Critical velocities of persistent currents of superfluid helium

James R. Clow

Jukka P Pekola

I. Rudnick, H. Kojima, W. Veith +1 more

Haruo Kojima

Abstract : Fourth sound techniques were extensively developed into a powerful tool for investigating persistent current states in rotating superfluid helium contained in a superleak. Unique features of these techniques are: (1) persistent superfluid component velocity is directly determined by the Doppler-shifted splitting of a fourth sound resonant mode in an annular cavity; (2) the superfluid component density is determined simultaneously and independently by measuring the velocity of fourth sound; (3) persistent current states are observed from a rotating frame. The fourth sound techniques allows observations of persistent current states which are otherwise inaccessible in a stationary measurement. As a consequence the authors show that every phenomenon observed in rotating locked superfluid helium (Bose superfluid) has an analogous counterpart in an irreversible type-II superconductor (Fermi superfluid). (Author)

Asaf Paris-Mandoki, J. Shearring, Francesco Mancarella +3 more

Superfluidity and superconductivity have been widely studied since the last century in many different contexts ranging from nuclear matter to atomic quantum gases. The rigidity of these systems with respect to external perturbations results in frictionless motion for superfluids and resistance-free electric current flow in superconductors. This peculiar behaviour is lost when external perturbations overcome a critical threshold, i.e. above a critical magnetic field or a critical current for superconductors. In superfluids, such as liquid helium or ultracold gases, the corresponding quantities are a critical rotation rate and a critical velocity respectively. Enhancing the critical values is of great fundamental and practical value. Here we demonstrate that superfluidity can be completely restored for specific, arbitrarily large flow velocities above the critical velocity through quantum interference-induced resonances providing a nonlinear counterpart of the Ramsauer-Townsend effect occurring in ordinary quantum mechanics. We illustrate the robustness of this phenomenon through a thorough analysis in one dimension and prove its generality by showing the persistence of the effect in non-trivial 2d systems. This has far reaching consequences for the fundamental understanding of superfluidity and superconductivity and opens up new application possibilities in quantum metrology, e.g. in rotation sensing.

George Kukich, R. P. Henkel, J. D. Reppy

Superfluid persistent currents are observed to decay with a logarithmic time dependence for flow velocities near the critical velocity. The temperature dependence of the decay rate and the critical velocity obtained from the experiment are discussed in terms of a thermal activation picture.