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 X42: Spin Phenomena in 2D SystemsLive
|
Hide Abstracts |
Sponsoring Units: DCMP Chair: Mehmet Noyan, United States Naval Research Laboratory |
Friday, March 19, 2021 8:00AM - 8:12AM Live |
X42.00001: Valley magnetism, density waves, and nematicity in twisted bilayer graphene Dmitry Chichinadze, Laura Classen, Andrey Chubukov We analyze density wave and Pomeranchuk orders in twisted bilayer graphene. We assume that near half filling of either conduction or valence band, the Fermi level is close to Van Hove points, where the density of states diverges, and study potential instabilities in the particle-hole channel within a patch model with two valley degrees of freedom. The hexagonal symmetry of twisted bilayer graphene allows for either six or twelve Van Hove points. We consider both cases and find the same two leading candidates for particle-hole order. One is an SU(2)-breaking spin state with ferromagnetism within a valley. Another state is valley-polarized charge order. In the absence of additional perturbations these terms are degenerate. Introducing degeneracy-lifting terms we consider potential ground states. For the magnetic state, a subleading intervalley hopping induces antiferromagnetism between the valleys. The same state has also been obtained in strong-coupling approaches, indicating that this order is robust. For valley-polarized charge order there is a coexistence phase with magnetism which realizes valley-polarized magnetic order. In addition, we find a weaker but still attractive interaction in nematic channels and discuss the type of a nematic order. |
Friday, March 19, 2021 8:12AM - 8:24AM Live |
X42.00002: Nematic spin density wave in magic-angle twisted bilayer graphene Artem Sboychakov, Alexander Rozhkov, Alexander Rakhmanov, Franco Nori We study theoretically many-body properties of magic-angle twisted bilayer graphene for different doping levels. At vanishing interactions, the low-energy spectrum of the system studied consists of four almost-flat almost-degenerate bands. Electron-electron repulsion lifts this degeneracy. To account for such an interaction effect, a numerical mean-field theory is used. Assuming that the ground state has spin-density-wave-like order, we introduce a multicomponent order parameter. Structure of this order parameter depends on the doping level. We show that doping away from the charge neutrality point reduces the rotational symmetry of the ordered state, indicating the appearance of an electron nematic state. Manifestations of the nematicity can be observed in the spatial distribution of the spin magnetization within a moirè cell, as well as in the single-electron band structure. The nematicity is strongest at half-filling (two extra electron or holes per supercell). We argue that nematic symmetry breaking is a robust feature of the system ground state, stable against model parameters variations. [1] |
Friday, March 19, 2021 8:24AM - 8:36AM Live |
X42.00003: Spin-fluctuation-induced pairing in twisted bilayer graphene Ammon Fischer, Lennart Klebl, Carsten Honerkamp, Dante Kennes We investigate the interplay of magnetic fluctuations and Cooper pairing in twisted bilayer graphene from a purely microscopic model within a large-scale tight-binding approach resolving the Ångström scale. For local onsite repulsive interactions and using the random-phase approximation for spin fluctuations, we derive a microscopic effective pairing interaction that we use for self-consistent solutions of the Bogoliubov-de-Gennes equations of superconductivity. We study the predominant pairing types as function of interaction strength, temperature and band filling. For large regions of this parameter space, we find chiral d-wave pairing regimes, spontaneously breaking time-reversal symmetry, separated by magnetic instabilities at integer band fillings. Interestingly, the d-wave pairing is strongly concentrated in the AA regions of the moiré unit cell and exhibits phase windings of integer multiples of 2π around these superconducting islands, i.e. pinned vortices. The spontaneous circulating current creates a distinctive magnetic field pattern. This signature of the chiral pairing should be measurable by state-of-the-art experimental techniques. |
Friday, March 19, 2021 8:36AM - 8:48AM Live |
X42.00004: Flat Bands in Twisted Bilayer Transition Metal Dichalcogenide with Strong Spin-orbit Interaction Sudipta Kundu, Mit Naik, Hulikal Krishnamurthy, Manish Jain Since the discovery of flat bands at magic angle in twisted bilayer graphene, the formation of flat bands has been predicted for several other twisted bilayers, especially twisted transition metal dichalcogenides. We study the effect of spin-orbit coupling on the formation of flat bands in twisted WSe2, which possesses strong spin-orbit interaction. The spin-orbit interaction affects the flat bands with twist angle close to 0° differently than those near 60°. The flat bands at valence and conduction band edge arise from K and Q points of the unit cell Brillouin zone respectively. For twist angle near 60°, we find that the flattening of the bands arising from K point is a result of the atomic reconstruction in the individual layers and not due to interlayer coupling. On the other hand, for twist angle close to 0°, the interlayer interaction between the two layers becomes important. Additionally, we also find flat bands folded from the Γ point of the unit cell Brillouin zone. The wave function localization of the flat bands matches well with STM measurements from existing literature [1]. Furthermore, we find that the atomistic spin-orbit splitting in moiré remains unchanged from the monolayer. |
Friday, March 19, 2021 8:48AM - 9:00AM Live |
X42.00005: Time-reversal symmetry breaking vs spin-polarized phase from screening of Coulomb interaction in twisted bilayer graphene Jose Gonzalez, Tobias Stauber Relying on a self-consistent Hartree-Fock approach in real space, we discuss the way in which the on-site Hubbard repulsion and the screened long-range Coulomb interaction compete to drive twisted bilayer graphene into opposite symmetry breaking patterns. While keeping constant the Hubbard coupling but allowing for variations in the strength of the screened Coulomb interaction, we obtain in general a phase with time-reversal symmetry breaking at strong coupling, which then gives way to a phase with spin-SU(2) symmetry breaking as the on-site Hubbard interaction prevails for sufficiently large screening. We illustrate this trend in the case of magic-angle twisted bilayer graphene at half-filling of the first valence band, where we find a transition from valley symmetry breaking to spin-SU(2) symmetry breaking as the screening of the Coulomb interaction is increased. A similar competition can also be observed in twisted graphene bilayers with flat bands at larger twist angles under pressure, where the strong-coupling regime of the screened Coulomb interaction favors a Chern insulator phase, which turns into a spin-polarized phase as the Hubbard interaction prevails for very large screening. |
Friday, March 19, 2021 9:00AM - 9:12AM Live |
X42.00006: Valley Spirals in Magnetically Encapsulated Twisted Bilayer Graphene Tobias Wolf, Oded Zilberberg, Johann W. (Gianni) Blatter, Jose Lado Van der Waals heterostructures provide a rich platform for emergent physics due to their tunable hybridization of layers, orbitals, and spin. In this presentation, we show that twisted bilayer graphene stacked between ferromagnetic insulators features flat electronic bands due to the interplay between twist, exchange proximity, and spin-orbit coupling. Crucially, these flat bands are nearly degenerate in valley and are effectively described by a triangular superlattice model. At half-filling, we find that interactions induce spontaneous valley-correlations that favor spiral order and derive a low-energy valley-Heisenberg model with symmetric and antisymmetric exchange couplings. We also show how electric interlayer bias lifts the valley degeneracy and tunes these couplings. Our results put forward magnetic van der Waals heterostructures as platform to explore valley-correlated states in graphene. |
Friday, March 19, 2021 9:12AM - 9:24AM Live |
X42.00007: First-principles calculation of gate-tunable magnetism in twisted bilayer graphene 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-Tc 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 electron/holes doped per moiré unit cell. |
Friday, March 19, 2021 9:24AM - 9:36AM Live |
X42.00008: Competition between spin and charge excitations in graphene with long-range Coulomb interactions HO-KIN TANG, Indra Yudhistira, Maksim Ulybyshev, Pinaki Sengupta, Fakher Assaad, Shaffique Adam Graphene is the paradigm to study the interplay between the contact Hubbard interaction and the long-range Coulomb interaction [1]. This is mostly because of the experimental flexibility in tuning the interaction strength, for example, by forming a moiré superpotential by twisting, or by exerting strain. Despite the extensive research on the many-body physics of graphene, the spin and charge response to interactions remains largely unexplored. Here, we use the numerically exact projective quantum Monte Carlo with a maximum entropy method for analytical continuation to find the dynamical spin and charge response of strongly interacting graphene. We find that the long-range Coulomb interaction suppresses the quasi-particle nature, enhances charge-density wave excitations (CDW), and only weakly modifies the anti-ferromagnetic spin-wave excitations (AFM). We also find that long-range Coulomb interaction induces stiffer excitations in both CDW excitation and AFM excitation near Gamma point. We discuss how these predictions can be tested in an electron loss experiment. |
Friday, March 19, 2021 9:36AM - 9:48AM Live |
X42.00009: Spin/Valley Flavor Symmetry Breaking in Magic Angle Twisted Bilayer Graphene Ming Xie, Allan MacDonald The spin/valley flavor symmetry of the nearly flat low-energy bands in magic angle twisted bilayer graphene (MAtBG) is often seen experimentally to be broken at partial band fillings. The pattern of flavor symmetry breaking is quite distinct from what one would expect when the flat bands are completely dispersionless, different for example from what occurs at partial filling of the N=0 Landau levels in Bernal-stacked bilayer graphene. We analyze how details of the flat band dispersion influence the flavor symmetry breaking revealed by weak-field-Hall and Shubnikov-de Haas magneto-transport measurements [1]. Additionally, we study the unique thermodynamic characteristics of the flavor symmetry breaking transition observed in recent compressibility and entropy measurements. |
Friday, March 19, 2021 9:48AM - 10:00AM Live |
X42.00010: Aharonov-Bohm Oscillations in Minimally Twisted Bilayer Graphene Christophe De Beule, Fernando Dominguez, Patrik Recher We investigate transport in the network of valley Hall states that emerges in minimally twisted bilayer graphene under interlayer bias. To this aim, we construct a scattering theory that captures the network physics. In the absence of forward scattering, symmetries constrain the network model to a single parameter that interpolates between one-dimensional chiral zigzag modes and pseudo-Landau levels. Moreover, we show how the coupling of zigzag modes affects magnetotransport. In particular, we find that scattering between parallel zigzag channels gives rise to Aharonov-Bohm oscillations that are robust against temperature, while coupling between zigzag modes propagating in different directions leads to Shubnikov–de Haas oscillations that are smeared out at finite temperature. |
Friday, March 19, 2021 10:00AM - 10:12AM Live |
X42.00011: Momentum resolved ground/excited states and the ultra-fast excited state dynamics of monolayer MoS2 Woojoo Lee, Yi Lin, Li-Syuan Lu, Wei-Chen Chueh, Mengke Liu, Xiaoqin (Elaine) Li, Wen-Hao Chang, Robert A Kaindl, Chih-Kang Shih The emergence of transition metal dichalcogenides (TMD) as crystalline atomically thin semiconductors has created a tremendous amount of scientific and technological interest. Many novel device concepts have been proposed and realized. Nonetheless, progress in k-space investigations of ground/excited state electronic structures has been slow due to the challenge to create large scale, laterally homogeneous samples. Taking advantage of recent advancements in chemical vapor deposition, here we create a wafer-size MoS2 monolayer with well-aligned lateral orientation. The ground state and excited state electronic structures are probed using scanning tunneling spectroscopy (STS), angle-resolved photoemission (ARPES) and time-resolved (tr-)ARPES. In addition to mapping out the momentum-space quasiparticle band structure in the valence and conduction bands, we unveil ultrafast excited state dynamics, including inter- and intra-valley carrier scattering and a rapid downward energy shift by ~ 0.2eV lower than the initial free carrier state at sigma point. |
Friday, March 19, 2021 10:12AM - 10:24AM Live |
X42.00012: Optotwistronics in van der waals multilayers Zhurun Ji, Eugene John Mele, Ritesh Agarwal
|
Friday, March 19, 2021 10:24AM - 10:36AM On Demand |
X42.00013: Effect of long-ranged Coulomb interactions on magnetic ordering in twisted bilayer graphene Zachary Goodwin, Lennart Klebl, Arash A Mostofi, Dante Kennes, Johannes Lischner Using an atomistic approach, we investigate the magnetic phase diagram of twisted bilayer graphene as function of twist angle and doping. Specifically, we calculate the magnetic susceptibility and determine the critical value of the Hubbard parameter at which the susceptibility diverges. As a starting point, we use either a non-interacting tight-binding model or a Hartree approach that captures effects of the long-range part of the Coulomb interaction. We find leading instabilities that correspond to ferromagnetic and anti-ferromagnetic states. Importantly, the different approaches result in qualitatively different phase diagrams. In particular, the band structure of twisted bilayer graphene from Hartree theory depends sensitively on the doping level and exhibits an interaction-induced band flattening which enhances magnetic instabilities and increases the twist-angle window where strong correlation effects can be observed. We also study quasiparticle properties of the magnetic phases using a mean-field Hubbard model. |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700