Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session M60: Correlated States IIFocus Recordings Available
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Chair: Liuyan Zhao, University of Michigan Room: Hyatt Regency Hotel -DuSable C |
Wednesday, March 16, 2022 8:00AM - 8:36AM |
M60.00001: Atomic-scale structure and electronic properties of twisted double bilayer graphene: topological edge states and nematic order Invited Speaker: Carmen Rubio Verdú Atomically thin van der Waals materials stacked with an interlayer twist are an excellent platform towards achieving gate-tunable correlated phenomena linked to the formation of flat electronic bands. We demonstrate the formation of emergent correlated phases in twisted double bilayer graphene (tDBG) in two regimes of twist angle: minimally twisted (<0.1°) and 1.1°. Minimally twisted tDBG hosts large regions of uniform rhombohedral four-layer (ABCA) graphene where scanning tunneling spectroscopy reveals unprecedentedly sharp flat band of 3-5 meV half-width. We demonstrate that, when this flat band straddles the Fermi level, a correlated many-body gap emerges. Moreover, under certain experimental conditions, topological helical edge states appear at the natural interface between rhombohedral and Bernal graphene domains. On the other hand, scanning tunneling microscopy on tDBG at a regime of twist angles (~1.1°) at which moiré physics play an important role, reveals the presence of van Hove singularities whose spatial distribution within the moiré unit cell is determined by the inequivalent stacking sites. Tuning the electron filling as well as the displacement field reveals broken C3 symmetry that emerges when the Fermi level is brought in the flat band. This symmetry breaking is manifested as long-range commensurate stripes along a high-symmetry moiré crystallographic direction, distinctive of nematic correlations of electronic origin. Comparing our experimental data with a combination of microscopic and phenomenological modeling, we show that the nematic instability is not associated with the local scale of the graphene lattice, but is an emergent phenomenon at the scale of the moiré lattice, pointing to the universal character of this ordered state in flat band moiré materials. |
Wednesday, March 16, 2022 8:36AM - 8:48AM |
M60.00002: Cascade of isospin phase transitions in Bernal bilayer graphene at zero magnetic field Sergio C de la Barrera, Samuel H Aronson, Zhiren Zheng, Kenji Watanabe, Takashi Taniguchi, Qiong Ma, Pablo Jarillo-Herrero, Raymond C Ashoori Bernal bilayer graphene has long been predicted to host interaction-driven symmetry-breaking states arising from large densities of states near the band edges. While subtle evidence of gapped symmetry-breaking states have been demonstrated at charge neutrality in suspended samples and exchange-driven symmetry breaking often manifests in large magnetic fields, no gapless spin or valley polarized states have been reported within the low-energy flat bands of the system in the absence of a magnetic field. Here, we show that ultra-clean bilayer graphene in fact hosts a series of isospin magnetic phase transitions for both signs of carriers at zero magnetic field. The phase boundaries and degeneracy of each isospin ordered state are further tunable by an external electric displacement field, replacing the role of magnetic field in the analogous quantum Hall ferromagnet. Quantum oscillations of the symmetry-broken states reveal discrete transitions in the degeneracy of each zero-field state and parallel field measurements hint at the underlying isospin order. |
Wednesday, March 16, 2022 8:48AM - 9:00AM |
M60.00003: Layer and spin signatures of spontaneous symmetry breaking in Bernal bilayer graphene Samuel H Aronson, Sergio C de la Barrera, Zhiren Zheng, Kenji Watanabe, Takashi Taniguchi, Qiong Ma, Pablo Jarillo-Herrero, Raymond C Ashoori Bernal-stacked bilayer graphene in the presence of an out-of-plane electric displacement field develops a band gap surrounded by flat bands near the band edges with large densities of states. Using capacitance techniques, we study the compressibility of the flat bands at low carrier densities as a function of displacement field. We observe a series of peaks in the compressibility as a function of carrier density that we identify as a sequence of spontaneous isospin symmetry-broken states separated by well-defined phase transitions. Via combined measurements of quantum oscillations, in-plane magnetic fields, and layer-sensitive capacitance techniques, we probe the character of these symmetry-broken states, track the evolution of the phase transitions, and unravel the spin, valley, and layer ordering of each phase. Rhombohedral trilayer graphene also exhibits an analogous sequence of spontaneously symmetry-broken states, though the isospin ordering differs between the two systems. |
Wednesday, March 16, 2022 9:00AM - 9:12AM |
M60.00004: Coexistence of Canted Antiferromagnetism and Bond-order in ν=0 Graphene Ankur Das, Ribhu K Kaul, Ganpathy N Murthy Motivated by experimental studies of graphene in the quantum Hall regime, we revisit the phase diagram of a single sheet of graphene at charge neutrality. Because of spin and valley degeneracies, interactions play a crucial role in determining the nature of the ground state. We show that, generically, in the regime of interest there is a region of coexistence between magnetic and bond orders in the phase diagram. We demonstrate this result both in continuum and lattice models and argue that the coexistence phase naturally provides an explanation for unreconciled experimental observations on the quantum Hall effect in graphene. |
Wednesday, March 16, 2022 9:12AM - 9:24AM |
M60.00005: Broken flavor symmetries in rhombohedral multilayer graphene Tobias M Wolf, Chunli Huang, Nemin Wei, Wei Qin, Allan H MacDonald Graphene multilayers have proven to be a rich platform to realize and control exotic emergent physics by engineering stacking arrangement, external fields, and proximity effects. Most observations of strong correlation effects in these systems are related to moiré flat bands, but recent experiments have demonstrated that even commensurately stacked graphene multilayers can feature a variety of strongly correlated phases, including superconductivity. These phases break spin and valley flavor symmetries. I will explain why the broken symmetry favored at large displacement fields in AB-bilayer and ABC-trilayer graphene is spontaneous valley coherence. This conclusion is based on a realistic lattice tight-binding description that faithfully captures inter-valley interactions, and a self-consistent Hartree-Fock mean field approximation for interaction. I will comment on the Fermi surface reconstructions and key electronic properties of the broken symmetry phase, and the relationship between superconductivity in ABC and AB bilayers and in magic-angle twisted bilayer graphene. |
Wednesday, March 16, 2022 9:24AM - 9:36AM |
M60.00006: Orbital excitations on the cusp of Mott-band insulator crossover in 1T-TaS2 Xun Jia, Yue Cao The nature of electron itinerancy is a fundamental descriptor of a solid-state material. In the case of insulators, band and Mott insulators represent two different regimes: the former can be described using spatially extended electron wavefunctions and well-defined quasiparticles, while the latter features localized electron wavefunctions with strong local correlations. Recently the nature of the insulator behavior has been under intense investigation in a number of 5d transition metal compounds. Here we perform resonant inelastic X-ray scattering studies of 1T-TaS2 which has till recently been long considered a prototypical Mott insulator. We observed five electronic excitations arising from the interband transitions of the Ta 5d/5p orbitals and the S 3p ligand state at the Ta L3 edge. These excitations cannot be explained within the framework of standard molecular orbital multiplet calculations that are based on a localized picture with strong electronic correlations. Instead, calculations from the band dispersions can reasonably capture the number and energies of the orbital excitations with small deviations. We argue that the insulating phase of 1T-TaS2 is on the cusp of band-Mott insulator crossover and leans heavily towards the band insulator limit. |
Wednesday, March 16, 2022 9:36AM - 9:48AM |
M60.00007: Atomic Touchstone Distinguishing a Mott Insulator from a Band Insulator Jinwon Lee, Kyung-Hwan Jin, Han Woong Yeom In an electronic system with various interactions intertwined, revealing the origin of its many-body ground state is challenging and a direct experimental way to verify the correlated nature of an insulator has been lacking. Here we demonstrate an experimental way to unambiguously distinguish a paradigmatic correlated insulator, a Mott insulator, from a trivial band insulator based on their distinct chemical behavior for a surface adsorbate. Using scanning tunneling microscopy, we observed, in a microscopic field of view, two distinct insulating states with different energy gaps on 1T-TaS2 surface, depending on the interlayer stacking of the surface layer. We also revealed that these two insulating states have distinguished adsorption behaviors of surface K adatoms, and the adatoms, furthermore, play totally different roles on the electronic states of the surface layer. On one insulating state, an extra electron from each K adatom is highly localized at the adsorption site and kills the empty state, forming a totally different spectral feature. On the other insulating state, electrons from K adatoms are widely spread and induce the global doping, shifting but keeping the overall spectral features. This can be straightforwardly understood from the fundamental distinction between Mott and band insulators: an electron filling, the half- and full-filled electrons, respectively. This work not only resolves a mystery of the insulating ground state in 1T-TaS2, but also provides a simple and unambiguous experimental way to identify the Mott insulating state. |
Wednesday, March 16, 2022 9:48AM - 10:00AM |
M60.00008: Graphite Gate induced Quantum Oscillations in 2D Insulators Jiacheng Zhu Recently, Wang et al. [1] reported magnetoresistance (MR) oscillations near the insulating state of monolayer WTe2 encapsulated between thin hexagonal boron nitride (hBN) dielectrics (< 10 nm) and top and bottom graphite gates. They interpreted the results as evidence for charge-neutral fermions, engendering further theoretical proposals to explain the results. However, we demonstrate a capacitive mechanism by which MR oscillations in the sample are generated by oscillations of density of states in the graphite gate. We measure simultaneously resistances in graphite gate and monolayer WTe2 and find oscillations in resistance coincide. In addition, we demonstrate ubiquity of MR in various 2D materials, including bilayer WTe2, angle-aligned MoTe2/WSe2 heterobilayers and Bernal-stacked bilayer graphene. Furthermore, quantitative measurement on bilayer graphene reveals a 180-degree phase shift in the MR oscillations between slightly electron and hole doped bilayer graphene, in consistent with our proposed capacitive mechanism. |
Wednesday, March 16, 2022 10:00AM - 10:12AM |
M60.00009: Quantum anomalous Hall phases in Bernal stacked bilayer graphene driven by orbital magnetism Thomas R Weitz, Anna Seiler The exchange interaction can lead to correlated states in low dimensional systems including the graphene family. Its strength depends in part on the dielectric constant of the surrounding of the system of investigation. Here, we discuss our recent quantum transport epxeriments on high-quality dually-gated bilayer graphene, where the low-k environment leads to comparably large exchange interactions compared to non-suspended samples. Indeed, we have identified clear transport signatures that are consistent with a novel exchange driven quantum anomalous Hall (QAH) nu=2 state that exhibits quantized charge Hall conductance close to zero magnetic field as well as spin, valley and spin-valley anomalous quantum Hall effects and out-of-plane ferroelectricity [1]. The orbital magnetism that leads to the QAH effect arises from the spontaneous chiral symmetry breaking [2]. |
Wednesday, March 16, 2022 10:12AM - 10:24AM |
M60.00010: Ising paramagnons in two dimensional 2H-NbSe2 Joaquin Fernandez-Rossier, Marcio Costa, Antonio T Costa Paramagnons are the collective modes that govern the spin response of nearly ferromagnetic conductors. Their interactions with quasiparticles can induce spin-triplet superconductivity, a scenario that may occur in spin-valley coupled two dimensional transition metal dichalcogenides such as 2H-NbSe2. This motivates this work exploring paramagnons in systems with spin split bands due to spin orbit coupling leading to spin-valley coupling. We use both a Kane-Mele-Hubbard model and a tight-binding model derived from DFT calculations for a monolayer of 2H-NbSe2. We find paramagnons with energies around 1 meV that feature a colossal magnetic anisotropy. In the longitudinal (spin preserving) channel, we obtain the conventional paramagnon enhancement of the spin response, whereas spin response is quenched in the transverse channel. We discuss a possible connection of these Ising paramagnon with Ising superconductivity observed in these materials. |
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