Singlet superfluidity in³He film on⁴He film

It is predicted that spin-singlet superfluidity can be realized in a submonolayer ³He film on the surface of thin film of superfluid ⁴He. The basic mechanism of the superfluidity is the attraction between ³He quasi particles arising from exchange of virtual third sound quanta. It is argued that the superfluid transition of ³He film is of Kosterlitz-Thouless nature if the ⁴He film is thick, and changes its nature as the ⁴He thickness is reduced, with a crossover thickness of ∼ 2 atomic layers. Several experiments are suggested to detect the new phase transition, to test whether the proposed mechanism is really working, and to examine the nature of the transition. The transition temperature is estimated to be in a temperature range attainable with a dilution refrigerator.