Dipole superfluid hydrodynamics II

TL;DR

This paper develops a dissipative hydrodynamic theory for s-wave dipole superfluids, revealing unique sound, shear, and magnon-like modes, and explores their response properties and equilibrium states with dipole superflow.

Contribution

It introduces a new hydrodynamic framework for dipole superfluids, including a novel derivative counting scheme and analysis of their linear response and equilibrium states.

Findings

Identification of characteristic sound, shear, and magnon-like modes.

Existence of equilibrium dipole superflow states.

Derivation of response functions and transport coefficients.

Abstract

We present a dissipative hydrodynamic theory of "s-wave dipole superfluids" that arise in phases of translation-invariant and dipole-symmetric models in which the U(1) symmetry is spontaneously broken. The hydrodynamic description is subtle on account of an analogue of dangerously irrelevant operators, which requires us to formalize an entirely new derivative counting scheme suitable for these fluids. We use our hydrodynamic model to investigate the linearized response of such a fluid, characterized by sound modes $\omega \sim \pm k - ik^2$, shear modes $\omega\sim-ik^2$, and magnon-like propagating modes $\omega \sim \pm k^2 - ik^4$ that are the dipole-invariant version of superfluid "second sound" modes. We find that these fluids can also admit equilibrium states with "dipole superflow" that resemble a polarized medium. Finally, we couple our theory to slowly varying background fields, which allows us to compute response functions of hydrodynamic operators and Kubo formulas for hydrodynamic transport coefficients.