Breaking the superfluid speed limit in a fermionic condensate

Abstract

An experiment reports the unexpected behaviour of an object in uniform motion in superfluid helium-3 above the Landau critical velocity — the limit above which it can generate excitations at no energy cost. Coherent condensates appear as emergent phenomena in many systems1,2,3,4,5,6,7,8. They share 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 scatterer frame to become gapless, can create excitations at no energy cost, initiating the breakdown of the condensate1,9,10,11,12,13—the well-known Landau velocity9. Whereas, 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 dissipation12,13, 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. Given the importance of the Landau velocity for our understanding of superfluidity, this result is unexpected, with implications for dissipative effects of moving objects in all coherent condensate systems.