TRANSPARENCY OF A YBA2CU3O7-FILM SUBSTRATE INTERFACE FOR THERMAL PHONONS MEASURED BY MEANS OF VOLTAGE RESPONSE TO RADIATION

The transparency of a film/substrate interface for thermal phonons was investigated for YBa2Cu3O7 thin films deposited on MgO, Al2O3, LaAlO3 , NdGaO3, and ZrO2 substrates. Both voltage response to pulsed-visible and to continuously modulated far-infrared radiation show two regimes of heat escape from the film to the substrate. That one dominated by the thermal boundary resistance at the film/substrate interface provid es an initial exponential decay of the response. The other one prevail ing at longer times or smaller modulation frequencies causes much slow er decay and is governed by phonon diffusion in the substrate. The tra nsparency of the boundary for phonons incident from the film on the su bstrate and also from the substrate on the film was determined separat ely from the characteristic time of the exponential decay and from the time at which one regime was changed to the other. Taking into accoun t the specific heat of optical phonons and the temperature dependence of the group velocity of acoustic phonons, we show that the body of ex perimental data agrees with acoustic mismatch theory rather than with the model that assumes strong diffusive scattering of phonons at the i nterface.