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.
We consider a two-component Bose-Einstein condensate with and without synthetic "spin-orbit" interactions in two dimensions. Density-and phase-fluctuations of the condensate are included, allowing us to study the impact of thermal fluctuations and density-density interactions on the physics originating with spin-orbit interactions. In the absence of spin-orbit interactions, we find that inter-component density interactions deplete the minority condensate. The thermally driven phase transition is driven by coupled density and phase-fluctuations, but is nevertheless shown to be a phase-transition in the Kosterlitz-Thouless universality class with close to universal amplitude ratios irrespective of whether both the minority-and majority condensates exist in the ground state, or only one condensate exists. In the presence of spin-orbit interactions we observe three separate phases, depending on the strength of the spin-orbit coupling and inter-component density-density interactions: a phasemodulated phase with uniform amplitudes for small intercomponent interactions, a completely imbalanced, effectively single-component, condensate for intermediate spin-orbit coupling strength and suficciently large inter-component interactions, and a phase-modulated and amplitude-modulated phase for sufficiently large values of both the spin-orbit coupling and the inter-component density-density interactions. The phase which is modulated by a single q-vector only is observed to transition into an isoptropic liquid through a strong depinning transition with periodic boundary conditions, which weakens with open boundaries. arXiv:1608.03300v1 [cond-mat.quant-gas]
The N -component London U(1) superconductor is expressed in terms of integer-valued supercurrents. We show that the inclusion of inter-band Josephson couplings introduces monopoles in the current fields, which convert the phase transitions of the charge-neutral sector to crossovers. The monopoles only couple to the neutral sector, and leave the phase transition of the charged sector intact. The remnant non-critical fluctuations in the neutral sector influence the one remaining phase transition in the charged sector, and may alter this phase transition from a 3DXY inverted phase transition into a first-order phase transition depending on what the values of the gauge-charge and the inter-component Josephson coupling are. This preemptive effect becomes more pronounced with increasing number of components N , since the number of charge-neutral fluctuating modes that can influence the charged sector increases with N . We also calculate the gauge-field correlator, and by extension the Higgs mass, in terms of current-current correlators. We show that the onset of the Higgsmass of the photon (Meissner-effect) is given in terms of a current-loop blowout associated with going into the superconducting state as the temperature of the system is lowered.
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