Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session M57: Correlated Phases in Moire bilayers and Monolayer grapheneFocus
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Sponsoring Units: DMP Chair: Matthew Yankowitz, Columbia University Room: Mile High Ballroom 3A |
Wednesday, March 4, 2020 11:15AM - 11:27AM |
M57.00001: Tunneling Spectroscopy of Superconducting States in Magic-Angle Twisted Bilayer Graphene Jeong Min Park, Yuan Cao, Daniel Rodan-Legrain, Kenji Watanabe, Takashi Taniguchi, Pablo Jarillo-Herrero The discovery of correlated insulating states and unconventional superconductivity in the twisted bilayer graphene (TBG) systems has instigated numerous follow-up experiments and theories in the past year. However, the origin of the superconducting states, including the nature of the correlated insulating states, parent metallic states, pairing mechanism, size of the superconducting gap, etc. still remain greatly unknown. Recent scanning tunneling spectroscopy experiments have analyzed the local density of states in the insulating states, revealing the splitting of the van Hove singularities near half-filling and the breaking of rotational symmetry. Here, we develop highly tunable magic-angle TBG systems with various gate geometries that allow several different tunneling transport regimes within each device. The results of our tunneling spectroscopy experiments on the superconducting states are presented. |
Wednesday, March 4, 2020 11:27AM - 11:39AM |
M57.00002: Nature of superconductivity in highly-homogeneous twisted bilayer graphene Yu Saito, Jingyuan Ge, Takashi Taniguchi, Kenji Watanabe, Andrea Young Twisted bilayer graphene with a twist angle of around 1.1 degree hosts superconducting, ferromagnetic and insulating states at partial band filling, all of which are associated with the formation of a nearly-flat electronic band. The experimental difficulty of this system is to fabricate highly angle-homogeneous samples, a necessary prerequisite for exploring the intrinsic properties. In this talk, I will report the results of transport measurements in several homogeneous twisted bilayer graphene at or near the flat band condition. We find that superconductivity near negative half-filling robustly appears regardless of the existence of a resistive state while the optimal critical temperature is very sensitive to the twist angle and is maximized around 1.1 degree. |
Wednesday, March 4, 2020 11:39AM - 11:51AM |
M57.00003: Proximity induced Josephson effect in twisted bilayer graphene near the magic angle Yiran Zhang, Robert Polski, Harpreet Singh Arora, Youngjoon Choi, Hechen Ren, Kenji Watanabe, Takashi Taniguchi, Stevan Nadj-Perge Twisted bilayer graphene (TBG) near the magic angle (~1.1°) has been established to be a highly tunable platform that hosts superconducting, ferromagnetic and correlated insulating states associated with the formation of flat bands. Though numerous models have been proposed to explain basic physics of flat bands and correlations in this system, many questions related the precise origin of insulating and superconducting states are still elusive. Here we report on the observation of the Josephson effect in correlated states of TBG near the magic angle. The detailed measurements of critical current as a function of gate voltage, temperature and magnetic field provide novel insights into the interplay between superconductivity and insulating states in this correlated system. |
Wednesday, March 4, 2020 11:51AM - 12:03PM |
M57.00004: On the electronic phases of magically twisted bilayer graphene Chuan Chen, Antonio Helio Castro Neto, Vitor Manuel Pereira We explore in detail the electronic phases of a system consisting of three non-colinear arrays of coupled quantum wires, each rotated 120 degrees with respect to the next. A perturbative renormalization-group analysis reveals that multiple correlated states can be stabilized: a smectic or d ± id superconductor, a charge density wave insulator, a two-dimensional Fermi liquid, and a 2D Luttinger liquid (also known as smectic metal or sliding Luttinger liquid). The model provides an effective description of electronic interactions in small-angle twisted bilayer graphene and we discuss its implications in relation to the recent observation of correlated and superconducting ground states near commensurate densities, as well as the “strange metal” behaviour at finite temperatures as a natural outcome of the 2D Luttinger liquid phase. |
Wednesday, March 4, 2020 12:03PM - 12:15PM |
M57.00005: Interacting domain-wall network model in twisted bilayer graphene Yang-Zhi Chou The interacting domain-wall network systems, which are naturally realized in the twisted bilayer graphene (TBLG) at sub-degree twist angles, are discussed in this talk. Motivated by the superconductor-insulator transition in the TBLG at the magic angle, a simplified model that is composed of a collection of crossed one-dimensional quantum wires whose intersections form a superlattice. At each superlattice point, we place a locally superconducting puddle which can exchange Cooper pairs with the quantum wires. We show that for a range of repulsive intrawire interactions, the system is superconducting at `generic' incommensurate fillings, with the superconductivity being `interrupted' by an insulating phase at commensurate fillings. We further show that the gapped insulating states at commensurate fillings give way to gapless states upon application of Zeeman fields. These features are consistent with experimental observations in magic-angle TBLG despite the distinct microscopic details. We also discuss novel phases driven by the interplay of small velocity and network structure. |
Wednesday, March 4, 2020 12:15PM - 12:27PM |
M57.00006: Prominent Cooper Pairing Away From the Fermi Level and its Spectroscopic Signature in Twisted Bilayer Graphene Alex Aperis, Fabian Schrodi, Peter Oppeneer We investigate phonon-mediated Cooper pairing in flat electronic band systems by solving the full-bandwidth multiband Eliashberg equations for superconductivity in magic angle twisted bilayer graphene using a realistic tight-binding model [1]. We find that Cooper pairing away from the Fermi level [2] contributes decisively to superconductivity by enhancing the critical temperature and ensures a robust finite superfluid density. We show that this pairing yields particle-hole asymmetric superconducting domes in the temperature--gating phase diagram and gives rise to distinct spectroscopic signatures in the superconducting state. We predict several such features in tunneling and angle resolved photoemission spectra for future experiments. |
Wednesday, March 4, 2020 12:27PM - 1:03PM |
M57.00007: Quantum simulation of the Hubbard model in a moiré superlattice Invited Speaker: Kin Fai Mak The Hubbard model, first formulated by physicist John Hubbard in the 1960s, is a simple theoretical model of interacting quantum particles in a lattice. The model is thought to capture the essential physics of high-temperature superconductors, magnetic insulators, and other complex emergent quantum many-body ground states. Although the Hubbard model is greatly simplified as a representation of most real materials, it has nevertheless proved difficult to solve accurately except in the one-dimensional case. Physical realizations of the Hubbard model in two or three dimensions, which can act as quantum simulators, therefore have a vital role to play in solving the strong-correlation puzzle. In this talk, I will discuss a recent experimental realization of the two-dimensional triangular lattice Hubbard model in angle-aligned WSe2/WS2 bilayers, which form moiré superlattices because of the difference in lattice constant between the two 2D materials. We obtain a quantum phase diagram of the two-dimensional triangular lattice Hubbard model near the half filling by probing both the charge and magnetic order of the system. Implications for future studies will also be discussed. |
Wednesday, March 4, 2020 1:03PM - 1:15PM |
M57.00008: Imaging Viscous Flow of the Dirac Fluid in Graphene Using a Quantum Spin Magnetometer Mark Jen-Hao Ku, Tony Zhou, Qing Li, Young J. Shin, Jing Shi, Claire Burch, Huiliang Zhang, Francesco Casola, Yonglong Xie, Andrew Pierce, Takashi Taniguchi, Kenji Watanabe, Philip Kim, Amir Yacoby, Ronald L Walsworth The electron-hole plasma in charge-neutral graphene, known as the Dirac fluid, is predicted to realize a quantum critical system whose transport features a universal hydrodynamic description, even at room temperature. In this work[1], we directly image viscous Dirac fluid flow at room temperature via measurement of the associated stray magnetic field. Nanoscale magnetic imaging[2,3] is performed using quantum spin magnetometers realized with NV centers in diamond. Our measurement reveals a parabolic Poiseuille profile for electron flow in a graphene channel near the charge neutrality point, establishing the viscous transport of the Dirac fluid. Measurement of viscosity indicates that a nearly-ideal electron fluid presides in graphene at room temperature. Our results pave the way to study hydrodynamic transport in quantum critical fluids relevant to strongly-correlated electrons in superconductors. |
Wednesday, March 4, 2020 1:15PM - 1:27PM |
M57.00009: Mott insulation and superconductivity in the cluster Hubbard model for magic-angle twisted bilayer graphene Shin-Ming Huang, Yi-Ping Huang, Ting-Kuo Lee We investigate the strongly correlated electronic system in magic-angle twisted bilayer graphene. Owing to the extended figure of Wannier orbitals, we study the two-orbital cluster Hubbard model with spin-valley fourfold degeneracy, focusing around half filling of valence bands below the neutrality point. The theory shows relatively impotent long-ranged hoppings after renormalization and predicts multiple Mott insulator phases at fractional filling, not only for integer charges per moire site. From second-order perturbation, spin-valley fluctuations give rise to the pairing attraction, exhibiting superconducting domes adjacent to Mott insulator phases. The cluster interaction generates high entanglement among clusters, implying plenty possibilities of nontrivial states. |
Wednesday, March 4, 2020 1:27PM - 1:39PM |
M57.00010: Chiral symmetry breaking induced gap and flat band in a correlated graphene Changhua Bao, Hongyun Zhang, Laipeng Luo, Shaohua Zhou, Qian Li, Wei Yao, Shuyun Zhou The low-energy excitations of graphene are relativistic massless Dirac fermions with opposite chiralities at K and K' valleys. Breaking the chiral symmetry could lead to gap opening (in analogy to mass generation in quantum electrodynamics) and other intriguing phenomena. Despite many theoretical predictions, so far experimental realization of chiral symmetry breaking (CSB) in a real honeycomb lattice has been missing. Here I will present our recent experimental work on graphene with large CSB induced gap. In addition, a flat band is observed near the Fermi energy, opening up new opportunity for investigating correlated physics and interaction driven superconductivity. |
Wednesday, March 4, 2020 1:39PM - 1:51PM |
M57.00011: Phonon-mediated superconductivity in graphene and hexagonal boron nitride Even Thingstad, Akashdeep Kamra, Justin Wells, Asle Sudbo Insight into why superconductivity in pristine and doped monolayer graphene seems strongly suppressed has been central for the recent years' various creative approaches to realize superconductivity in graphene and graphene-like systems. We provide further insight by studying superconductivity in doped monolayer graphene based on intrinsic phonon modes and solving the gap equation using a detailed model for the effective attraction based on electron tight binding and phonon force constant models. The various system parameters can be tuned at will, and our results show that the Coulomb interaction induces non-uniform gap textures along the Fermi surface and plays a main role in suppressing superconductivity at realistic dopings. We also perform similar calculations in the gapped hexagonal boron nitride (h-BN), which has direct onset of a large density of states. Somewhat counter-intuitively, however, the dimensionless electron-phonon coupling strength cannot capitalize on this for small charge dopings. |
Wednesday, March 4, 2020 1:51PM - 2:03PM |
M57.00012: Ultrafast dynamics of electron-phonon coupling in correlated graphene revealed by time- and angle-resolved photoemission spectroscopy Hongyun Zhang, Changhua Bao, Shaohua Zhou, Laipeng Luo, Qian Li, Shuyun Zhou Electron-phonon coupling plays important roles in many intriguing physics, including superconductivity, charge and spin density waves. Graphene is a model material with novel electronic properties and provide an ideal platform to study electron-phonon interactions. Despite many researches on non-equilibrium graphene electron system, the direct observation of momentum-resolved electron-phonon coupling dynamics is still missing. Here, by using time- and angle-resolved photoemission spectroscopy, we directly observe multiple electron-phonon coupling and ultrafast dynamics in correlated graphene. In addition, we observe significant suppression of decay rate induced by self-energy effects, which provide important information for controlling and understanding many-body interactions in graphene. |
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M57.00013: Gate tunable exciton superfluid and topological phases in van der Waals heterobilayer Qizhong Zhu, Wei-Yuan Tu, Qingjun Tong, Huazheng Sun, Yong Wang, Wang Yao Van der Waals heterostructures of 2D materials provide a powerful approach toward engineering various quantum phases of matter. In one work, we show that heterobilayers of 2D valley semiconductors can be tuned through interlayer bias between an exciton superfluid, a quantum anomalous Hall insulator, and a QSH insulator. The tunability between these distinct phases results from the competition of Coulomb interaction with the interlayer quantum tunneling that has a chiral form in valley semiconductors. In another work, we realize electrically tunable QSH insulator with large gap in van der Waals heterobilayer of monolayer transition metal dichalcogenide and hexagonal boron arsenide (BAs), in particular the WSe2/BAs heterobilayer. Our findings point to exciting opportunities for harnessing both protected topological edge channels and bulk superfluidity in an electrically configurable platform. |
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