Bose-Einstein Condensation in liquid $^4$He films

Neutron scattering measurements of Bose-Einstein condensation in liquid $^4$He fils will be presented. The measurements were carried out on the MARI time-of-flight spectrometer at the CCLRC ISIS Facility, Rutherford Appleton Laboratory, UK. The goal is to determine whether the condensate fraction, $n_0$, is enhanced above the bulk liquid value at a liquid $^4$He surface. It is also to determine $n_0$ in 2D thin films and, by varying the film thickness, observe a 2D to 3D cross-over. Data for films on a flat MgO substrate [1] and on carbon-black will be presented. The data shows, that $n_0$ is indeed significantly enhanced at a film - vapor surface suggesting a lower liquid density there [2]. \newline \newline [1] J.V. Pearce {\em et al.}, {\em J. Phys. Condens. Matter} {\bf 16}, 4391 (2004) \newline [2] E.W. Drager and D.M. Ceperley, {\em Phys. Rev. Lett.} {\bf 89}, 15301 (2002)