Abstract
We use classical field simulations of the homogeneous Bose gas to study the breakdown of superflow due to vortex nucleation past a cylindrical obstacle at finite temperature. Thermal fluctuations modify the vortex nucleation from the obstacle, turning anti-parallel vortex lines (which would be nucleated at zero temperature) into wiggly lines, vortex rings and even vortex tangles. We find that the critical velocity for vortex nucleation decreases with increasing temperature, and scales with the speed of sound of the condensate, becoming zero at the critical temperature for condensation.
Highlights
A defining feature of superfluids is the absence of excitations when the flow is slower than a critical velocity; above this velocity, the flow becomes dissipative
In this work we study the motion of a cylindrical Gaussianshaped obstacle through a 3D homogeneous Bose gas at finite temperature via classical field simulations
We have analyzed the nucleation of vortices past a moving cylindrical obstacle in a finite-temperature homogeneous Bose gas
Summary
A defining feature of superfluids is the absence of excitations when the flow (relative to some obstacle or boundary) is slower than a critical velocity; above this velocity, the flow becomes dissipative. The breakdown of superfluidity has been experimentally probed by introducing a localized repulsive obstacle, engineered via the repulsive force generated by a focused blue-detuned laser beam and moving the condensate relative to the obstacle [2,3,4,5,6,7,8,9] This has enabled the measurement of the critical velocity and the direct observation of the ensuing excitations, that is, pairs of quantized vortex lines with opposite polarity. Kwon et al undertook a systematic experimental analysis of the critical velocity for vortex shedding, exploring the dependence of the nucleation on height and width of the penetrable obstacle and the crossover from penetrable to impenetrable obstacles [8] Their results, obtained in a condensate with temperature much lower than the critical temperature for condensation, are in agreement with previous zero-temperature predictions based on the Gross-Pitaevskii equation. Tangle, and we indicate the occurrence of these structures in the parameter space of condensate fraction and flow speed
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