Interactions of Phonons and Rotons with Interfaces in Superfluid Helium

TL;DR

This paper develops a unified non-local hydrodynamic theory to analyze how phonons and rotons in superfluid helium interact with solid interfaces, detailing transmission, reflection, and mode conversion phenomena.

Contribution

It introduces a comprehensive theoretical framework for quasiparticle-interface interactions in superfluid helium, including creation probabilities and angular dependencies.

Findings

Derived creation probabilities for quasiparticles at interfaces.

Analyzed backward reflection and refraction of R^- rotons.

Developed a unified non-local hydrodynamic model.

Abstract

We solve the problem of beams of phonons and rotons incident on, and interacting with, solid surfaces. Phonons and rotons are the quasiparticles of superfluid helium and have a unique dispersion curve. The dispersion curve controls the transmission, reflection and mode change of these quasiparticles at the interface with another medium. We develop a non-local hydrodynamic theory in a consistent and unified way. The structure of the solutions in the quantum fluid is discussed. The creation probabilities of all quasiparticles are derived when any one of them is incident on the interface. The dependencies on frequency and angular are analysed and the backward reflection and refraction for $R^-$ rotons are discussed.