Fluctuation effects in rotating Bose-Einstein condensates with broken SU(2) andU(1)×U(1)symmetries in the presence of intercomponent density-density interactions

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 U(1) × U(1) broken symmetries and intercomponent density-density interactions, as well as the case with a larger SU(2) broken symmetry obtainable from the U(1) × 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 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.

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