Dynamics of Supersolid state: normal fluid, superfluid, and supersolid velocities

Abstract

Landau's excitation-based argument for superfluids -- that at temperature $T=0$ the normal fluid density $\rho_{n}$ is zero -- should also apply to supersolids. Further, for a total mass density $\rho$, Leggett argues that the superfluid fraction $\rho_{s}/\rho<1$. These arguments imply that there is a missing mass. We attribute this to a supersolid density $\rho_{L}$, with $\rho_{L}\equiv \rho-\rho_{s}-\rho_{n}$, and a momentum-bearing supersolid velocity $v_{Li}$. Using Onsager's irreversible thermodynamics we derive the macroscopic dynamical equations for this system. We find that $v_{Li}$ is subject to the force of elasticity, to the negative gradient of the chemical potential per mass $\mu$ (as for the superfluid velocity $v_{si}$), and to drag against the normal fluid (leading to the interpretation of $L$ as lattice). Thus both the superfluid and supersolid components are associated with the ground state. The normal modes for such a system have a crossover in frequency, above which the normal fluid velocity $v_{ni}$ is an independent variable and below which it is locked to $v_{Li}$. For an isotropic lattice we study both the transverse response and longitudinal response. The ring geometry for atomic gas supersolid states may provide a geometry for testing these predictions.