Neutron Reflection from the Surface of Liquid 4He with and without a Layer of 3He

TL;DR

This study presents the first neutron reflection measurements on liquid helium surfaces, revealing differences in surface roughness, a surface layer in superfluid helium, and effects of 3He impurities, advancing understanding of quantum surface phenomena.

Contribution

It provides novel neutron reflection data on helium surfaces, identifying a surface layer in superfluid helium and effects of 3He impurities on surface states.

Findings

Surface roughness varies across the lambda transition.

A ~200 Å surface layer forms in superfluid helium.

3He impurities lead to the formation of Andreev levels.

Abstract

We report and discuss the first neutron reflection measurements from the free surface of normal and superfluid 4He and of liquid 3He-4He mixture. In case of liquid 4He the surface roughness is different above and below the lambda transition, being smoother in the superfluid state. For the superfluid, we also observe the formation of a surface layer ~200 A thick which has a subtly different neutron scattering cross-section. The results can be interpreted as an enhancement of Bose-Einstein condensate fraction close to the helium surface. We find that the addition of 3He isotopic impurities leads to the formation of Andreev levels at low temperatures.