Topological Triplon Modes and Bound States in a Shastry-Sutherland Magnet

TL;DR

This paper demonstrates that the dimerized quantum magnet SrCu₂(BO₃)₂ exhibits topological triplon bands with nonzero Chern numbers, leading to protected chiral edge states, thus extending topological insulator physics to magnetic excitations.

Contribution

It confirms that SrCu₂(BO₃)₂ is a bosonic topological insulator with topologically protected edge states in a weak magnetic field.

Findings

Identification of nonzero Chern numbers in triplon bands.

Observation of topologically protected chiral edge excitations.

Confirmation of the bosonic topological insulator phase in a magnetic system.

Abstract

The twin discoveries of the quantum Hall effect, in the 1980's, and of topological band insulators, in the 2000's, were landmarks in physics that enriched our view of the electronic properties of solids. In a nutshell, these discoveries have taught us that quantum mechanical wavefunctions in crystalline solids may carry nontrivial topological invariants which have ramifications for the observable physics. One of the side effects of the recent topological insulator revolution has been that such physics is much more widespread than was appreciated ten years ago. For example, while topological insulators were originally studied in the context of electron wavefunctions, recent work has led to proposals of topological insulators in bosonic systems: in photonic crystals, in the vibrational modes of crystals, and in the excitations of ordered magnets. Here we confirm the recent proposal that, in a weak magnetic field, the dimerized quantum magnet SrCu$_{2}$(BO$_{3}$)$_2$ is a bosonic topological insulator with nonzero Chern number in the triplon bands and topologically protected chiral edge excitations.