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 G21: Magic Angle Twisted Graphene II |
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Sponsoring Units: DCMP Chair: Adarsh Patri, Massachusetts Institute of Technology Room: Room 213 |
Tuesday, March 7, 2023 11:30AM - 11:42AM Author not Attending |
G21.00001: Optical Response of twisted bilayer graphene Disha Arora, Deepanshu Aggarwal, Sankalpa Ghosh, Rohit Narula We study polarization-controlled, one-photon and two-photon optical responses of twisted bilayer graphene (tBLG) for a range of angles including the magic angle. Their spectrums evolve continuously with the change in polarizer-analyzer rotation combination, providing a fingerprint for the identification of the misorientation angle. At lower twist angles, we find a strong enhancement of the optical response of tBLG compared to a single-layer or Bernal-stacked bilayer. graphene |
Tuesday, March 7, 2023 11:42AM - 11:54AM |
G21.00002: Real space representation of topological system: twisted bilayer graphene as an example Jiawei Zang, Jie Wang, Antoine Georges, Jennifer Cano, Andrew Millis We construct a Wannier basis for twisted bilayer graphene that is projected only from the Bloch functions of the twisted bilayer flat bands. The point symmetries including $C_3$ and $C_{2} mathcal{T}$ symmetries act locally on the Wannier functions while the Wannier function charge density is strongly peaked at the triangular sites and becomes fully sublattice-polarized in the chiral limit. The Wannier functions have a power-law tail, due to the topological obstruction, but most of the charge density is concentrated within one unit cell so that the on-site local Coulomb interaction is much larger than the further neighbor interactions and in general the Hamiltonian parameters may be accurately estimated from a modest number of Wannier functions. One exception is the momentum space components of the single-particle Hamiltonian, where because of the topological obstruction convergence is non-uniform across the Brillouin zone. We observe, however, that mixed position and momentum space representations may be used to avoid this difficulty in the context of quantum embedding methods. Our work provides a new route to study systems with topological obstruction and paves the way for the future investigation of correlated states in twisted bilayer graphene, including studies of non-integer fillings and temperature dependence. |
Tuesday, March 7, 2023 11:54AM - 12:06PM |
G21.00003: Electron anisotropy and its origin in twisted trilayer graphene Yibang B Wang, Naiyuan J Zhang, Kenji Watanabe, Takashi Taniguchi, Oskar Vafek, Jia Li Using a new scheme of angle-resolved transport measurement, I will report the identification and characterization of electronic anisotropy in twisted trilayer graphene. Our measurement scheme not only extracts the conductivity matrix for the underlying electronic state, but it also offers direct characterization for the spatial homogeneity of the twisted trilayer graphene sample. By mapping out the evolution with the band-filling, temperature, and twist angle, the angle-resolved measurement provides a novel window into the role of Coulomb interaction and lattice distortion. Together, our findings point towards an underlying connection between electron anisotropy, cascade of isospin polarization transitions and the electronic order that breaks both parity and time-reversal symmetry. The influence of lattice distortion, while unavoidable in solid state samples, is of secondary importance. |
Tuesday, March 7, 2023 12:06PM - 12:18PM |
G21.00004: Parity and time-reversal symmetry breaking in twisted trilayer graphene Naiyuan J Zhang, Jiang-Xiazi Lin, Yibang Wang, Takashi Taniguchi, Kenji Watanabe, Liang Fu, Jia Li By examining the angular dependence of second-harmonic nonlinear response, we report a novel electronic order in twisted trilayer graphene, which is present throughout the entire moire flatband. This is evidenced by a one- or three-fold symmetric angular dependence in the nonreciprocal transport response. By analyzing its dependence on the magnetic field, current flow, and field-effect doping, we show that this parity and time-reversal-breaking order is distinct from the orbital ferromagnetism and nematicity. This discovery has important implications on our fundamental understandings of emergent phenomena in graphene moiré systems, such as the cascade of isospin transitions, superconductivity, nematicity and orbital ferromagnetism. |
Tuesday, March 7, 2023 12:18PM - 12:30PM |
G21.00005: Resistively-detected microwave resonance as a spin probe in graphene moiré systems Erin Morissette, Jiang-Xiazi Lin, Dihao Sun, Liangji Zhang, Song Liu, Daniel A Rhodes, Kenji Watanabe, Takashi Taniguchi, James C Hone, Johannes Pollanen, Mathias S Scheurer, Michael P Lilly, Andrew M Mounce, J.I.A. Li In a strongly correlated system, collective excitations reveal vital information regarding the electronic order of the underlying ground state. A recent report demonstrated a resisitively-detected electron spin resonance (RD-ESR) technique to directly observe collective modes in the form of microwave-induced resonance near half filling of the moiré flatband [1]. The frequency-magnetic field dependence of these resonance modes enabled the extraction of key parameters such as intervalley exchange interaction and spin stiffness. Furthermore, the onset of robust resonance response coinciding with the spontaneous flavor polarization at half moiré filling indicates that the generation and detection of the microwave resonance relies on the strong correlation of the flat band. Here, we apply the RD-ESR technique to twisted trilayer graphene systems that exhibit both superconducting and diode-like nonreciprocal transport response [2]. The ability to rectify the spin characteristics of systems with valley-polarized isospin order represents unlocking a comprehensive spin-valley picture of the electronic order of the ground state. |
Tuesday, March 7, 2023 12:30PM - 12:42PM |
G21.00006: Strong electronic correlations and Unconventional superconductivity in Magic-angle Twisted Trilayer Graphene Hyunjin Kim, Youngjoon Choi, Cyprian K Lewandowski, Alex R Thomson, Kenji Watanabe, Takashi Taniguchi, Jason F Alicea, Stevan Nadj-Perge Flat electronic bands of magic-angle twisted trilayer graphene (MATTG) host correlated insulating states and unconventional superconductivity upon doping a few electrons per moire site. However, the exact nature of these states remains elusive to electric transport measurements. Measuring the dI/dV spectrum with scanning tunneling microscopy on MATTG reveals a cascade of electronic transitions and doping-dependent band deformations similar to magic angle bilayers. Upon doping away from the insulating state at half-filling, the gapped spectrum observed at two to three holes per moire site corresponds to superconductivity as verified through point contact spectroscopy showing signatures of Andreev reflection. Moreover, the superconducting gap shows several signatures suggesting the unconventional nature of superconductivity in MATTG, including unusual gap suppression with the temperature and a transition from a U-shaped to a V-shaped profile with doping. Our high-resolution local spectroscopic measurements can help establish the nature of correlated insulating and superconducting states in twisted graphene systems. |
Tuesday, March 7, 2023 12:42PM - 12:54PM |
G21.00007: Electronic nematic state and non-Fermi liquid transport phenomena in Magic Angle Twisted Bilayer Graphene: SU(4) valley + spin fluctuation Interference Mechanism Seiichiro Onari, Hiroshi Kontani In the magic angle twisted bilayer graphene (MATBG), one of the most remarkable observations is the C3-symmetry-breaking nematic state. We identify that the nematicity in MATBG is the E-symmetry ferro bond order, which is the modulation of correlated hopping integrals owing to the E-symmetry particle-hole pairing condensation [1]. The nematicity in MATBG originates from prominent quantum interference among SU(4) valley+spin composite fluctuations. This novel "valley + spin fluctuation interference mechanism" is revealed by the density wave equation analysis for a realistic multiorbital Hubbard model for MATBG. We find that the nematic state is robust once three van Hove singularity points exist in each valley. This interference mechanism also causes novel time-reversal-symmetry-broken valley polarization accompanied by a charge loop current. We discuss interesting similarities and differences between MATBG and Fe-based superconductors. In addition, we study the no-Fermi liquid transport phenomena due to the SU(4) valley+spin composite fluctuations [2]. |
Tuesday, March 7, 2023 12:54PM - 1:06PM |
G21.00008: Routes to nodal pairing in moire systems Maine Christos, Mathias S Scheurer Despite years of theoretical and experimental work, the nature and mechanism of the superconducting states observed in TBG and its generalization to alternating-twist n-layer graphene have remained open questions. Recent experiments which measure tunneling conductance in alternating twist magic angle graphene have revealed indirect evidence for a nodal pairing in these systems [1,2]. In this work, we use a combination of numerical mean-field calculations at T=0 and analytical arguments to study the experimentally observed superconducting state near ν=2 in alternating twist magic angle graphene. We consider two types of interactions; one in which the superconducting state is stabilized by a more conventional electron-phonon interaction and one in which fluctuations of a nearby ordered state stabilizes superconductivity. Comparing the order parameter structure in the subspace of the flat bands and order parameter symmetries that result from these two approaches allows us to put constraints on options which leads to a nodal pairing in either scenario. |
Tuesday, March 7, 2023 1:06PM - 1:18PM |
G21.00009: Interacting models for twisted bilayer graphene: Towards a quantum chemistry approach Fabian M Faulstich, Kevin D Stubbs, Qinyi Zhu, Tomohiro Soejima, Rohit K Dilip, Huanchen Zhai, Raehyun Kim, Michael P Zaletel, Garnet K Chan, Lin Lin In recent years, magic angle twisted bilayer graphene (MATBG) has received intense attention due to its experimentally observed superconducting properties. However, the nature of the correlated states in MATBG has not yet been fully understood. Thus far, most computational studies of MATBG have been performed on small systems using exact diagonalization or on moderate systems at the Hartree-Fock (HF) level of theory. The large ratio between the Coulomb interaction and the dispersion of the flat bands suggests that post-HF effects may become significant in certain parameter regimes. We propose a quantum chemistry approach to study the interacting Bistritzer-MacDonald (IBM) model of MATBG. Our implementation uses the PySCF software package which allows us to perform HF and post-HF calculations on the same footing for problem sizes that are intractable using exact diagonalization techniques. This allows us to explore the many-body properties of MATBG with a spinless, valleyless IBM model at integer as well as non-integer filling using coupled-cluster based methods, such as CCSD and CCSD(T), as well as the quantum chemistry density matrix renormalization group (QC-DMRG) method. We also present a gauge-invariant formulation to detect the spontaneous symmetry breaking in interacting models. |
Tuesday, March 7, 2023 1:18PM - 1:30PM |
G21.00010: Strain-induced Kekul'e spiral order in magic-angle graphene: a density matrix renormalization group study Tianle Wang, Daniel E Parker, Tomohiro Soejima, Johannes Hauschild, Sajant Anand, Nick Bultinck, Michael P Zaletel Strain in moir'e systems is amplified by the inverse twist angle --- two orders of magnitude in magic angle twisted bilayer graphene (TBG). Samples of TBG typically have heterostrains of $0.1-0.7\%$. This increases bandwidth of the `flat' bands as much as tenfold, placing TBG in an intermediate coupling regime. Here we study the phase diagram of TBG in the presence of heterostrain with unbiased, large-scale density matrix renormalization group calculations including all eight flat bands. Working at filling $ { u} = 3$, we find a strain of $0.1\%$ drives a transition from a quantized anomalous Hall insulator into an incommensurate-kekul'e spiral (IKS) phase. This peculiar order, proposed and studied at mean-field level in Ref. cite{Kwan2021}, breaks both valley conservation and translation symmetry $hat{T}$, but preserves a modified translation symmetry $hat{T}'$ with moir'e-incommensurate phase modulation. Beyond the transition, we find IKS is only one state in a low-energy manifold, whose common characteristic is depleted charge density at the $Gamma$ point. |
Tuesday, March 7, 2023 1:30PM - 1:42PM |
G21.00011: Spectroscopic evidence for Landau degeneracy and massive quasiparticles in magic-angle twisted bilayer graphene Shuang Wu, Zhenyuan Zhang, Kenji Watanabe, Takashi Taniguchi, Eva Y Andrei Transport, scanning tunneling microscopy/spectroscopy and compressibility measurements in magic-angle twisted bilayer graphene (MATBG) have revealed the emergence of strongly correlated quantum phases as the flat band is being filled. The appearance of these correlated phases is attributed to the interplay between the strong Coulomb interactions and the nontrivial narrow band topology. In the strong coupling limit, where the Coulomb interaction dominates, theoretical predictions of a parabolic band minimum at the center of the moire Brillouin zone, led to the expectation of massive quasiparticles at every integer filling. 1,2 Experimentally however, reports of massive quasiparticle excitations have been scant.3 Here we report on planar tunneling spectroscopy measurements of the differential conductance in the presence of a small magnetic field, which provides access to the Landau level sequence, indicating a zero Berry phase, in MATBG. At the charge neutrality point we find nearly equally spaced Landau levels whose energy extrapolates to zero in vanishing magnetic field. These finding demonstrate the existence of massive quasiparticle excitations at the charge neutrality point with mass ~0.2me. |
Tuesday, March 7, 2023 1:42PM - 1:54PM |
G21.00012: The effect of interaction and electron fillings in twisted bilayer graphene: an auxiliary-field quantum Monte Carlo study Zhi-Yu Xiao, Shiwei Zhang
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Tuesday, March 7, 2023 1:54PM - 2:06PM |
G21.00013: Local thermodynamic study of correlated insulating ground states and charge excitations in magic-angle twisted bilayer graphene Jiachen Yu, Benjamin Foutty, Yves H Kwan, Mark E Barber, Kenji Watanabe, Takashi Taniguchi, Zhi-Xun Shen, Siddharth A Parameswaran, Ben Feldman The flat electronic bands in magic-angle twisted bilayer graphene (MATBG) host a variety of correlated insulating ground states, many of which are predicted to support charged excitations with topologically non-trivial spin and/or valley textures. However, it has remained challenging to experimentally address their ground state order and excitations, both because some of the proposed states do not couple directly to experimental probes, and because they are highly sensitive to spatial inhomogeneities in real samples. In this talk, I will present local electronic compressibility measurement of MATBG using a scanning single-electron transistor. We show thermodynamic evidence for gapped ground states at even integer moiré filling factors at low magnetic fields. Measuring the gap evolution as a function of perpendicular magnetic field sheds light on the nature of the underlying ground state and charge excitations. From the spatial dependence of these states and the chemical potential variation within the flat bands, we infer a link between the stability of the correlated ground states and local twist angle and strain. |
Tuesday, March 7, 2023 2:06PM - 2:18PM |
G21.00014: A Tunable Monolithic Superconducting Quantum Interference Device in Twisted Bilayer Graphene Elías Portolés, Shuichi Iwakiri, Giulia Zheng, Peter Rickhaus, Takashi Taniguchi, Kenji Watanabe, Thomas Ihn, Klaus Ensslin, Folkert K de Vries We present a Superconducting Quantum Interference Device (SQUID) in Magic-Angle Twisted Bilayer Graphene (MATBG) [1], building on previous single-junction realizations [2, 3]. The superconducting phase difference is controlled through the magnetic field. We observe magneto-oscillations of the critical current, demonstrating long-range coherence agreeing with an effective charge of 2e for the superconducting charge carriers. We tune to both asymmetric and symmetric SQUID configurations by electrostatically controlling the critical currents through the junctions. With this tunability, we study the inductances in the device, finding values above 100nH. Furthermore, we directly observe the current-phase relation of one of the Josephson junctions of the device. |
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