Layer- and bulk roton excitations of 4He in porous media

TL;DR

This paper investigates the energetics of bulk and layer-roton excitations of helium-4 in porous media, revealing how substrate interactions influence layer-roton energies and how bulk rotons reflect internal matrix structure and negative pressures.

Contribution

It provides new insights into how substrate strength affects layer-roton energies and distinguishes layer modes as independent excitations from two-dimensional rotons.

Findings

Layer-roton energy depends on substrate strength.

Bulk roton energy is largely independent of medium interaction.

Bulk rotons reflect the internal structure and negative pressure effects.

Abstract

We examine the energetics of bulk and layer-roton excitations of 4He in various porous medial such as aerogel, Geltech, or Vycor, in order to find out what conclusions can be drawn from experiments on the energetics about the physisorption mechanism. The energy of the layer-roton minimum depends sensitively on the substrate strength, thus providing a mechanism for a direct measurement of this quantity. On the other hand, bulk-like roton excitations are largely independent of the interaction between the medium and the helium atoms, but the dependence of their energy on the degree of filling reflects the internal structure of the matrix and can reveal features of 4He at negative pressures. While bulk-like rotons are very similar to their true bulk counterparts, the layer modes are not in close relation to two-dimensional rotons and should be regarded as a third, completely independent kind of excitation.