Binary superfluids: Low-energy properties and dissipative processes from spontaneous emission of massive phonons

TL;DR

This paper explores the low-energy excitations and dissipative phenomena in binary superfluids with density-dependent interactions, revealing the presence of both massless and massive phonons, and analyzing Hawking-like emission effects at acoustic horizons.

Contribution

It introduces a detailed analysis of symmetry-breaking effects and acoustic horizons in binary superfluids, including the computation of viscosity ratios and violation of the Kovtun-Son-Starinets bound.

Findings

Identification of a massless and a massive (pseudo) Nambu-Goldstone boson in the spectrum.

Discovery of two acoustic horizons in transonic binary superfluids and their associated Hawking-like emission.

Violation of the viscosity-to-entropy ratio bound near the acoustic horizons.

Abstract

We discuss the low-energy properties of binary superfluids with density-dependent interactions. Adding an intra-species coupling that induces an explicit soft symmetry-breaking, we determine the background pressure and we show that the low-energy spectrum consists of a massless Nambu- Goldstone boson and a massive (pseudo) Nambu-Goldstone boson. When the background velocities of the two superfluids are transonic, the system is characterized by two distinct acoustic horizons: the hydrodynamic analogue of the black hole event horizon. The Hawking-like emission occurring at these horizons produces an effective friction on the fluids. We compute the viscosity-to-entropy ratios close to the two acoustic horizons, finding that the emission of pseudo Nambu-Goldstone bosons violates the bound conjectured by Kovtun, Son and Starinet.