2020/12/31 by Johannes Hofmann, Wilhelm Zwerger · 1 citation
Physics and Astronomy · #cond-mat.quant-gas #cond-mat.stat-mech
paper · pdf · doi:10.1088/1742-5468/abe598
published as J. Stat. Mech. (2021) 033104 · 20 pages
arxiv created 2021/04/05 · arxiv updated 2021/04/07
We determine the hydrodynamic modes of the superfluid analog of a smectic-A phase in liquid crystals, i.e., a state in which both gauge invariance and translational invariance along a single direction are spontaneously broken. Such a superfluid smectic provides an idealized description of the incommensurate supersolid state realized in Bose-Einstein condensates with strong dipolar interactions as well as of the stripe phase in Bose gases with spin-orbit coupling. We show that the presence of a finite normal fluid density in the ground state of these systems gives rise to a well-defined second-sound type mode even at zero temperature. It replaces the diffusive permeation mode of a normal smectic phase and is directly connected with the classic description of supersolids by Andreev and Lifshitz in terms of a propagating defect mode. An analytic expression is derived for the two sound velocities that appear in the longitudinal excitation spectrum. It only depends on the low-energy parameters associated with the two independent broken symmetries, which are the effective layer compression modulus and the superfluid fraction.