Excitations in confined helium

TL;DR

This paper models helium in porous materials, calculating the dynamic structure function to identify various excitations and compare with neutron scattering data, highlighting the effects of confinement and pore structure.

Contribution

It provides a microscopic model for helium in porous matrices and identifies different excitations, including layer modes and ripplons, as functions of geometry and scattering angle.

Findings

Good agreement with neutron scattering data

Identification of coexisting layer and bulk-like excitations

Sensitivity of layer modes to scattering angle

Abstract

We design models for helium in matrices like aerogel, Vycor or Geltech from a manifestly microscopic point of view. For that purpose, we calculate the dynamic structure function of 4He on Si substrates and between two Si walls as a function of energy, momentum transfer, and the scattering angle. The angle--averaged results are in good agreement with the neutron scattering data; the remaining differences can be attributed to the simplified model used here for the complex pore structure of the materials. A focus of the present work is the detailed identification of coexisting layer modes and bulk--like excitations, and, in the case of thick films, ripplon excitations. Involving essentially two--dimensional motion of atoms, the layer modes are sensitive to the scattering angle.