The Higgs mode in a superfluid of Dirac fermions

TL;DR

This paper investigates the Higgs amplitude mode in a superfluid of Dirac fermions on a honeycomb lattice, revealing stable collective excitations across a quantum phase transition, measurable via cold atom experiments.

Contribution

It demonstrates the existence and evolution of the Higgs mode and related collective excitations in a Dirac fermion superfluid near a quantum phase transition, highlighting their stability and experimental detectability.

Findings

Collective modes are stable against decay into quasiparticle pairs.

Modes evolve smoothly across the quantum phase transition.

Bragg scattering can measure these collective excitations.

Abstract

We study the Higgs amplitude mode in the s-wave superfluid state on the honeycomb lattice inspired by recent cold atom experiments. We consider the attractive Hubbard model and focus on the vicinity of a quantum phase transition between semi-metal and superfluid phases. On either side of the transition, we find collective mode excitations that are stable against decay into quasiparticle-pairs. In the semi-metal phase, the collective modes have "Cooperon" and exciton character. These modes smoothly evolve across the quantum phase transition, and become the Anderson-Bogoliubov mode and the Higgs mode of the superfluid phase. The collective modes are accommodated within a window in the quasiparticle-pair continuum, which arises as a consequence of the linear Dirac dispersion on the honeycomb lattice, and allows for sharp collective excitations. Bragg scattering can be used to measure these excitations in cold atom experiments, providing a rare example wherein collective modes can be tracked across a quantum phase transition.