The Surface Region of Superfluid ${}^4$He as a Dilute Bose-Condensed Gas

Abstract

In the low-density surface region of superfluid ${}^4$He, the atoms are far apart and collisions can be ignored. The only effect of the interactions is from the long-range attractive Hartree potential produced by the distant high-density bulk liquid. As a result, at $T=0$, all the atoms occupy the same single-particle state in the low-density tail. Striking numerical evidence for this 100\% surface BEC was given by Pandharipande and coworkers in 1988. We derive a generalized Gross-Pitaevskii equation for the inhomogeneous condensate wave function $\hat{I}¦(z)$ in the low-density region valid at all temperatures. The overall amplitude of $\hat{I}¦(z)$ is fixed by the bulk liquid, which ensures that it vanishes everywhere at the bulk transition temperature.

6 pages, paper submitted to Low Temperature Conference (LT21), Prague, Aug., 1996; to appear in proceedings