Phases of Superfluid Helium in Smooth Cylindrical Pores

TL;DR

This study investigates the microscopic phases of superfluid helium in smooth cylindrical pores, revealing how different local densities influence excitation spectra and phase behavior.

Contribution

It provides experimental evidence of distinct superfluid phases and their excitations in confined geometries, linking local density to phonon-roton spectra.

Findings

Observation of modified phonon-roton spectrum in thin films

Identification of 2D surface roton in dense films

Coexistence of bulk-like modes and surface rotons in saturated pores

Abstract

Two different microscopic phases have been observed via inelastic neutron scattering for superfluid helium confined within highly monodisperse, smooth, and unidimensional silica pores. When the helium forms a thin film on the pore walls, it supports a dramatically modified phonon-roton spectrum as well as a 2D surface roton. The energies of these modified phonon-roton modes are consistent with predictions for a dilute, low-density film, while the energy of the 2D surface roton corresponds to that of a dense film. The relatively smooth and weak surface potential of the substrate permits the formation of a film with large dilute regions and only small compressed regions. When the pores are saturated with liquid, the modified phonon-roton spectrum disappears, and bulk-like modes coexist with the 2D surface roton. Presumably, bulk-like liquid occupies the core volume of the pores and high density liquid layers are present at the liquid-solid interface. These findings clearly connect the nature of the excitations to the local density of the liquid.