POLARON-TYPE THEORY FOR THE SPECIFIC-HEAT OF HE-3 IN SUPERFLUID HE-4

The difference between the specific heat of a dilute He-3-He-4 He mixt ure and the specific heat of pure superfluid He-4 at temperatures belo w the superfluid transition temperature is usually explained by consid ering the He-3 as an ideal gas of quasiparticles with an energy spectr um that is not quadratic in the momentum, but which is difficult to ju stify on a theoretical basis. On the basis of a polaron-type model, in troduced by three of the present authors, the specific heat of the mix ture can be calculated, taking into account the emission and absorptio n of the phonon-roton excitations. In a temperature range where the He -3 particles follow Boltzmann statistics, we find that the model accur ately explains the experimental data for the specific heat. Therefore, the difference between the specific heat of the mixture and the speci fic heat of pure superfluid He-4 can be explained as the result of the interaction of He-3 with the excitations of the superfluid. Moreover, it is one of the most clearly measured ''polaron'' effects.