Bulletin of the American Physical Society
APS March Meeting 2021
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session P57: 2D Materials: Metals, Superconductors, and Correlated Materials  3Live

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Sponsoring Units: DMP Chair: Jörn Venderbos, Drexel Univ 
Wednesday, March 17, 2021 3:00PM  3:12PM Live 
P57.00001: Coexistence of superconductivity and exciton condensation in graphene multilayers Igor Blinov, Allan MacDonald Recent moiré material experiments have demonstrated that twisted bilayer graphene can be a superconductor 
Wednesday, March 17, 2021 3:12PM  3:24PM Live 
P57.00002: Firstprinciples strategy for electronphonon coupling in twisted bilayer graphene Zhenglu Li, Mit Naik, Yanghao Chan, Meng Wu, Steven G Louie Twisted bilayer graphene (tBLG) systems at certain twisted angles host exotic phenomena such as superconductivity and correlated insulator state. One key question is whether the superconductivity in such systems is phonondriven or not. However, a moiré unit cell at small twisted angles usually contains over ten thousand of atoms, limiting any quantitative firstprinciples investigations on the electronphonon (eph) coupling in tBLG. Here we propose a firstprinciples strategy to accurately compute the eph coupling in tBLG where the eph matrix elements in the moiré cells are mapped to those in fewatom unitcells, with controlled approximations. The eph coupling is computed using densityfunctional perturbation theory and GW perturbation theory [1], with the latter includes electron correlation effects in the eph matrix elements. This strategy can be generalized to other interactions in different moiré systems while incorporating both intralayer and interlayer effects. 
Wednesday, March 17, 2021 3:24PM  3:36PM Live 
P57.00003: Dirac Fermion Renormalization near Topological Lifshitz Transitions Valeri Kotov, Bruno Uchoa, Oleg Sushkov We study Coulomb interaction effects near a Lifshitz transition, where two Dirac cones with winding numbers +1 and 1 annihilate to form a semiDirac dispersion, with quadratic momentum dependence in one direction and linear in the other (winding number 0). The longrange Coulomb interaction leads to an extraordinary strong spectrum renormalization at weak coupling, with unconventional log squared contributions proliferating even at first order in the interaction. We show that the renormalization group (RG) analysis, which effectively resums these logs, leads to restoration of the linear dispersion in all directions at low energies. These exotic renormalized Dirac fermions are effectively anisotropic Dirac particles with winding number 0, as required by topological arguments and confirmed by our calculations. Moreover, our weak coupling RG fixed point is charactered by longrange Coulomb vertex where screening is virtually nonexistent. Hence, the fixed point is not continuously connected to the regime of “strongcoupling”, or ‘’largeN”, where nonFermi liquid scenarios has been previously advocated. Therefore the analysis of experimental data for various semiDirac materials would depend strongly on the intrinsic value of the Coulomb coupling constant. 
Wednesday, March 17, 2021 3:36PM  3:48PM Live 
P57.00004: Lowfrequency Quantum Oscillations in Bilayers and Quasitwodimensional Systems Andrew Allocca, Nigel R Cooper In a quasitwodimensional (Q2D) system, the weak coupling of two dimensional layers produces a Fermi surface with crosssectional area that modulates along the direction of stacking. In agreement with the predictions of standard LifshitzKosevich theory, quantum oscillation studies on this sort of system see clear oscillations at two frequencies corresponding to the extremal areas of the Fermi surface cross section. However, recent experimental studies have found additional slow oscillations at the frequency corresponding to the difference of these high frequencies, which suggests effects beyond LifshitzKosevich theory. Here we explore the causes of such low frequency oscillations. We first examine a simple bilayer model then extend the analysis to infinitelayer Q2D systems, finding that a sufficient ingredient to produce this phenomenon in both cases is to fix the total particle number in the system rather than the chemical potential, as is standard theoretical practice. We also examine the role of interlayer interactions. 
Wednesday, March 17, 2021 3:48PM  4:00PM 
P57.00005: Deconfinement of Mott localized electrons into topological and spin–orbitcoupled Dirac fermions José Pizarro, Severino Elia Adler, Karim Zantout, Thomas Mertz, Paolo Barone, Roser Valenti, Giorgio Sangiovanni, Tim Wehling We show that stacking 1TTaSe_{2} into bilayers can deconfine electrons from a deep Mott insulating state in the monolayer to a system of correlated Dirac fermions subject to sizable spin–orbit coupling in the bilayer. 1TTaSe_{2} develops a StarofDavid charge density wave pattern in each layer. When the StarofDavid centers belonging to two adjacent layers are stacked in a honeycomb pattern, the system realizes a generalized Kane–Mele–Hubbard model in a regime where Dirac semimetallic states are subject to significant Mott–Hubbard interactions and spin–orbit coupling. At charge neutrality, the system is close to a quantum phase transition between a quantum spin Hall and an antiferromagnetic insulator. We identify a perpendicular electric field and the twisting angle as two knobs to control topology and spin–orbit coupling in the system. Their combination can drive it across hitherto unexplored grounds of correlated electron physics, including a quantum tricritical point and an exotic firstorder topological phase transition. 
Wednesday, March 17, 2021 4:00PM  4:12PM Live 
P57.00006: Competing orders at highorder Van Hove points: doped graphene Laura Classen, Andrey Chubukov, Carsten Honerkamp, Michael M Scherer Van Hove points are special points in the energy dispersion, where the density of states exhibits singularities. When a Van Hove point is close to the Fermi level, tendencies towards density wave, Pomeranchuk, and superconducting orders can all be enhanced leading to unconventional ground states. Here we consider effects from highorder Van Hove points, around which the dispersion is exceptionally flat and the density of states has a powerlaw divergence. We argue that such points are present in intercalated graphene and other materials. We use an effective lowenergy model for electrons near highorder Van Hove points and analyze the competition between different ordering tendencies using an unbiased renormalization group approach. For purely repulsive interactions, we find that two key competitors are ferromagnetism and chiral superconductivity. For a small attractive exchange interaction, we find a new type of spin Pomeranchuk order, in which the spin order parameter winds around the Fermi surface. The supermetal state, predicted for a single highorder Van Hove point, is an unstable fixed point in our case. 
Wednesday, March 17, 2021 4:12PM  4:24PM Live 
P57.00007: Weak Coupling Instabilities in Chern Bands with Multiple Van Hove Singularities Daniel Shaffer, Luiz Santos, Jian Wang With the advent of fabrication of systems with nanometer scale superlattices, such as nanopatterned superlattices and moiré systems, large magnetic fluxes Φ per unit cell (on the order of the flux quantum Φ_{0} = h/e) have become accessible. Such systems exhibit the Hofstadter single particle spectrum composed of Chern bands that generalize the Landau levels, raising the question of potential instabilities due to interactions. We consider systems with square and hexagonal lattices with Φ= (p/q) Φ_{0} for various coprime integers p and q that have 2q and 3q Van Hover singularities in each Chern band respectively at which the density of states diverges. We further assume that the chemical potential is tuned to the Van Hove singularities, which corresponds to halffilled Landau levels in the limit of infinite q. We study the resulting instabilities using a parquet renormalization group approach and analyze the competition between superconductivity and charge and spin density wave instabilities. 
Wednesday, March 17, 2021 4:24PM  4:36PM Live 
P57.00008: Ab initio phonon selfenergies and fluctuation diagnostics of phonon anomalies: Lattice instabilities from Dirac pseudospin physics in transition metal dichalcogenides Jan Berges, Erik van Loon, Arne Schobert, Malte Roesner, Tim Wehling We present an ab initio approach for the calculation of phonon selfenergies and their fluctuation diagnostics, which allows us to identify the electronic processes behind phonon anomalies. Application to the transitionmetaldichalcogenide monolayer 1HTaS_{2} reveals that coupling between the longitudinal–acoustic phonons and the electrons from an isolated lowenergy metallic band is entirely responsible for phonon anomalies such as the mode softening and associated chargedensity waves observed in this material. Our analysis allows us to distinguish between different modesoftening mechanisms including matrixelement effects, Fermisurface nesting, and Van Hove scenarios. We find that matrixelement effects originating from a peculiar type of Dirac pseudospin textures control the chargedensitywave physics in 1HTaS_{2} and similar transition metal dichalcogenides. 
Wednesday, March 17, 2021 4:36PM  4:48PM Live 
P57.00009: MagicAngle Twisted Trilayer Graphene: Part 1 Jeong Min Park, Yuan Cao, Kenji Watanabe, Takashi Taniguchi, Pablo JarilloHerrero Moiré superlattices have recently become a playground for exploring correlated physics and superconductivity. Despite the presence of correlated effects in several moiré systems, magicangle twisted bilayer graphene has remained the only one with robust superconductivity. Here we present a new moiré superconductor, magicangle twisted trilayer graphene with dramatically richer properties and tunability. By exploring the entire phase space as a function of carrier density, electric and magnetic fields, and temperature, we determine the system’s tunable phase boundaries and reveal the intimate connection between the superconducting state and the broken symmetry phase at two carriers per moiré unit cell. The suppression and bounding of superconductivity at the Van Hove singularities is difficult to reconcile with the weakcoupling BCS theory. More strikingly, we can tune the system to be in the ultrastrong coupling regime close to the twodimensional BCSBEC crossover, where the GinzburgLandau coherence length reaches the average interparticle distance and T_BKT/T_F ratios are in excess of 0.1. Our system establishes a new generation of moiré platform where we can investigate correlated states, strong coupling superconductivity, and more, with unprecedented tunability. 
Wednesday, March 17, 2021 4:48PM  5:00PM Live 
P57.00010: MagicAngle Twisted Trilayer Graphene: Part 2 Yuan Cao, Jeong Min Park, Kenji Watanabe, Takashi Taniguchi, Pablo JarilloHerrero Moiré superlattices have recently become a playground for exploring correlated physics and superconductivity. Despite the presence of correlated effects in several moiré systems, magicangle twisted bilayer graphene has remained the only one with robust superconductivity. Here we present a new moiré superconductor, magicangle twisted trilayer graphene with dramatically richer properties and tunability. By exploring the entire phase space as a function of carrier density, electric and magnetic fields, and temperature, we determine the system’s tunable phase boundaries and reveal the intimate connection between the superconducting state and the broken symmetry phase at two carriers per moiré unit cell. The suppression and bounding of superconductivity at the Van Hove singularities is difficult to reconcile with the weakcoupling BCS theory. More strikingly, we can tune the system to be in the ultrastrong coupling regime close to the twodimensional BCSBEC crossover, where the GinzburgLandau coherence length reaches the average interparticle distance and T_BKT/T_F ratios are in excess of 0.1. Our system establishes a new generation of moiré platform where we can investigate correlated states, strong coupling superconductivity, and more, with unprecedented tunability. 
Wednesday, March 17, 2021 5:00PM  5:12PM Live 
P57.00011: Effective Extended BoseHubbard Model for Helium on Graphene Jiangyong Yu, Mohamed Marwan Elsayed, Kenneth Shepherd Jr, Ethan Lauricella, Todd Lombardi, Sang Wook Kim, Juan M Vanegas, Taras Lakoba, Carlos Wexler, Valeri Kotov, Adrian G Del Maestro The possibility of twodimensional (2D) helium superfluidity mitigated by adsorption on novel 2D quantum materials, is an exciting development. For helium on graphene, superfluidity competes with insulating (1/3 filled) states, and the phase boundary between them is highly sensitive to the details of the van der Waals interactions between system components. We present a mapping of the full, complex manybody problem onto an effective extended BoseHubbard (tVV’) model, which describes how the Helium atoms hop (t) on the graphene lattice and interact at nearestneighbor (V) and nextnearest neighbor (V’) sites. Helium atoms behave effectively as hardcore bosons (U=∞) and V/t controls the nature of the emergent manybody state. We compare the results of a variety of accurate techniques: largescale Monte Carlo simulations, band structure calculations and abinitio calculations on finite systems. We find that V/t is large enough across all techniques to favor the insulating 1/3filled state. Therefore our effective model provides an accurate description which can be also used as a starting point in more complex situations where atomic and materials parameters are modified and consequently new phases could emerge. 
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