Transport in clean quantum critical superfluids

 The hydrodynamic regime of superfluids in systems like 4He has been well understood since the seminal work of Landau and Tisza. A fundamental assumption in much subsequent work is the low-temperature dominance of the superfluid phonon which leads to universal behavior in transport that is characterized by the phonon sound speed. I will discuss investigations of the two-fluid model of superfluids near an underlying quantum critical phase and establish criteria for the breakdown of this assumption which leads to novel transport behavior. A surprising result is the non-vanishing of the charge density of the fluid’s normal component at zero temperature. I will illustrate these results with some holographic models.