Fluctuation effects in rotating Bose-Einstein condensates with broken $\mathrm{SU}(2)$ and $\mathrm{U}(1)\times \mathrm{U}(1)$ symmetries in the presence of intercomponent density-density interactions

TL;DR

This paper explores how thermal fluctuations influence vortex states in two-component Bose-Einstein condensates with broken symmetries, revealing unique phase transitions and states not seen in single-component systems.

Contribution

It provides the first detailed analysis of thermal effects on vortex matter in superfluids with $ ext{U(1)} imes ext{U(1)}$ and $ ext{SU(2)}$ symmetries, uncovering novel phase behaviors.

Findings

Discovery of thermally driven phase transitions between square and hexagonal vortex lattices.

Identification of a non-topological vortex state where superfluid coherence persists without a regular vortex lattice.

Observation of transitions between nearly-degenerate vortex states induced by thermal fluctuations.

Abstract

Thermal fluctuations and melting transitions for rotating single-component superfluids have been intensively studied and are well understood. In contrast, the thermal effects on vortex states for two-component superfluids with density-density interaction, which have a much richer variety of vortex ground states, have been much less studied. Here, we investigate the thermal effects on vortex matter in superfluids with $\mathrm{U(1)}\times \mathrm{U(1)}$ broken symmetries and intercomponent density-density interactions, as well as the case with a larger $\mathrm{SU(2)}$ broken symmetry obtainable from the $\mathrm{U(1)}\times \mathrm{U(1)}$-symmetric case by tuning scattering lengths. In the former case we find that, in addition to first-order melting transitions, the system exhibits thermally driven phase transitions between square and hexagonal lattices. Our main result, however, concerns the case where the condensate exhibits $\mathrm{SU(2)}$-symmetry, and where vortices are not topological. At finite temperature, the system exhibits effects which do not have a counter-part in single component systems. Namely, it has a state where thermally averaged quantities show no regular vortex lattice, yet the system retains superfluid coherence along the axis of rotation. In such a state, the thermal fluctuations result in transitions between different (nearly)-degenerate vortex states without undergoing a melting transition. Our results apply to multi-component Bose-Einstein condensates, and we suggest how to experimentally detect some of these unusual effects in such systems.