| Brief Bio of the Speaker |
Dr. Zhongyi Zhang obtained his B.S. from Shandong University in 2018, and his Ph.D. from the Institute of Physics, Chinese Academy of Sciences, in 2023. He subsequently worked as a Postdoctoral Fellow at the Hong Kong University of Science and Technology. His research centers on the theoretical study of condensed matter physics, with a particular emphasis on the full classification, topological, magnetic, and superconducting properties of exotic quantum systems. |
Abstract |
Topological superconductivity is widely believed to require unconventional pairing, yet most superconductors have s-wave-like pairing, which transforms trivially under the crystal symmetry. It thus looks trivial in two senses: topologically, and because its symmetry label cannot tell a sign-changing s_pm gap from a uniform one. I show that crystalline symmetry makes it nontrivial in both senses, because the gap’s distribution over symmetry-labeled band sectors decides its topology and its experimental fingerprints. First, I construct a universal scheme for topological superconductivity from conventional BCS s-wave pairing: where space-group symmetry enforces high-dimensional band degeneracies, magnetic order that splits them yields various topological phases, independent of any details of system. With time reversal preserved, symmetry indicators are silent for s-wave-like pairing, so I derive Fermi-surface formulas from the momentum-space Atiyah-Hirzebruch spectral sequence that read the invariants from the signs of the Fermi velocity and intraband pairing on symmetry-labeled Fermi surfaces, covering all layer and space groups. Second, the bilayer nickelate La3Ni2O7 belongs to the recently discovered third family of high-Tc superconductors, and its widely proposed interlayer pairing is a symmetry-enforced spm state: s-wave-like in pairing symmetry, yet forced by the mirror exchanging the two NiO2 layers to change sign between the bonding and antibonding sectors. This makes bilayer nickelates a natural platform for both kinds of nontriviality, and I will show that this crystalline mirror symmetry plays crucial role in several aspects. To begin with, quasiparticle interference must take the mirror into account when probing the pairing sign; otherwise an spm pairing can appear to have no sign change at all. Next, I show that a point contact coupled unevenly to the two layers introduces a mirror-odd potential that binds in-gap Andreev states, so that nodeless spm pairing displays d-wave-like Andreev signals. Finally, the classification of its space group forces this spm state to be topologically trivial, but the layer asymmetry that a substrate induces in thin films can drive it into a time-reversal-invariant nodal topological superconductor. Its Majorana Kramers pair on the edge produces a zero-bias peak, while the edge shows none, again mimicking d-wave signatures.
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