Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session D67: Nematicity in Topological and Superconducting MaterialsInvited
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Sponsoring Units: DCMP Chair: Qimiao Si, Rice Univ Room: Four Seasons 2-3 |
Monday, March 2, 2020 2:30PM - 3:06PM |
D67.00001: Interacting multi-channel topological boundary modes in a quantum Hall valley system Invited Speaker: Mallika Randeria Two-dimensional quantum Hall systems offer a versatile platform to explore the interplay between topology and symmetry breaking facilitated by Coulomb interactions. Spontaneous valley ordering of bismuth surface states in a large magnetic field gives rise to the emergence of a nematic phase. We can directly visualize the rotational symmetry breaking of this nematic phase using a scanning tunneling microscope (STM) [1]. We further use an STM to image the formation of topological boundary modes in between nematic quantum Hall domains. By changing the valley flavor and number of modes at the domain wall, we realize strikingly different regimes where these boundary modes are either metallic or insulating. This behavior is a consequence of Coulomb interactions constrained by the valley flavor, making these channels a new class of interacting symmetry-protected Luttinger liquids [2,3]. |
Monday, March 2, 2020 3:06PM - 3:42PM |
D67.00002: Odd-parity multipolar orders in the spin–orbit-coupled metal Cd2Re2O7 Invited Speaker: Zenji Hiroi The pyrochlore oxide Cd2Re2O7 is the only superconductor (Tc = 1.0 K) in the family of α-pyrochlore oxides [1]. Moreover, it exhibits two characteristic structural transitions losing the inversion symmetry below 200 K. Recently, it has attracted increasing attention as a candidate spin–orbit-coupled metal (SOCM), in which specific Fermi liquid instability can lead to odd-parity multipolar orders with spontaneous inversion-symmetry breaking [2] and parity-mixing superconductivity [3, 4]. More recent theoretical classifications based on the crystal symmetry reveal that the two parity-breaking phases are described by unconventional odd-parity multipoles, that are the electric toroidal quadrupoles (ETQs) with different components, x2−y2 and 3z2−r2 [5]. These ETQs are unique as they are cluster multipoles defined on the tetrahedral unit of the pyrochlore lattice, distinguished from conventional atomic multipoles realized in f-electron systems [6]. Here we report our recent experimental progress and review the present understanding of the SOCM Cd2Re2O7. |
Monday, March 2, 2020 3:42PM - 4:18PM |
D67.00003: Nematic Superconductivity Invited Speaker: Vladyslav Kozii Nematic superconductivity refers to the spontaneous breaking of rotational symmetry of a system in superconducting state. Nematic superconductivity in topological insulator Bi2Se3 doped with Cu, Nb, or Sr atoms was observed in numerous experiments, including Kinght shift, upper critical field, specific heat, magnetic torque, and STM measurements. Remarkably, such a nematic state appears to be a clear signature of topological superconductivity. In this talk, I will discuss the theoretically predicted properties of these compounds and their relation to the existing experiments. Furthermore, the recent measurements of the upper critical field in twisted bilayer graphene revealed strong two-fold anisotropy of the superconducting state, indicating nematic superconductivity. Motivated by these experiments, I will also briefly describe some theoretical proposals aiming to explain such an exotic state. |
Monday, March 2, 2020 4:18PM - 4:54PM |
D67.00004: Nematicity in normal and superconducting states of twisted bilayer graphene Invited Speaker: Pablo Jarillo-Herrero In this talk I will review our recent experiments on magic-angle twisted bilayer graphene which show normal state and superconducting state anisotropies in the resistivity and critical magentic field, respectively. These represent evidence for nematicity in this novel correlated system. |
Monday, March 2, 2020 4:54PM - 5:30PM |
D67.00005: Spin-polarized correlated insulator and superconductor in twisted double bilayer graphene. Invited Speaker: Xiaomeng Liu In flat electronic bands, the effect of interaction is elevated, which may lead to the emergence of correlated electronic states. Recently, the discoveries of correlated insulator and interaction-driven superconductivity in the magic-angle twisted bilayer graphene (MATBG) sparked the search of flat band materials enabled by moire superlattices. In this talk, I will present our experimental study of moire flat band in twisted double bilayer graphene (TDBG). In TDBG, the flatness of the band is tunable by perpendicular electric fields, adding another experimental knob and eliminating the requirement of a precise twist angle. Correlated insulators and superconductivity are discovered in TDBG with a much larger energy scale than in MATBG. Intriguingly, these emergent phases are found to be spin-polarized by in-plane magnetic field dependence. Since ferromagnetism and superconductivity typically compete with each other, the finding of spin-polarization in a superconductor carries an implication of exotic paring mechanism and pairing symmetry. |
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