Roton-Phonon Interactions in Superfluid 4He

TL;DR

This study investigates how rotons in superfluid helium-4 interact with phonons, revealing a temperature-dependent energy shift due to complex many-particle processes, and suggests a repulsive interaction at low temperatures.

Contribution

It provides a detailed diagrammatic analysis of roton-phonon interactions, highlighting the role of three- and four-particle processes enabled by broken gauge symmetry.

Findings

Roton energy and lifetime have different temperature dependencies.

Roton-phonon interactions are predominantly repulsive at low temperatures.

The analysis identifies the significance of three-particle processes in the interaction.

Abstract

High-resolution neutron resonance spin-echo measurements of superfluid 4He show that the roton energy does not have the same temperature dependence as the inverse lifetime. Diagrammatic analysis attributes this to the interaction of rotons with thermally excited phonons via both four- and three-particle processes, the latter being allowed by the broken gauge symmetry of the Bose condensate. The distinct temperature dependence of the roton energy at low temperatures suggests that the net roton-phonon interaction is repulsive.