Thomas P. Billam, Matthew T. Reeves, Brian P. Anderson, Ashton S. Bradley
A negative-temperature statistical equilibrium state of two-dimensional quantum vortices can exhibit clustering of same-circulation vortices and a macroscopic accumulation of kinetic energy at the system length scale, which we term an Onsager-Kraichnan condensate (OKC). We develop a method for microcanonical sampling of equilibrium vortex configurations in the homogeneous 2D Gross-Pitaevskii theory. Varying the system energy at fixed vortex number, we characterize the possible states of vortex matter in a bulk superfluid, from the vortex-dipole gas at positive temperature, to negative-temperature OKC states exhibiting macroscopic vortex clustering and kinetic energy condensation. Dynamical simulations reveal that negative-temperature OKC states emerge as the end states of decaying two-dimensional quantum turbulence. Our work provides the first realization of Onsager's point-vortex model, and its negative temperature states, in a microscopic theory of a 2D superfluid.
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http://arxiv.org/abs/1307.6374
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