Breakdown of sound in superfluid helium

TL;DR

This paper presents neutron scattering experiments demonstrating that quasiparticle decays cause the damping of high-energy phonon-roton sound waves in superfluid helium, revealing the importance of quasiparticle stability in quantum fluids.

Contribution

The study provides experimental evidence linking quasiparticle decay processes to sound wave damping in superfluid helium, advancing understanding of quasiparticle dynamics.

Findings

Quasiparticle decay explains sound wave damping.

Damping increases near crystallization and superfluid transition.

Results clarify previous experimental puzzles.

Abstract

Like elementary particles carry energy and momentum in the Universe, quasiparticles are the elementary carriers of energy and momentum quanta in condensed matter. And, like elementary particles, under certain conditions quasiparticles can be unstable and decay, emitting pairs of less energetic ones. Pitaevskii proposed that such processes exist in superfluid helium, a quantum fluid where the very concept of quasiparticles was borne, and which provided the first spectacular triumph of that concept. Pitaevskii's decays have important consequences, including possible breakdown of a quasiparticle. Here, we present neutron scattering experiments, which provide evidence that such decays explain the collapsing lifetime (strong damping) of higher-energy phonon-roton sound-wave quasiparticles in superfluid helium. This damping develops when helium is pressurized towards crystallization or warmed towards approaching the superfluid transition. Our results resolve a number of puzzles posed by previous experiments and reveal the ubiquity of quasiparticle decays and their importance for understanding quantum matter.