Person: Vishwanath, Ashvin
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Vishwanath
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Ashvin
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Vishwanath, Ashvin
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Publication Tunable spin-polarized correlated states in twisted double bilayer graphene(Springer Science and Business Media LLC, 2020-07-08) Liu, Xiaomeng; Hao, Zeyu; Khalaf, Eslam; Lee, Jong Yeon; Ronen, Yuval; Yoo, Hyobin; Haei Najafabadi, Danial; Watanabe, Kenji; Taniguchi, Takashi; Vishwanath, Ashvin; Kim, PhilipReducing the energy bandwidth of electrons in a lattice below the long-range Coulomb interaction energy promotes correlation effects. Created by stacking van der Waals (vdW) heterostructures with a controlled twist angle1–3, moire ́ superlattices enable the engineering of electron band structure. In an engineered moire ́ flat band, exotic quantum phases can emerge. The correlated insulator, superconductivity, and quantum anomalous Hall ef- fect found in the flat band of the magic angle twisted bilayer graphene (MA-TBG) 4–8 have sparkled exploration of correlated electron states in other moire ́ systems 9–11. The electronic properties of vdW moire ́ superlattices can further be tuned by adjusting the interlayer cou- pling 6 or the band structure of constituent layers 9. Here, employing vdW heterostructures of twisted double bilayer graphene (TDBG), we demonstrate a flat electron band that is tun- able by perpendicular electric fields in a range of twist angles. Similar to the MA-TBG, TDBG exhibits energy gaps at the half- and quarter-filled flat bands, indicating the emergence of correlated insulating states. We find that the gaps of these insulating states increase with in-plane magnetic field, suggesting a ferromagnetic order. Upon doping the half-filled insulator, a sudden drop of resistivity is observed with lowering temperature. This critical behavior is confined in a small area in the density-electric field plane, and is attributed to a phase transition from a normal metal to a spin-polarized correlated state. Spin-polarized correlated states discovered in the electric field tunable TDBG provide a new route to engineering interaction-driven quantum phases.Publication Nishimori transition across the error threshold for constant-depth quantum circuits(Springer Science and Business Media LLC, 2024-12-16) Chen, Edward H.; Zhu, Guo-Yi; Verresen, Ruben; Seif, Alireza; Bäumer, Elisa; Layden, David; Tantivasadakarn, Nathanan; Zhu, Guanyu; Sheldon, Sarah; Vishwanath, Ashvin; Trebst, Simon; Kandala, AbhinavPublication Emergent Dirac fermions and broken symmetries in confined and deconfined phases of Z2 gauge theories(Springer Nature, 2017) Gazit, Snir; Randeria, Mohit; Vishwanath, AshvinLattice gauge theories are ubiquitous in physics, describing a wide range of phenomena from quark confinement to quantum materials. At finite fermion density, gauge theories are notoriously hard to analyze due to the fermion sign problem. Here, we investigate the Ising gauge theory in 2+1 dimensions, a problem of great interest in condensed matter, and show that it is free of the sign problem at arbitrary fermion density. At generic filling, we find that gauge fluctuations mediate pairing leading to a transition between a deconfined BCS state to a confined BEC. At half-filling, a π-flux phase is generated spontaneously with emergent Dirac fermions. The deconfined Dirac phase, with a vanishing Fermi surface volume is a non-trivial example of violation of Luttinger’s theorem due to fractionalization. At strong coupling, we find a single continuous transition between the deconfined Dirac phase and the confined BEC, in contrast to the expected split transition.Publication Observation of a discrete time crystal(Springer Science and Business Media LLC, 2017-03-09) Hess, P.W.; Zhang, J.; Kyprianidis, A.; Becker, P.; Lee, A.; Smith, J.; Pagano, G.; Potirniche, I.-D.; Potter, A.C.; Vishwanath, Ashvin; Yao, N.Y.; Monroe, C.Spontaneous symmetry breaking is a fundamental concept in many areas of physics, ranging from cosmology and particle physics to condensed matter1. A prime example is the breaking of spatial translation symmetry, which underlies the formation of crystals and the phase transition from liquid to solid. Analogous to crystals in space, the breaking of translation symmetry in time and the emergence of a “time crystal” was recently proposed2,3, but later shown to be forbidden in thermal equilibrium4–6. However, nonequilibrium Floquet systems subject to a periodic drive can exhibit persistent time-correlations at an emergent sub-harmonic frequency7–10. This new phase of matter has been dubbed a “discrete time crystal” (DTC)10 (This phase is also referred to as a π-spin glass7 or a Floquet time crystal8). Here, we present the first experimental observation of a discrete time crystal, in an interacting spin chain of trapped atomic ions. We apply a periodic Hamiltonian to the system under many-body localization (MBL) conditions, and observe a sub-harmonic temporal response that is robust to external perturbations. Such a time crystal opens the door for studying systems with long-range spatial-temporal correlations and novel phases of matter that emerge under intrinsically non-equilibrium conditions7.Publication Nematic Order by Disorder in Spin-2 Bose-Einstein Condensates(American Physical Society (APS), 2007) Turner, Ari M.; Barnett, Ryan; Demler, Eugene; Vishwanath, AshvinWe show that quantum and thermal fluctuations in spin-2 Bose-Einstein condensates lift the accidental degeneracy of the mean-field phase diagram. Fluctuations select the uniaxial (square biaxial) nematic state for scattering lengths a4 > a2 (a4 < a2). Paradoxically, the order is stronger at higher temperatures. For spin-2 87Rb and 23Na, a continuous Ising-type transition is predicted on raising the magnetic field, from a fluctuation stabilized uniaxial state to a field stabilized square biaxial order state. This is a promising experimental system to realize the ‘‘order-by-disorder’’ phenomenon.Publication Comprehensive search for topological materials using symmetry indicators(Springer Nature, 2019-02) Tang, Feng; Po, Hoi Chun; Vishwanath, Ashvin; Wan, XiangangTopological materials have attracted much attention in the past decade. While several theoret- ically proposed topological materials have been experimentally confirmed, extensive experimental exploration of topological properties as well as applications in realistic devices have been held back due to the lack of excellent topological materials in which interference from trivial Fermi surface states are minimized. Here we tackle this problem by applying our recently developed method of symmetry indicators to all non-magnetic compounds in the 230 space groups. An exhaustive database search reveals thousands of topological materials candidates. Of these, we highlight the excellent candidates, the 241 topological insulators and 142 topological crystalline insulators which have either noticeable full band gap or a considerable direct gap together with small trivial Fermi pockets. We also give a list of 692 topological semimetals with the band crossing points located near the Fermi level. All predictions obtained through standard generalized gradient approximation calculations were cross-checked with the modified Becke-Johnson potential calculations, appropriate for narrow gap materials. These newly found topological materials candidates open wide possibilities for realizing the promise of topological materials in next-generation electronic devices.Publication Fractional Chern Insulators in Magic-Angle Twisted Bilayer Graphene(Springer Science and Business Media LLC, 2021-12-15) Xie, Yonglong; Pierce, Andrew; Park, Jeong Min; Parker, Daniel E.; Khalaf, Eslam; Ledwith, Patrick; Cao, Yuan; Lee, Seung Hwan; Chen, Shaowen; Forrester, Patrick R.; Watanabe, Kenji; Taniguchi, Takashi; Vishwanath, Ashvin; Jarillo-Herrero, Pablo; Yacoby, AmirAbstractFractional Chern insulators (FCIs) are lattice analogues of fractional quantum Hall states that may provide a new avenue towards manipulating non-Abelian excitations. Early theoretical studies1–7 have predicted their existence in systems with flat Chern bands and highlighted the critical role of a particular quantum geometry. However, FCI states have been observed only in Bernal-stacked bilayer graphene (BLG) aligned with hexagonal boron nitride (hBN)8, in which a very large magnetic field is responsible for the existence of the Chern bands, precluding the realization of FCIs at zero field. By contrast, magic-angle twisted BLG9–12 supports flat Chern bands at zero magnetic field13–17, and therefore offers a promising route towards stabilizing zero-field FCIs. Here we report the observation of eight FCI states at low magnetic field in magic-angle twisted BLG enabled by high-resolution local compressibility measurements. The first of these states emerge at 5 T, and their appearance is accompanied by the simultaneous disappearance of nearby topologically trivial charge density wave states. We demonstrate that, unlike the case of the BLG/hBN platform, the principal role of the weak magnetic field is merely to redistribute the Berry curvature of the native Chern bands and thereby realize a quantum geometry favourable for the emergence of FCIs. Our findings strongly suggest that FCIs may be realized at zero magnetic field and pave the way for the exploration and manipulation of anyonic excitations in flat moiré Chern bands.Publication Erratum: Symmetry-based indicators of band topology in the 230 space groups(Nature Publishing Group UK, 2017) Po, Hoi Chun; Vishwanath, Ashvin; Watanabe, Haruki