Calculations Show Ideal BEC Layer at Liquid Helium Surface

The Path Integral Monte Carlo method was used by David Ceperley's group at the University of Illinois to calculate the Bose-Einstein condensate fraction at the surface of a helium film. The results show how the amount of helium in the lowest state, the condensate fraction, varies with position near the surface of a film of liquid helium. Moving from the center of the slab to the surface, the condensate fraction was found to initially increase with decreasing density to a maximum value of 0.9. Long-wavelength density correlations were observed in the static structure factor at the surface of the slab, indicating the existence of capillary waves that reduce the condensate at the surface. The calculations support the picture that there is a thin layer of nearly ideal BEC at the surface of liquid helium.

A paper describing these calculations, published in Physical Review Letters 89 015301 (2002), was titled "Bose-Einstein Condensation at a Helium Surface" by E. W. Draeger and D. M. Ceperley. This research was done as a part of the NASA-supported thesis research of Erik Draeger, who received his PhD in Dec. 2001.