Superfluidity and excitations at unitarity

TL;DR

This paper presents lattice simulations of spin-1/2 fermions at unitarity, revealing superfluid ground state properties and characterizing low-energy excitations through correlation functions.

Contribution

It provides new lattice results for unitarity fermions, analyzing superfluidity and low-energy excitations using Euclidean time projection and correlation functions.

Findings

Long-range order indicating superfluidity at large Euclidean times

Exponential decay due to a low-energy excitation at intermediate times

Identification of a chain-like low-energy excitation extending across the lattice

Abstract

We present lattice results for spin-1/2 fermions at unitarity, where the effective range of the interaction is zero and the scattering length is infinite. We measure the spatial coherence of difermion pairs for a system of 6, 10, 14, 18, 22, 26 particles with equal numbers of up and down spins in a periodic cube. Using Euclidean time projection, we analyze ground state properties and transient behavior due to low-energy excitations. At asymptotically large values of t we see long-range order consistent with spontaneously broken U(1) fermion-number symmetry and a superfluid ground state. At intermediate times we see exponential decay in the t-dependent signal due to an unknown low-energy excitation. We probe this low-energy excitation further by calculating two-particle correlation functions. We find that the excitation has the properties of a chain of particles extending across the periodic lattice.