Tunneling critical velocity of a superfluid. [Lifshitz-Kogan theory]

Masakazu Ichiyanagi

Miodrag L. Kulić, Armen Sedrakian, Dirk H. Rischke

A rotating ultracold $S$-wave superfluid Fermi gas is studied when the mismatch in chemical potentials $\ensuremath{\delta}\ensuremath{\mu}$ (and the population imbalance $\ensuremath{\delta}n$) corresponds to the Larkin-Ovchinnikov-Fulde-Ferrell state in the vicinity of the Lifshitz critical point. It is shown that under these conditions the critical angular velocity ${\ensuremath{\Omega}}_{c2}$ in two-dimensional systems is an oscillating function of temperature and $\ensuremath{\delta}n$ giving rise to reentrant superfluid phases. The reason for this behavior is the population by Cooper pairs of the Landau levels $(n\ensuremath{\ge}1)$ above the lowest one $(n=0)$.

Francesco Piazza, Lee A. Collins, Augusto Smerzi

Yi‐Cai Zhang, Shizhong Zhang

Abstract In this paper, we develop an appropriate set of hydrodynamic equations for a U(N) invariant superfluid that couple the dynamics of superflow and magnetization. In the special case when both the superfluid and normal velocities are zero, the hydrodynamic equations reduce to a generalized version of Landau–Lifshitz equation for ferromagnetism with U(N) symmetry. When both velocities are non-zero, there appears couplings between the superflow and magnetization dynamics, and the superfluid velocity no longer satisfies the irrotational condition. On the other hand, the magnitude of magnetization is no longer a constant of motion as was the case for the standard Landau–Lifshitz theory. In comparison with the simple superfluid, the first and second sounds are modified by a non-zero magnetization through various thermodynamic functions. For U(2) invariant superfluid, we get both (zero-) sound wave and a spin wave at zero temperature. It is found that the dispersion of spin wave is always quadratic, which is consistent with microscopic analysis. In the appendix, we show that the hydrodynamic equation for a U(N) invariant superfluid can be obtained from the general hydrodynamic equation with arbitrary internal symmetries.

Avinash Kumar, Stephen Eckel, Fred Jendrzejewski +1 more

We study how temperature affects the lifetime of a quantized, persistent current state in a toroidal Bose-Einstein condensate. When the temperature is increased, we find a decrease in the persistent current lifetime. Comparing our measured decay rates to simple models of thermal activation and quantum tunneling, we do not find agreement. We also measured the size of the hysteresis loops in our superfluid ring as a function of temperature, enabling us to extract the critical velocity. The measured critical velocity is found to depend strongly on temperature, approaching the zero-temperature mean-field solution as the temperature is decreased. This indicates that an appropriate definition of critical velocity must incorporate the role of thermal fluctuations, something not explicitly contained in traditional theories.

F. Barranco, P. F. Bortignon, A. Bracco +4 more