Hydrodynamics of a superfluid smectic

Abstract

We determine the hydrodynamic modes of the superfluid analog of a smectic-A liquid crystal phase, 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.

Universal relations for dipolar quantum gases

“…It is negative but vanishes faster with density than C. A similar behavior is found in the two-body limit, where a bound state of the combined short-range plus dipolar interaction exists whenever a 2 > a d 2 [48]. Using the adi-…”

“…. (48) of the density along the y-direction with a fundamental reciprocal lattice vector q 0 . This type of ordering is observed for confined gases in the regime l z d beyond the Hansen-Verlet bound for the critical value of the dipolar interaction.…”

Supersolid phase of a spin-orbit-coupled Bose-Einstein condensate: A perturbation approach

“…It is well theoretically understood that a common feature of the excitation spectra of supersolids is the presence of two or more Goldstone modes [30][31][32][33][34][35][36][37][38][39][40][41][42]. In general, one of them is associated with the superfluid motion of particles through the crystalline structure, which stays at rest.…”

Dynamical preparation of stripe states in spin-orbit-coupled gases

“…The predicted spatial density modulations have only been unambiguously observed in [17], using orbital states in a superlattice as pseudo spin states, and very recently also in metastable states of a 87 Rb spinor gas [18] (for its realization in dipolar gases, see [19][20][21]). While sharing many properties with conventional supersolids, the nature of the stripe phases in gases with SOC is still debated [22], with current proposals focusing on probing its excitation spectrum. So far, most protocols to enhance the accessibility of the phase and the contrast of the stripes pursue an effective decrease of the intraspin interactions [23,24].…”