Microscopic Theory for Emission of Elementary Excitations into He II from a Heated Solid

Abstract

I develop here the microscopic quantum theory for description of creation of phonons and rotons in superfluid helium by a solid heater. Starting with correct transfer Hamiltonian describing a coupling between the solid and liquid $^4$He the probabilities of transformation of a single phonon in the solid into i) single helium phonon, ii) two helium phonons, and iii) single helium roton are found out. All the obtained expressions account for different polarizations of phonons in the solid. The heat transfer associated with single phonon and single roton channels are calculated. Particularly, the obtained expression for heat flux via the single phonon channel calculated in the framework of present microscopic theory exactly coincides with the well known Khalatnikov formula obtained initially in the framework of acoustic-mismatch theory. The impossibility of direct creation of R$^{(-)}$ rotons becomes clear in the used framework due to accurate account of the boundary conditions at the solid -- liquid helium interface, which is in agreement with recent experimental results.