Breaking the superfluid speed limit

TL;DR

This paper challenges the traditional understanding of the Landau velocity as a strict limit for superfluid flow, showing that in certain conditions, dissipation can occur smoothly without a clear critical velocity.

Contribution

The study reveals that for uniform linear motion in superfluid 3He-B, dissipation does not exhibit a sharp transition at the Landau velocity, contradicting previous assumptions.

Findings

No discontinuity in dissipation at the Landau velocity

Dissipation occurs smoothly beyond the Landau velocity

Implications for superfluidity theory and coherent condensate systems

Abstract

Coherent condensates appear as emergent phenomena in many systems, sharing the characteristic feature of an energy gap separating the lowest excitations from the condensate ground state. This implies that a scattering object, moving through the system with high enough velocity for the excitation spectrum in the scatter frame to become gapless, can create excitations at no energy cost, initiating the breakdown of the condensate. This limit is the well-known Landau velocity. While, for the neutral Fermionic superfluid 3He-B in the T=0 limit, flow around an oscillating body displays a very clear critical velocity for the onset of dissipation, here we show that for uniform linear motion there is no discontinuity whatsoever in the dissipation as the Landau critical velocity is passed and exceeded. Since the Landau velocity is such a pillar of our understanding of superfluidity, this is a considerable surprise, with implications for the understanding of the dissipative effects of moving objects in all coherent condensate systems.