Fragility of Z2 topological invariant characterizing triplet excitations in a bilayer kagome magnet

Andreas Thomasen, Karlo Penc, Nic Shannon, and Judit Romhányi
Phys. Rev. B 104, 104412 – Published 7 September 2021

Abstract

The discovery by Kane and Mele of a model of spinful electrons characterized by a Z2 topological invariant had a lasting effect on the study of electronic band structures. Given this, it is natural to ask whether similar topology can be found in the bandlike excitations of magnetic insulators, and recently models supporting Z2 topological invariants have been proposed for both magnon [H. Kondo et al., Phys. Rev. B 99, 041110(R) (2019)] and triplet [D. G. Joshi and A. P. Schnyder, Phys. Rev. B 100, 020407(R) (2019)] excitations. In both cases, magnetic excitations form time-reversal (TR) partners, which mimic the Kramers pairs of electrons in the Kane-Mele model but do not enjoy the same type of symmetry protection. In this paper, we revisit this problem in the context of the triplet excitations of a spin model on the bilayer kagome lattice. Here the triplet excitations provide a faithful analog of the Kane-Mele model as long as the Hamiltonian preserves the TR×U(1) symmetry. We find that exchange anisotropies, allowed by the point group and typical in realistic models, break the required TR×U(1) symmetry and instantly destroy the Z2 band topology. We further consider the effects of TR breaking by an applied magnetic field. In this case, the lifting of spin degeneracy leads to a triplet Chern insulator, which is stable against the breaking of TR×U(1) symmetry. Kagome bands realize both a quadratic and a linear band touching, and we provide a thorough characterization of the Berry curvature associated with both cases. We also calculate the triplet-mediated spin Nernst and thermal Hall signals which could be measured in experiments. These results suggest that the Z2 topology of bandlike excitations in magnets may be intrinsically fragile compared to their electronic counterparts.

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  • Received 7 September 2020
  • Revised 7 July 2021
  • Accepted 23 August 2021

DOI:https://doi.org/10.1103/PhysRevB.104.104412

©2021 American Physical Society

Physics Subject Headings (PhySH)

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Andreas Thomasen1, Karlo Penc2, Nic Shannon1, and Judit Romhányi1,3

  • 1Theory of Quantum Matter Unit, Okinawa Institute of Science and Technology Graduate University, Onna-son, Okinawa 904-0395, Japan
  • 2Institute for Solid State Physics and Optics, Wigner Research Centre for Physics, H-1525 Budapest, P.O.B. 49, Hungary
  • 3Department of Physics and Astronomy, University of California, Irvine, California 92697, USA

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Issue

Vol. 104, Iss. 10 — 1 September 2021

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