Immiscibile two-component Bose Einstein condensates beyond mean-field approximation: phase transitions and rotational response

TL;DR

This paper investigates phase transitions in a two-component immiscible Bose-Einstein condensate beyond mean-field theory, revealing complex thermal and rotational behaviors including symmetry breaking, vortex interactions, and phase separation.

Contribution

It provides a detailed phase diagram analysis of the system at finite temperature and rotation, highlighting novel phase transitions and symmetry-breaking phenomena beyond mean-field approximation.

Findings

First-order transition from single-component to two-component normal fluid with temperature increase

Rotation induces a 'mixing' phase transition with broken U(1) symmetry

Strong inter-component interactions lead to split superfluid and Z2 phase transitions

Abstract

We consider a two-component immiscible Bose-Einstein condensate with dominating intra-species repulsive density-density interactions. In the ground-state phase of such a system only one condensates is present. This can be viewed as a spontaneous breakdown of $\mathbb{Z}_2$ symmetry. We study the phase diagram of the system at finite temperature beyond mean-field approximation. In the absence of rotation, we show that the system undergoes a first order phase transition from this ground state to a miscible two-component normal fluid as temperature is increased. In the presence of rotation, the system features a competition between vortex-vortex interaction and short range density-density interactions. This leads to a rotation-driven "mixing" phase transition in a spatially inhomogeneous state with additional broken $\mathrm{U}(1)$ symmetry. Thermal fluctuations in this state lead to nematic two-component sheets of vortex liquids. At sufficiently strong inter-component interaction, we find that the superfluid and $\mathbb{Z}_2$ phase transitions split. This results in the formation of an intermediate state which breaks only $\mathbb{Z}_2$ symmetry. It represents two phase separated normal fluids with density imbalance.