Fluctuation effects at the free surface of superfluid He-4

We report our results about the fluctuation effects on the surface static structure factor and on the local Bose-Einstein condensate (BEC) in the low density surface region of He-4. This information has been obtained by a variational Monte Carlo calculation for a slab of He-4 modelled by a novel shadow wave function with a glue term (glue-SWF). This glue-SWF describes the self-binding of He-4 only via local interparticle correlations. BEC increases from the bulk-like value well inside the slab to a much larger value in the surface region (up to about 50%). Further out of the surface the condensate decreases and correspondingly there is an enhanced population of small momentum states. This is very different from the full BEC that was predicted using a simplified variational treatment of these He-4 systems. This different behaviour is correlated with the presence in the glue-SWF of enhanced density fluctuations in the surface region as shown by the behaviour of the static structure factor at small wave vector. This is interpreted as an effect due to the zero point motion of ripplons.