Rigorous Density Functional Theory for Inhomogeneous Bose-Condensed Fluids

The density functional theory originally developed by Hohenberg, Kohn and Sham provides a rigorous conceptual framework for dealing with inhomogeneous interacting Fermi systems. We extend this approach to deal with inhomogeneous interacting Bose-condensed systems, limiting this presentation to setting up the formalism to deal with ground state $(T=0)$ properties. The key new feature is that one must deal with energy functionals of both the local density $n({\bf r})$ and the local complex macroscopic wavefunction $\Phi ({\bf r})$ associated with the Bose broken-symmetry (the local condensate density is $n_{c}({\bf r}) = \vert \Phi ({\bf r}) \vert ^{2}$). Implementing the Kohn-Sham scheme, we reduce the problem to a gas of weakly-interacting Bosons moving in self-consistent diagonal and off-diagonal one-body potentials. Our formalism should provide the basis for studies of the surface properties of liquid $^4$He as well as the properties of Bose-condensed atomic gases trapped in external potentials.Comment: 20 page

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

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 $\Phi(z)$ in the low-density region valid at all temperatures. The overall amplitude of $\Phi(z)$ is fixed by the bulk liquid, which ensures that it vanishes everywhere at the bulk transition temperature.Comment: 6 pages, paper submitted to Low Temperature Conference (LT21), Prague, Aug., 1996; to appear in proceeding