Higher-order topology and corner triplon excitations in two-dimensional quantum spin-dimer models

TL;DR

This paper explores higher-order topological phases in 2D quantum spin-dimer models, revealing corner triplon excitations as signatures of nontrivial bulk topology through theoretical calculations and proposing experimental detection methods.

Contribution

It introduces two 2D quantum dimer models with higher-order topological triplon bands and demonstrates the existence of corner modes using real-space calculations and topological invariants.

Findings

Identification of nontrivial higher-order topology in spin-dimer models

Existence of mid-bandgap corner triplon modes as topological signatures

Phase transitions driven by parameter tuning in the models

Abstract

The concept of free fermion topology has been generalized to $d$-dimensional phases that exhibit $(d-n)$-dimensional boundary modes, such as zero-dimensional (0D) corner excitations. Motivated by recent extensions of these ideas to magnetic systems, we consider 2D quantum paramagnets formed by interacting spin dimers with dispersive triplet excitations. We propose two examples of such dimer models, where the spin-gapped bosonic triplon excitations are shown to host bands with nontrivial higher-order topology. We demonstrate this using real-space Bogoliubov--de Gennes calculations that reveal the existence of mid-bandgap corner triplon modes as a signature of higher-order bulk topology. We provide an understanding of the higher-order topology in these systems via a computation of bulk topological invariants as well as the construction of edge theories, and study their phase transitions as we tune parameters in the model Hamiltonians. We also discuss possible experimental approaches for detecting the emergent corner triplon modes.