisons, we also study graphene on boronnitride (GBN) possessing a different Moire pattern, and singlelayer graphene (SLG) without a Moire pattern. We will also present novel properties emanating from similar bilayer systems. 

B51.00015: Symmetry breaking states at commensurate fillings of the Moire bands of the twisted bilayer graphene Yi Zhang, Kun Jiang, Ziqiang Wang, FuChun Zhang We study the twisted bilayer graphene system with the continuous model together with the longrange Coulomb interaction. Within the Hartree Fock approximation, we are able to find some symmetry breaking ground states at commensurate fillings of the Moire bands, which will gap out the Dirac points at the reduced Brillouin Zone that are otherwise protected by the symmetry. 
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