Collective Modes in Supersolid 4He

TL;DR

This paper derives hydrodynamic equations for solid and supersolid helium-4, predicting unique collective modes, including propagating defect modes and superfluid-related modes, which serve as experimental signatures of the supersolid phase.

Contribution

It introduces modified hydrodynamic equations for supersolid helium-4, revealing new collective modes associated with superfluidity and defect propagation.

Findings

Propagating defect mode observed in experiments.

Two additional superfluid-related modes identified.

Hydrodynamic framework explains supersolid collective excitations.

Abstract

We derive the hydrodynamic equations of motion of solid and supersolid 4He, that describe the collective modes of these phases. In particular, the usual hydrodynamics is modified in such a way that it leads to the presence of a propagating instead of a diffusive defect mode. The former is appropriate for a quantum crystal and observed in recent experiments. Furthermore, we find that in supersolid helium there are two additional modes associated with the superfluid degrees of freedom. The observation of these additional modes is a clear experimental signature of the supersolid phase.