Bosonic Topological Excitations from the Instability of a Quadratic Band Crossing

TL;DR

This paper explores how interactions in ultracold bosonic atoms on a 2D optical lattice can induce topological excitations by destabilizing quadratic band crossings, leading to a novel superfluid phase with protected edge states.

Contribution

It introduces two realizations of quadratic band crossings in optical lattices and demonstrates how interactions induce a topological superfluid phase with edge excitations.

Findings

Interaction induces a topological gap in the excitation spectrum.

Spontaneous time-reversal symmetry breaking occurs in the superfluid phase.

Protected edge excitations are observed in finite systems.

Abstract

We investigate the interaction-driven instability of a quadratic band crossing arising for ultracold bosonic atoms loaded into a two-dimensional optical lattice. We consider the case when the degenerate point becomes a local minimum of both crossing energy bands such that it can support a stable Bose-Einstein condensate. Repulsive contact interaction among the condensed bosons induces a spontaneously time-reversal symmetry broken superfluid phase and a topological gap is opened in the excitation spectrum. We propose two concrete realizations of the desired quadratic band crossing in lattices with either fourfold or sixfold rotational symmetries via suitable tuning of the unit cell leading to reduced Brillouin zones and correspondingly folded bands. In either case, topologically protected edge excitations are found for a finite system.