TRANSMISSION OF HIGH-FREQUENCY BALLISTIC PHONONS IN SUPERCONDUCTING IN, SN, AND PH FILMS

Phonon transmission through superconducting In, Sn, and Pb films (with thicknesses of 4 and 8 mu m) is studied with a heat-pulse technique using a semiconductor Si:P detachable bolometer as the phonon detector. Below the critical temperature of the films, ballistic-phonon propagation is observed. From the dependence of the phonon transmission on the incident power and film thickness, we calculate the mean free path of phonons at the gap frequency. The phonon transmission amount changes when a magnetic field is applied and is also different if the field is parallel or perpendicular to the film. By a simple model based on the Landau laminar model for intermediate states, the critical field for the films can be calculated. Good agreement (for In and Sn) with the theoretical values is obtained.