Entropy driven formation of a half-quantum vortex lattice

TL;DR

This paper demonstrates that a lattice of half-quantum vortices can be stabilized at finite temperatures due to entropy gain, even if it is not energetically favored at zero temperature, highlighting a novel entropy-driven phase transition.

Contribution

It introduces the concept that entropy can stabilize a half-quantum vortex lattice at finite temperature, even when it is not the ground state at zero temperature.

Findings

HQV lattice has an optical phonon branch absent in full vortex lattice.

Finite temperature stabilizes HQV lattice through entropy gain.

HQV lattice structure can differ between T=0 and T>0.

Abstract

Half-quantum vortices (HQVs) can exist in a superconductor or superfluid with an exact or approximate U(1) \times U(1) symmetry, for instance in spinor condensates, 3He-A, Sr2RuO4, and possibly cuprate superconductors with stripe order. In this paper, we show that a lattice of HQVs can be stabilized at finite temperature even when it does not have lower energy than the lattice of full vortices at T = 0 since there is a gain in configurational entropy when a full vortex fractionalizes into a pair of HQVs. Specifically, the lattice of HQVs has an optical branch of phonon modes absent in the lattice of full vortices. Moreover, the HQV lattice at T > 0 can have a different structure than the HQV lattice at T = 0.