Bulletin of the American Physical Society
APS March Meeting 2023
Volume 68, Number 3
Las Vegas, Nevada (March 5-10)
Virtual (March 20-22); Time Zone: Pacific Time
Session F21: Magic Angle Twisted Graphene I & Goeppert Mayer Award TalkFocus
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Sponsoring Units: DCMP Chair: Talat Rahman, University of Central Florida; Aaron Sharpe, Sandia National Laboratories Room: Room 213 |
Tuesday, March 7, 2023 8:00AM - 8:12AM |
F21.00001: Twisted bilayered graphenes at magic angles and their Casimir interactions Dai Nam Le, Pablo Rodriguez-Lopez, Maria J Calderon, Elena Bascones, Lilia M Woods Magic-angle twisted bilayered graphene (MATBG) is emerging as a material with unusual physics resulting from the interplay of long-range periodicity from its moiré patterns and the short-range lattice structure periodicity from each graphene monolayer. Strong correlation effects in MATBG lead to breaking of various symmetries, considered to be an inherent reason for the emergence of superconductivity and nematicity among others. In this work, we consider Casimir phenomena as a means to probe the electronic and optical response of MATBG. Using a generalized Lifshitz approach and advanced electronic structure models for the optical response, the ubiquitous Casimir force is found to exhibit rich physics, such as different scaling laws, repulsion, and quantization for its different phases. A sizable Casimir torque is found for nematic MATBG directly related to the anisotropy of the Drude optical conductivity. Possible experimental measurements of Casimir effects are also discussed. Casimir effects are one of the few macroscopic manifestations of quantum mechanics and they broaden our understanding of light-matter interactions in general. |
Tuesday, March 7, 2023 8:12AM - 8:24AM Author not Attending |
F21.00002: Correlation and superconductivity in magic-angle twisted graphene: Part 2 Shuwen Sun, Jeong Min Park, Kenji Watanabe, Takashi Taniguchi, Pablo Jarillo-Herrero The abundance of emergent phenomena in twisted and untwisted van der Waals heterostructures has opened a new door to study strongly correlated physics in two-dimensional systems. In particular, the magic-angle twisted graphene family, which consists of systems where the adjacent graphene layers are twisted in an alternating fashion, has shown robust superconducting states. These superconducting states display orders of magnitude higher transition temperatures than the superconducting states shown in other graphene-based systems, despite the low carrier density. There have been striking observations, such as the violation of Pauli limit and the strong coupling strength of the Cooper pairs, which may limit the possible superconducting order parameters. Furthermore, the presence of nearby correlated resistive states and the topologically nontrivial phases has given rise to various theoretical interpretations on the ground state underlying the superconducting state. However, understanding the microscopic mechanism for such superconducting states and the relation to the nearby correlated phases is still far from complete. In this talk, we will present our most recent experimental results on the nature of the correlated and superconducting phases and their interactions in the extended magic-angle twisted graphene family. |
Tuesday, March 7, 2023 8:24AM - 8:36AM |
F21.00003: Imaging Magic-Angle Twisted Bilayer Graphene: Part I Ryan L Lee, Myungchul Oh, Kevin P Nuckolls, Dillon Wong, Tomohiro Soejima, Jung Pyo Hong, Jonah Herzog-Arbeitman, Dumitru Calugaru, Kenji Watanabe, Takashi Taniguchi, Nicolas Regnault, Andrei B Bernevig, Michael P Zaletel, Ali Yazdani Magic-angle twisted bilayer graphene (MATBG) has long been known to host flat electronic bands that produce a wide variety of correlated states including superconducting, correlated insulating, and magnetic states [1-4]. However, several key properties of the correlated insulating states remain poorly understood, including the symmetries they break, the natures of their ground states, and the relationship between these states and superconductivity. In this first talk in a series of three presentations, I will discuss theoretical candidate ground states of MATBG and their relevant symmetries, and will introduce how we can use scanning tunneling microscopy (STM) experiments to distinguish among these candidates. The power of STM lies in its ability to resolve local electronic structure as a function of energy, allowing us to determine its spatial symmetries. By imaging these correlated phases at various length scales, we narrow down the candidate ground states based on these spatial symmetries. |
Tuesday, March 7, 2023 8:36AM - 8:48AM |
F21.00004: Imaging Magic-Angle Twisted Bilayer Graphene: Part II Myungchul Oh, Ryan L Lee, Kevin P Nuckolls, Dillon Wong, Tomohiro Soejima, Jung Pyo Hong, Jonah Herzog-Arbeitman, Dumitru Calugaru, Kenji Watanabe, Takashi Taniguchi, Nicolas Regnault, Andrei B Bernevig, Michael P Zaletel, Ali Yazdani The flat bands in magic-angle twisted bilayer graphene (MATBG) host exotic quantum phases, including correlated insulating states, topologically non-trivial phases, and unconventional superconductors [1,2]. Strong interactions between electrons modulate the ground state from which such correlated phases emerge, and reconstruct the local electronic structure along with spatial symmetry breaking [3,4]. The ability to measure the local electronic excitation spectrum allows scanning tunneling microscopy (STM) to investigate the spatial dependence of the local electronic density of states (LDOS) and visualize the spatial characteristics of correlated phases using a variety of imaging techniques. |
Tuesday, March 7, 2023 8:48AM - 9:00AM |
F21.00005: Imaging Magic-Angle Twisted Bilayer Graphene: Part III Kevin P Nuckolls, Myungchul Oh, Ryan L Lee, Dillon Wong, Tomohiro Soejima, Jung Pyo Hong, Jonah Herzog-Arbeitman, Dumitru Calugaru, Kenji Watanabe, Takashi Taniguchi, Nicolas Regnault, Andrei B Bernevig, Michael P Zaletel, Ali Yazdani Magic-angle twisted bilayer graphene (MATBG) hosts many correlated ground states, including correlated insulating, superconducting, and magnetic phases [1]. Theoretical studies have predicted an intense energetic competition among several candidate ground states in MATBG, but it appears to be very difficult to differentiate among them without spatially resolved experiments [2, 3]. Fortunately, scanning tunneling microscopy (STM) has the capability to access key microscopic observables predicted by theory that are crucial for understanding the origin of these phases. In this final talk in a series of three presentations, I will discuss how we combine STM imaging experiments with a unified symmetry-based analysis framework to extract quantitative information about gapped phases in MATBG. In large-scale imaging experiments, we identify universal real-space features that are shared across devices, and identify more subtle features that are highly sample-dependent. This approach allows us to directly compare candidate ground states to our experimental observations, which we use to distinguish the nature of the insulating phases near v = +-2 in MATBG, ruling out leading theoretical contenders for these states on the basis of symmetry. |
Tuesday, March 7, 2023 9:00AM - 9:12AM |
F21.00006: Generalized Peierls substitution for the tight-binding model of twisted graphene systems in a magnetic field Thi-Nga Do, Po-Hsin Shih, Hsin Lin, Danhong Huang, Godfrey Gumbs, Tay-Rong Chang This work addresses an important advanced methodology for twisted layered graphene in the presence of applied magnetic field by combining the Bloch-basis tight-binding model (TBM) and the generalized Peierls substitution. We investigate extensively the band structures, Landau levels (LLs), and quantum Hall conductivity (QHC) of twisted bilayer graphene and twisted double-bilayer graphene at large angles and the magic angles. Our theoretical model opens up an opportunity for comprehension of the interplay between an applied magnetic field and the twisting effect associated with layered graphene. The proposed method is expected to be applicable for the calculation of magnetic quantization problems of other complex systems. |
Tuesday, March 7, 2023 9:12AM - 9:24AM |
F21.00007: Spin-valley locking and universal mechanism of Ising superconductivity in twisted bilayer, trilayer, and quadrilayer graphene Jose Gonzalez, Tobias Stauber We show that the superconductivity in twisted bilayer, trilayer, and quadrilayer graphene originates from a common feature, which is the strong valley symmetry breaking characteristic of these moiré systems at the magic angle. This leads to a breakdown of the inversion symmetry of the flat moiré bands and to ground states with broken time-reversal symmetry for each spin projection. However, this symmetry can be recovered with the exchange of spin-up and spin-down electrons, as we illustrate by means of a self-consistent Hartree-Fock resolution where the two spin projections acquire opposite signs of the valley polarization. This implies a spin-valley locking in which the Fermi lines for spin-up and spin-down electrons are different and related by inversion symmetry, lending protection to the superconductivity against magnetic fields. In the twisted multilayers, the pairing glue is shown to be given by the nesting between parallel segments of the Fermi lines.This induces a strong Kohn-Luttinger instability, which is dominant until the Fermi level crosses the van Hove singularity in the second valence band. Then a Lifshitz transition occurs, leading to more isotropic Fermi lines and to a regime with vanishing pairing instability. |
Tuesday, March 7, 2023 9:24AM - 9:36AM |
F21.00008: A Monte Carlo study of the electron-phonon problem in twisted bilayer graphene Johannes S Hofmann, Juan F Mendez-Valderrama, Jong Yeon Lee, Debanjan Chowdhury, Erez Berg Recent progress in experiment and theory constrains the pairing symmetry of the superconducting order parameter in twisted bilayer graphene (TBG); however, the pairing mechanism remains more elusive. We study the effects of electron-phonon interactions in a continuum model of TBG near the magic angle, addressing the possibility of phonon-mediated superconductivity. Additionally, we critically examine the possibility of other symmetry-broken states, such as charge-density-wave and nematic phases. The model is free of the infamous fermion sign problem and is thus amenable to solution by the unbiased, numerically exact determinant quantum Monte Carlo method. |
Tuesday, March 7, 2023 9:36AM - 9:48AM |
F21.00009: Kondo Lattice Approach for Magic-Angle Twisted Bilayer Graphene Liam L.H. L Lau, Piers Coleman Magic-angle twisted bilayer graphene (MATBG) has emerged as a tunable platform for low temperature correlation driven phases. Experiments have observed seemingly contrasting characteristics: delocalized electrons and localized quantum-dot behavior. Song and Bernevig have recently been able to capture both aspects by exactly mapping the Bistrizer—MacDonald model into a topological heavy fermion model with a simple real space picture. The topological heavy fermion model for MATBG opens the door for heavy-fermion machinery to be applied. Hence, starting from the topological heavy fermion model, we obtain and investigate the low energy Kondo Lattice model for MATBG. Harnessing the theoretical tools of the heavy fermion community, we solve the model using mean-field theory and explore the low-energy phases. In particular, we will show how the local moment formation mechanism is altered by the relativistic dispersion of the topological conduction electrons, resulting in modified strong coupling limits for the Kondo Lattice. We investigate the observable consequences of local moment and Kondo physics in MATBG. We finish by exploring further analogies between known phases heavy fermion materials and the diverse phase diagram of MATBG. |
Tuesday, March 7, 2023 9:48AM - 10:00AM |
F21.00010: A DMRG study of a lattice model for Twisted bilayer graphene aligned with hexagonal Boron Nitride Rafael Miksian Magaldi In this work, we study a lattice model for twisted bilayer graphene aligned with hexagonal boron nitride (TBG/hBN) at quarter filling. We employ the density matrix renormalization group algorithm in the matrix product state formulation (DMRG-MPS) to obtain the ground state of the system and study its phases. In agreement with previous experimental results, which showed a large anomalous Hall effect for the system, we find that strong interactions lead to spontaneous polarization in the spin/valley space, giving a Chern insulator. It is interesting to observe how this evolves as a function of the bandwidth, which can be tuned via the hopping parameters, as well as the interaction strength. |
Tuesday, March 7, 2023 10:00AM - 10:12AM Author not Attending |
F21.00011: Band structure sensitive photoresponse in twisted bilayer graphene proxtimitized with WSe2 Aparna Parappurath, Bhaskar Ghawri, Saisab Bhowmik, Arup Singha, Kenji Watanabe, Takashi Taniguchi, Arindam Ghosh The ability to tune the twist angle between different layers of two-dimensional (2D) materials has enabled the creation of flat bands artificially, leading to exotic quantum phases. An emerging direction in this field is twisted bilayer graphene (tBLG) van der Waals coupled to a layer of semiconducting transition metal dichalcogenide, such as WSe2, which leads to unique electronic and structural properties arising from moiré superlattice potential, proximity-induced spin-orbit interaction, etc. Although different transport measurements have shed light on the rich-phase diagram of WSe2/tBLG devices, understanding light-matter interaction in such systems remains elusive. Here we have leveraged WSe2/tBLG heterostructure to perform photoresponse measurements, where the mis-orientation angle of the tBLG layer was chosen to lie close to the magic angle of 1.10. Our experiments show that the photoresponse is extremely sensitive to the band structure of tBLG. We demonstrate that photogating emerges as a primary mechanism for photoresponse in the tBLG layer prevailing above the moiré band edge. In contrast, strong suppression of photoresponse is observed as the Fermi level is tuned inside moiré flat bands at low temperatures. Our observations suggest that the screening effects from moiré flat bands strongly affect the charge transfer process at the WSe2/tBLG interface, which is further supported by time-resolved photo-resistance measurements. With the enhanced photo responsivity arising from the photogating effect, our device architecture opens up new possibilities to optoelectronically probe the rich physics of WSe2 proximitized tBLG. |
Tuesday, March 7, 2023 10:12AM - 10:48AM |
F21.00012: Maria Goeppert Mayer Award Winner: Ab initio approaches to nonequilibrium dynamics in quantum matter Invited Speaker: Prineha Narang In this talk, I will present theoretical and computational approaches to describe excited-states in quantum matter, and predicting emergent states created by external drives. Understanding the role of such light-matter interactions in the regime of correlated electronic systems is of paramount importance to fields of study across condensed matter physics, quantum optics, and ultrafast dynamics. The simultaneous contribution of processes that occur on many time and length-scales have remained elusive for state-of-the-art calculations and model Hamiltonian approaches alike, necessitating the development of new methods in computational physics. I will discuss our work at the intersection of ab initio cavity quantum-electrodynamics and electronic structure methods to treat electrons, photons and phonons on the same quantized footing, accessing new observables in strong light-matter coupling. Current approximations in the field almost exclusively focus on electronic excitations, neglecting electron-photon effects, for example, thereby limiting the applicability of conventional methods in the study of nonequilibrium interactions in quantum matter, which requires understanding the coupled dynamics of electronic spins, nuclei, phonons and photons. With our approach we can access correlated electron-photon and photon-phonon dynamics, essential to our latest work on driving quantum materials far out-of-equilibrium to control the coupled electronic and vibrational degrees-of-freedom. In the second part of my talk, I will demonstrate how the same approach can be generalized in the context of control of molecular quantum matter and quantum transduction with implications for scalable quantum architectures. |
Tuesday, March 7, 2023 10:48AM - 11:00AM Author not Attending |
F21.00013: Moiré bands in Twisted Bilayer Graphene under externally imposed periodic potential Deepanshu Aggarwal, Rohit Narula, Sankalpa Ghosh Applying a periodic external potential in magic-angle twisted bilayer graphene (MATBG) having the same periodicity as the moiré pattern changes the curvature of either the conduction or valence band. However, the bandwidth at the centre of the moiré Brillouin zone remains unaffected. We study the MATBG in such external periodic potentials such that the bandwidth can be decreased further to enhance the flatness of both the conduction and valence bands simultaneously, which consequently enhances the strong correlation effects.
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