Neutron reflection from the surface of a liquid ⁴He- ³He mixture

We have used neutron reflection from the liquid surface at ultra-low temperatures to study the surface properties of liquid helium. We measured neutron scattering from the free surfaces of commercially pure ⁴He (0.3 ppm of ³He impurities) and of a ³He-⁴He mixture with a ³He concentration of 0.5% for temperatures in the range from 340mK to 2.2K We compare the reflected neutron intensity for different temperatures and we fit a model that describes the collected data. The data are described well by a diffusive ³He layer of a few hundred Angstrom thickness on the bulk ⁴He liquid surface. Even at high temperatures (∼2K) there is an increased concentration of ³He atoms near the liquid surface. The distribution does not change very much with temperature. At low temperatures the neutron absorption increases significantly, which might be an indication of the formation of Andreev states. However, the shapes of the curves do not change very much which seems to suggest that the layer formed by the ³He atoms in Andreev states is very thin ∼ 10Å. The experimental method, based on neutron reflectometry, opens up new opportunities for the study of the surfaces and interfaces of quantum fluids and solids.