Berezinskii-Kosterlitz-Thouless transitions in an easy-plane ferromagnetic superfluid

TL;DR

This paper explores the two distinct Berezinskii-Kosterlitz-Thouless (BKT) transitions in a 2D spin-1 Bose gas with easy-plane ferromagnetic order, revealing complex superfluid behaviors and vortex dynamics.

Contribution

It provides a detailed analysis of the two BKT transitions, including the superfluid phases, vortex binding, and the effects of intercomponent interactions in a 2D spinor Bose gas.

Findings

Identification of two separate BKT transitions for mass and spin superfluidity.

Analytical calculation of superfluid drag between spin components.

Phase diagram showing dependence on quadratic Zeeman energy q.

Abstract

A two-dimensional (2D) spin-1 Bose gas exhibits two Berezenskii-Kosterlitz-Thouless (BKT) transitions in the easy-plane ferromagnetic phase. The higher temperature transition is associated with superfluidity of the mass current determined predominantly by a single spin component. The lower temperature transition is associated with superfluidity of the axial spin current, quasi-long range order of the transverse spin density and binding of polar-core spin vortices (PCVs). Above the spin BKT temperature, the component circulations that make up each PCV spatially separate, suggesting possible deconfinement analogous to quark deconfinement in high energy physics. Intercomponent interactions give rise to superfluid drag between the spin components, which we calculate analytically at zero temperature. We present the mass/spin superfluid phase diagram as a function of quadratic Zeeman energy $q$. At $q=0$ the system is in an isotropic spin phase with $\mathrm{SO}(3)$ symmetry. Here the fluid response exhibits a system size dependence, suggesting the absence of a BKT transition. Despite this, for finite systems the decay of spin correlations changes from exponential to algebraic as the temperature is decreased.