Unconventional interaction between vortices in a polarized Fermi gas

TL;DR

This paper investigates vortices in a novel gapless superfluid state of a spin-imbalanced ultracold Fermi gas, revealing an unconventional, oscillating vortex interaction mediated by gapless fermions, with implications for vortex lattice structures.

Contribution

It introduces a symmetry-based effective field theory to describe vortex interactions in a gapless superfluid state, uncovering a spatially oscillating, RKKY-like potential caused by gapless fermions.

Findings

Vortex interactions exhibit spatial oscillations similar to RKKY interactions.

The effective theory parameters are linked to measurable experimental quantities.

Conditions for experimental verification of the unconventional vortex interactions are discussed.

Abstract

Recently, a homogeneous superfluid state with a single gapless Fermi surface was predicted to be the ground state of an ultracold Fermi gas with spin population imbalance in the regime of molecular Bose-Einstein condensation. We study vortices in this novel state using a symmetry-based effective field theory, which captures the low-energy physics of gapless fermions and superfluid phase fluctuations. This theory is applicable to all spin-imbalanced ultracold Fermi gases in the superfluid regime, regardless of whether the original fermion pairing interaction is weak or strong. We find a remarkable, unconventional form of the interaction between vortices. The presence of gapless fermions gives rise to a spatially oscillating potential, akin to the RKKY indirect-exchange interaction in non-magnetic metals. We compare the parameters of the effective theory to the experimentally measurable quantities and further discuss the conditions for the verification of the predicted new feature. Our study opens up an interesting question as to the nature of the vortex lattice resulting from the competition between the usual repulsive logarithmic (2D Coulomb) and predominantly attractive fermion-induced interactions.