Dynamics of liquid 4He in Vycor

TL;DR

This study investigates the behavior of liquid helium-4 confined in Vycor, revealing that phonon-roton excitations persist across the superfluid transition and suggesting a separation between Bose-Einstein condensation and superfluid transition temperatures in this confined system.

Contribution

It provides new insights into the excitations and phase transitions of helium-4 in porous Vycor, highlighting differences from bulk behavior and identifying a layer mode near the roton wave vector.

Findings

Phonon-roton excitations are observed across the superfluid transition in Vycor.

The p-r excitation intensity does not scale with the superfluid fraction.

A two-dimensional layer mode is detected near the roton wave vector.

Abstract

We have measured the dynamic structure factor of liquid 4He in Vycor using neutron inelastic scattering. Well-defined phonon-roton (p-r) excitations are observed in the superfluid phase for all wave vectors 0.3 < Q < 2.15. The p-r energies and lifetimes at low temperature (T = 0.5 K) and their temperature dependence are the same as in bulk liquid 4He. However, the weight of the single p-r component does not scale with the superfluid fraction (SF) as it does in the bulk. In particular, we observe a p-r excitation between T_c = 1.952 K, where SF = 0, and T_(lambda)=2.172 K of the bulk. This suggests, if the p-r excitation intensity scales with the Bose condensate, that there is a separation of the Bose-Einstein condensation temperature and the superfluid transition temperature T_c of 4He in Vycor. We also observe a two-dimensional layer mode near the roton wave vector. Its dispersion is consistent with specific heat and SF measurements and with layer modes observed on graphite surfaces.