Instabilities in a running superfluid: boosted superfluid and stripe supersolid

Abstract

The possible instabilities in a running superfluid has been a long-time historical problem since first studied by L. P. Landau. By constructing effective actions in terms of suitable order parameters, we revisit this outstanding open problem. We find that if the instability is driven by the SF Goldstone mode near $ k=0 $, then there is a quantum Lifshitz transition from the SF to a Boosted SF (BSF) with the dynamic exponent $ (z_x=3/2,z_y=3) $ subject to logarithmic corrections from a marginally irrelevant cubic term. This case may happen to exciton superfluids in bilayer quantum Hall systems or electron-hole bilayer systems, especially in weakly interacting Bose gas in cold atom systems. If the instability is driven by the roton mode near a finite momentum $ k=k_0 $, then there is a SF to a stripe supersolid transition with the dynamic exponent $ z=1 $ which is in the same universality class as the $ z=1 $ boosted Mott-SF transition studied previously in a different context. This case may apply to Helium 4 and also cold atom BECs where the rotons in the SF phase plays an important role. Driving a SF sufficiently fast may become an effective way to create a SS which is a long time sought novel state of matter.