Suppression of superfluid density in the superfluid-supersolid transition

TL;DR

This paper explains the pressure-dependent suppression of superfluid density near the superfluid-supersolid transition as a consequence of phonon mode softening during a second-order or weakly first-order quantum phase transition to a super-CDW state.

Contribution

It provides a theoretical understanding of the superfluid density suppression by linking it to phonon mode softening at the transition.

Findings

Superfluid density suppression is due to phonon mode softening.

Transition is likely second order or weakly first order.

Superfluid to supersolid transition involves a super-CDW state.

Abstract

We show that the rather unexpected pressure dependence of superfluid density observed near the superfluid-supersolid transition by Kim {\em et.al.}[M.H.W. Chan, {\em private communication}], can be understood if the transition from superfluid to supersolid state is a second order or weakly first order transition from the superfluid state to a super-CDW state with non-uniform Bose-condensation amplitude. The suppression of superfluid density is a direct consequence of softening of phonon mode at finite wave-vector $|\vec{Q}|\sim Q_0$ around the quantum phase transition.