Microscopic dynamics of superfluid $^4$He: a comprehensive study by inelastic neutron scattering

TL;DR

This comprehensive neutron scattering study reveals detailed microscopic dynamics of superfluid helium-4 across various pressures, highlighting modifications in excitation spectra and confirming theoretical predictions with high-resolution experimental data.

Contribution

The paper provides the first extensive experimental characterization of superfluid helium-4's dynamic structure factor over a wide energy and momentum range, validating advanced many-body theories.

Findings

Modification of single-excitation spectrum at high pressures

Observation of structured multi-excitation spectrum at low energies

Agreement between experimental data and dynamic many-body calculations

Abstract

The dynamic structure factor of superfluid $^4$He has been investigated at very low temperatures by inelastic neutron scattering. The measurements combine different incoming energies resulting in an unprecedentedly large dynamic range with excellent energy resolution, covering wave vectors $Q$ up to 5 \AA$^{-1}$ and energies $\omega$ up to 15 meV. A detailed description of the dynamics of superfluid $^4$He is obtained from saturated vapor pressure up to solidification. The single-excitation spectrum is substantially modified at high pressures, as the maxon energy exceeds the roton-roton decay threshold. A highly structured multi-excitation spectrum is observed at low energies, where clear thresholds and branches have been identified. Strong phonon emission branches are observed when the phonon or roton group velocities exceed the sound velocity. The spectrum is found to display strong multi-excitations whenever the single-excitations face disintegration following Pitaevskii's type a or b criteria. At intermediate energies, an interesting pattern in the dynamic structure factor is observed in the vicinity of the recoil energy. All these features, which evolve significantly with pressure, are in very good agreement with the Dynamic Many-body calculations, even at the highest densities, where the correlations are strongest.