Inelastic electron scattering at the interface between a conducting film and an insulating substrate provides a new channel for energy transfer from the film electrons to the substrate phonons. Its contribution to the Kapitza conductance is found to be $ħu\gamma {/k}_B{\tau }_{e-\mathrm{ph}},$ where $u$ is the sound velocity, γ is the Sommerfeld constant, and ${\tau }_{e-\mathrm{ph}}$ is the electron-phonon energy relaxation time in the film. This mechanism is significant for conductors with strong electron-phonon coupling, or for an interface with a small value of the phonon transparency. The results of the theory agree in order of magnitude with the observed decrease of the Kapitza conductance at the transition to the superconducting state. They can explain a universal minimal value of the conductance for pairs of materials with rather different acoustic impedances (metallic films on diamond substrate). This scattering mechanism results in the nonequilibrium component of the photoresponse with the ps decay time proportional to the film thickness that has been recently observed in YBaCuO ultrathin films.

• Received 30 March 1998

DOI:https://doi.org/10.1103/PhysRevB.58.R10199