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 Q38: 2D Materials: Strong Correlations and SuperconductivityFocus
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Sponsoring Units: DMP Chair: Salvador Barraza-Lopez, University of Arkansas; Yang Liu Room: Room 230 |
Wednesday, March 8, 2023 3:00PM - 3:36PM |
Q38.00001: Theory of magnetism and superconductivity in ABC trilayer graphene Invited Speaker: Chunli Huang The advent of moiré materials has led to unprecedent gate-tunable platforms for exploring topology, magnetism, superconductivity, and their interplay. Phase diagrams including Stoner-like half and quarter metals, spin-singlet and -triplet superconductors, and cascades of their phase transitions have been observed, first in the naturally occurring ABC-stacked trilayer graphene and later in AB-stacked bilayer graphene. Notably, all occurs in the absence of delicate moiré engineering, but under interplay between nonlinear Dirac bands and trigonal warping effects.Focusing on the trilayer graphene system, significant progress has been made by providing a theoretical framework that enables examination of the spontaneous spin-valley symmetry breaking and Fermi surface reconstructions observed in the fractional metallic phases and exploration of the tantalizing competitions between spin/orbital magnetism and singlet/triplet superconductivity. Specifically, the breaking of spin-valley symmetry is driven by momentum-space condensation that alters the Fermi surface topology from an annular disk to a circle and several crescents, by which the electrons can be compactly re-packed. This is consistent with the magneto-oscillation Fermi-surface-area measurements. Furthermore, the Cooper pairing can be generated by the repulsive Coulomb interaction, taking advantage of the large inter-pocket particle-hole susceptibility of an annular Fermi surface. In short, the inner and outer Fermi surfaces prefer d-wave spin-singlet and p-wave spin-triplet pairings, respectively. This allows not only elucidating the spin-singlet pairing observed at the present experiments but also predicting its transition to spin-triplet pairing. This framework [2] combining a Hartree-Fock mean-field theory and a functional renormalization group theory should turn out illuminating in understanding the collective behavior of correlated electrons in low-density, trigonally warped, few-layer graphene systems. |
Wednesday, March 8, 2023 3:36PM - 3:48PM |
Q38.00002: Interface-tunable charge transfer and correlation effects in bilayer 1T/1H-TaS2 Hyeonhu Bae, Igor I Mazin, Binghai Yan Layered heterostructures of TaS2, such as the bilayer 1T/1H-TaS2 and bulk 4Hb-TaS2 have recently been in the spotlight due to Mott physics, the putative Kondo effect and topological superconductivity. There, the filling of the flat band originating from the 1T charge density wave (Star of David) plays a vital role. However, the position of the flat band and its occupation are controversial between different reports. In this work, we examined the electronic structures of the bilayer 1T/1H-TaS2 and focused on the charge transfer between 1T and 1H layers. We found that the charge transfer is sensitive to the 1T/1H interlayer distance. Consequently, the flat band is empty in the equilibrium distance but turns to partially occupied when increasing the layer separation. Furthermore, we found that graphite substrate may also increase the flat band filling. Our work rationalizes that discrepancies in experiments may originate from the varied interlayer distance and/or underlying substrate and paves a practical pathway to tune the Mott physics. |
Wednesday, March 8, 2023 3:48PM - 4:00PM |
Q38.00003: Exact Many-Body Ground States from Decomposition of Ideal Higher Chern Bands: Applications to Chirally Twisted Graphene Multilayers Junkai Dong, Patrick J Ledwith, Eslam Khalaf, Jong Yeon Lee, Ashvin Vishwanath Motivated by the higher Chern bands of twisted graphene multilayers, we consider flat bands with arbitrary Chern number C with ideal quantum geometry. While C>1 bands differ from Landau levels, we show that these bands host exact fractional Chern insulator (FCI) ground states for short range interactions. We show how to decompose ideal higher Chern bands into separate ideal bands with Chern number 1 that are intertwined through translation and rotation symmetry. The decomposed bands admit an SU(C) action that combines real space and momentum space translations. Remarkably, they also allow for analytic construction of exact many-body ground states, such as generalized quantum Hall ferromagnets and FCIs, including flavor-singlet Halperin states and Laughlin ferromagnets in the limit of short-range interactions. In this limit, the SU(C) action is promoted to a symmetry on the ground state subspace. While flavor singlet states are translation symmetric, the flavor ferromagnets correspond to translation broken states and admit charged skyrmion excitations corresponding to a spatially varying density wave pattern. We confirm our analytic predictions with numerical simulations of ideal bands of twisted chiral multilayers of graphene, and discuss consequences for experimentally accessible systems such as monolayer graphene twisted relative to a Bernal bilayer. |
Wednesday, March 8, 2023 4:00PM - 4:12PM |
Q38.00004: Origin of Model Fractional Chern Insulators in All Topological Ideal Flatbands: Explicit Color-entangled Wavefunction and Exact Density Algebra Jie Wang, Semyon Klevtsov, Zhao Liu It is commonly believed that nonuniform Berry curvature ruins the Girvin-MacDonald-Platzman algebra and as a consequence destabilizes fractional Chern insulators. In this work we disprove such common sense by presenting a theory for all topological ideal flatbands with nonzero Chern number C. The smooth single-particle Bloch wavefunction is proved to admit an exact color-entangled form as a superposition of C lowest Landau level type wavefunctions distinguished by boundary conditions. Including repulsive interactions, Abelian and non-Abelian model fractional Chern insulators of Halperin type are stabilized as exact zero-energy ground states no matter how nonuniform Berry curvature is, as long as the quantum geometry is ideal and the repulsion is short-ranged. The key reason behind is the complete separation of the band-projected coordinate (guiding center) and the inter-band transition, in an unnormalized Hilbert space of ideal flatband where the Berry curvature is exactly flattened by scarifying normalization. In such Hilbert space, the flatband-projected density operator obeys a closed Girvin-MacDonald-Platzman type algebra, enabling an exact mapping to C-layered lowest Landau levels. In the end, we discuss applications of the theory, especially to the graphene and TMD based moire materials. |
Wednesday, March 8, 2023 4:12PM - 4:24PM |
Q38.00005: Chiral Kondo lattice in doped MoTe2/WSe2 bilayer Daniele Guerci, Jie Wang, Jiawei Zang, Jennifer Cano, Jed Pixley, Andrew Millis In this talk we theoretically analyse multiband physics in transition metal dicalcogenide bilayers where the density of carriers, the interaction and the energy offset between the orbitals can all be experimentally varied over wide ranges. We show theoretically that this tunability enables unprecedented control over the ground state, opening the door to the synthetic realization and systematic study of site/orbitally selective Mott transitions, heavy-Fermi liquids, Quantum anomalous Hall insulator and perhaps superconductivity. We present a detailed analysis of a new ``heavy fermion” regime in which we show that the combination of strong spin-orbit coupling and the non-local structure gives rise to a chiral Kondo coupling leading to a new twist on heavy fermion physics [1], discuss instabilities of the low-density electron gas in the background of ordered magnetic moments and present a nodal Kondo insulator state. Our theory is motivated by experiments in AB-stacked transition-metal-dichalcogenides bilayer MoTe2/WSe2 [2]. |
Wednesday, March 8, 2023 4:24PM - 4:36PM |
Q38.00006: Charge density wave order and commensuration in TaS2 heterostructures Samra Husremovic, Daniel K Bediako, Berit H Goodge, Sinead M Griffin, Alberto Mier, Katherine Inzani Two-dimensional charge density wave (CDW) materials are promising platforms for energy-efficient electronics. However, principles governing the formation and evolution of CDW quantum phases remain elusive. In this work, we examine novel TaS2 polymorphs and shed light on the interplay between atomic structure, CDW order and electronic properties. We thermally treat 2D 1T-TaS2flakes to make metastable polymorphs of TaS2 comprising 1H layers embedded in a 1T matrix. Different ratios of 1H to 1T layers can be realized with our thermal process, as revealed by differential-phase-contrast scanning transmission electron (DPC STEM) imaging. All synthesized heterostructures exhibit two types of CDW domains, characterized by different rotational tiling of CDW clusters. These so-called mirror domains, which are not commonly coeval, were characterized with room-temperature 4-dimensional (4D) STEM. 4D-STEM revealed that nanoscale mirror domains form in the out-of-plane direction, resulting in a CDW superlattice. The observed CDW structure is accompanied by increased commensuration of the CDW order parameter at room temperature. Further, temperature evolution of the CDW order was probed using a combination of cryogenic Raman spectroscopy and cryogenic TEM diffraction. Findings of these studies were correlated with variable-temperature quantum transport to establish how CDW structure shapes the electronic signatures of engineered TaS2 polytypes. |
Wednesday, March 8, 2023 4:36PM - 4:48PM |
Q38.00007: Engineering Superconductivity in Bernal Bilayer Graphene via Proximity-Induced Spin Orbit Coupling Isabelle Y Phinney, Andrew Zimmerman, Andres M Mier Valdivia, Jeffrey Kwan, Kenji Watanabe, Takashi Taniguchi, Philip Kim Graphene-based heterostructures have shown a wide variety of phases associated with strong electronic correlations, including electric and magnetic field stabilized superconductivity in bilayer and trilayer graphene[1][2]. Finding ways to understand and stabilize the pairing mechanism in these systems is of fundamental interest. Recently, the critical temperature of the electric field-induced superconductivity in Bernal bilayer graphene (BLG) was shown to be increased by proximity induced spin orbit coupling [3]. Here, we study the effects of spin orbit coupling on BLG by placing it on a transition metal dichalchogenide, WSe_2, with controlled stacking angles. We explore the varied phase diagram in electronic transport via doping and electric and magnetic field dependence. We can also alter the twist angle between BLG and WSe_2, which theoretically tunes the strength of the spin orbit coupling. We then fabricate a variety of Josephson junctions to probe properties of the observed superconducting phase. We will discuss our efforts to understand the phase diagram of BLG and extend our observations to the graphene-based family of superconductors. |
Wednesday, March 8, 2023 4:48PM - 5:00PM |
Q38.00008: Synthesis, Characterization and Optical Properties of Misfit Layer Chalcogenides [(PbSe)1+y]m(NbSe2)n and [(SnSe)1+y]m(VSe2)n Maureen Reedyk, Thilini Kuruppu Arachchilage, Paul Respicio Misfit layer dichalcogenide compounds [(MX)1+y]m(TX2)n consist of a stacking of rock salt (MX) and transition metal dichalcogenide (TX2) layers (M = Sn, Pb, Bi, rare earth; X = S, Se; T = Ti, V, Nb, Ta, Cr) where at least one lattice parameter differs between the two sublattices, and m and n give the stacking arrangement of the MX and TX2 layers.[1] We have synthesized single crystals of [(PbSe)1+y]m(NbSe2)n with m=1 and n=1, 2 and 3 via the iodine vapour transport technique. The structure and composition of the single crystals were investigated by X-ray diffraction and Energy Dispersive X-ray spectroscopy respectively and the crystals were further characterized by magnetization and resistivity measurements. The parent compound 2H-NbSe2 is a type II superconductor with a Tc of 7.2 K and a known charge density wave transition at 35 K.[2] As expected the superconducting transition temperature of [(PbSe)1+y]m(NbSe2)n was found to increase with increasing n, the number of dichalcogenide layers inserted between rock salt layers ( n=∞ for NbSe2). We measured the room temperature absolute reflectance over a wide wavenumber range with the goal of elucidating trends in the optical properties via Drude-Lorentz fitting and Kramers Kronig analysis. We found an approximate isosbestic point in the optical conductivity which is common in strongly correlated materials. We have also attempted to synthesize [(SnSe)1+y]m(VSe2)n using the same technique. The parent compound VSe2 exhibits a charge density wave transition near 110 K. We are interested to investigate the effect of the coupling between the two sublattices in the misfit layer compound on the charge density wave state. Results of our studies on these two misfit layer dichalcogenide systems will be presented. |
Wednesday, March 8, 2023 5:00PM - 5:12PM |
Q38.00009: Superconducting quantum criticality in monolayer WTe2 Tiancheng Song, Yanyu Jia, Guo Yu, Yue Tang, Pengjie Wang, Ratnadwip Singha, Xin Gui, Ayelet J. Uzan, Michael Onyszczak, Kenji Watanabe, Takashi Taniguchi, Robert Cava, Leslie M Schoop, N. Phuan Ong, Sanfeng Wu The study of quantum phase transitions in strongly correlated electron systems is of great interest due to the associated quantum critical behaviors. Among different kinds of material systems, tungsten ditelluride (WTe2) is a unique 2D semimetal, which exhibits quantum spin Hall insulator (QSHI), gate-tuned superconductivity, excitonic insulator, and ferroelectricity in a single material. In this talk, I will present the thermoelectric measurement of the QSHI to superconductor transition in monolayer WTe2. Our findings highlight monolayer WTe2 as a 2D model platform for investigating novel superconducting quantum criticality. |
Wednesday, March 8, 2023 5:12PM - 5:24PM |
Q38.00010: Correlation energy in bernal bilayer graphene under strong displacement field Tobias M Wolf, Chunli Huang, Shi-zeng Lin, Allan H MacDonald The discovery of spin-valley symmetry-broken states in graphene multilayers in a strong displacement field raises many questions about the phase diagram and the nature of electronic correlations in this system. While mean-field theory can often correctly explain the onset of symmetry-breaking and the order in which spin-valley symmetries are broken, it completely fails to describe some parts of the experimental phase diagram. In particular in bernal bilayer graphene, experiments found that the low-hole-density paramagnetic state (“Sym-12”) generally survives to much larger hole densities than predicted in mean-field theory. The discrepancies are particularly stark in some parts of the phase diagram; for example the paramagnetic state seems to be exceptionally stable on the low-density side of the van Hove singularity. In an effort to explain the systematic deviations from Hartree-Fock theory, we study the correlation energy contribution to the total energy using the time-dependent Hartree-Fock method, paying special attention to its relationship to the shape and the topology of the Fermi surface. |
Wednesday, March 8, 2023 5:24PM - 5:36PM |
Q38.00011: Competing superconducting and correlated phases in Bernal stacked bilayer graphene Ming Xie Owing to its unique electronic properties and a high degree of tunability, Bernal-stacked bilayer graphene (BLG) has been the subject of research from many different perspectives. Recent experimental observations of superconductivity under a weak magnetic field or in proximity to a spin-orbit coupled substrate brought another new perspective for studying BLG. Moreover, correlated insulating phases at low but finite hole dopings have also been observed shortly after. The correlated insulating states, which are attributed to the formation of Wigner crystals, signify strong electronic repulsion that is very likely not in favor of superconductivity. Here we study the competition between the superconducting and correlated phases and try to shed light on the mechanism underlying both phenomena. We treat the Coulomb interaction and effective attraction between electrons on the same footing and consider the effect of screening and spin-orbit coupling due to the proximity effect. Our approaches can pave the road for studying the completion between superconductivity and correlated states in twisted bilayer graphene with more complicated electronic properties. |
Wednesday, March 8, 2023 5:36PM - 5:48PM |
Q38.00012: Quantum Monte Carlo Simulations for 2D Moiré Flat Bands Xu Zhang, Kai Sun, Zi Yang Meng, Gaopei Pan, Heqiu Li, Xiao Yan Xu, Bin Bin Chen We report a study about moiré flat bands superconductor by projecting Coulomb repulsion and phenomenological attraction onto flat bands. Exact solution at flat band limit and Quantum Mento Carlo (QMC) results for finite band width are shown. With the development of fermion sign bounds theory for QMC, we show the polynomial decay of sign problem in chiral flat band twisted bilayer graphene at integer fillings so that finite temperature characters are ready to study in this limit with affordable sign problem. |
Wednesday, March 8, 2023 5:48PM - 6:00PM |
Q38.00013: Correlation-driven quantum ratchet in a layer-contrasting moiré structure Zhiren Zheng In electronic systems, electrons can come in two distinct flavors, itinerant and localized. Itinerant electrons in a dispersive band can be essential to phenomena, such as Dirac fermions and superconductivity. Meanwhile, localized electrons in a flat band tend to enhance electron correlation. Here, we discover that a layer-contrasting moiré potential can continuously convert the electron flavor between localized and itinerant in a unidirectional fashion, realizing a novel quantum ratchet. Specifically, by aligning a Bernal bilayer graphene (BLG) with two hexagonal boron nitride (BN) at vastly different angles, we create a strongly asymmetric moiré potential landscape. Experimental observation reveals a spontaneous dichotomy of localized and itinerant electrons. The enhanced Coulomb correlation in the localized system induces a continuous unidirectional charge conversion between the two systems. Its irreversible nature powers a highly gate-tunable hysteretic response. Theoretical investigation suggests many-body effects are important in mediating the ratcheting charge conversion and stabilizing a remnant polarization. Our study demonstrates engineered moiré landscape as a novel and generic approach to designing and studying the interplay of electrons with distinct natures. |
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