Dirac cones and mass terms in bosonic spectra

TL;DR

This paper extends the concept of Dirac cones and mass terms from fermionic to bosonic systems, demonstrating topological phenomena and protected edge states in magnon and triplon spectra using a new Clifford algebra framework.

Contribution

It introduces a novel Clifford algebra approach for bosonic spectra, enabling the analysis of Dirac cones, mass terms, and topological transitions in bosonic excitations.

Findings

Dirac cones appear in bosonic magnon spectra.

Mass terms can open gaps in bosonic bands.

Topological edge states emerge at band transitions.

Abstract

The notion of Dirac cones, wherein two or more bands become degenerate at a certain momentum, is the starting point for the study of topological phases. Dirac cones have been thoroughly explored in fermionic systems such as graphene, Weyl semimetals, etc. The underlying mathematical structure in these systems is a Clifford algebra -- a rule for identifying sets of matrices that span the Hamiltonian. This structure allows for the identification of suitable `mass' terms to open band gaps. In this article, we extend these ideas to bosonic systems. Due to the pseudo-orthogonal nature of eigenvectors, the algebra of matrices takes a very different form. Taking the honeycomb XY ferromagnet as a prototype, we show that a Dirac cone emerges in the magnon spectrum. A gap can be opened by a suitable mass term involving next-nearest neighbour interactions. We next construct a one-dimensional ladder model with triplon excitations. Using the new Clifford algebra, we define winding number as a topological invariant. In analogy with the Su-Schrieffer-Heeger model, topological transitions occur when the band gap closes, leading to the appearance (or disappearance) of protected edge states. Our results suggest a new route to studying band touching and band topology in bosonic systems.