Neutron reflection from the surface of normal and superfluid 4He

TL;DR

This study reports the first observation of neutron reflection from helium surfaces, revealing differences in surface smoothness and a distinct surface layer in superfluid 4He compared to normal helium.

Contribution

It provides the first experimental evidence of neutron reflection from helium surfaces and details surface differences between normal and superfluid states.

Findings

Superfluid 4He surface is smoother than normal helium surface.

A ~200 Angstrom thick surface layer with different neutron scattering properties was observed in superfluid 4He.

Enhanced Bose-Einstein condensate fraction may explain the surface layer's properties.

Abstract

The reflection of neutrons from a helium surface has been observed for the first time. The 4He surface is smoother in the superfluid state at 1.54 K than in the case of the normal liquid at 2.3 K. In the superfluid state we also observe a surface layer ~200 Angstroms thick which has a subtly different neutron scattering cross-section, which may be explained by an enhanced Bose-Einstein condensate fraction close to the helium surface.