Mixtures of Bose liquids at finite temperature

The superfluidity of two weakly interacting Bose gases at finite temperature is studied with the Bogoliubov model. At low temperatures, only the lowest energy normal modes are relevant, exciting particles of each species from their respective condensates. The low-temperature depletion of the zero-momentum condensate and superfluid density are calculated for arbitrary mixtures. For identical short-range interaction between all particles and low concentrations of one species, curves of constant depletion and superfluid density have a form similar to those of a free Bose gas. These considerations suggest that superfluid flow of the unstable isotope 6He at low concentrations (< or approx. =10-9) in liquid 4He would be detectable only at temperatures below approx.10-6 K. Other, more general interaction potentials predict similar behavior