Trapping of vortices by impurities in thin He-4 films

Quantum vortices are excitations which are spontaneously created in thin su perfluid He-4 films at low temperatures. They are trapped by impurity atoms . We have calculated microscopically the chemical potentials of He-3 and hy drogen isotope impurities as well as vortex creation using a variational ma ny-body wave function. Our results for the vortex core parameters agree wit h recent Monte Carlo calculations. Those chemical potentials are compared w ith the energy required to create the vortex excitation around the impurity . We find that the energy gained is approximate to 4 K at the saturation de nsity of the 2D He-4 fluid and increases with the density. The unknown para meter in our model is the vortex mass. We discuss two choices; the mass is set equal to the mass of the expelled superfluid volume and secondly the vo rtex kinetic energy is set equal to zero, which reduces the energy gain by 2 K. (C) 2000 Elsevier Science B.V. All rights reserved.