Conversion of electromagnetic into acoustic energy via indium films

Transverse acoustic waves were generated electromagnetically at 9 GHz using thin indium films on silicon substrates. We report measurements of the conversion efficiency α of electromagnetic into acoustic energy as a function of temperature, magnetic field, and microwave power. The highest conversion efficiency found above the superconducting transition temperature Tc of indium was 5×10-5 comparable to that of quartz transducers. Below Tc the conversion efficiency decreases rapidly and becomes magnetic field and microwave power dependent. At low powers a good agreement was found between the experiment and the theory of Abeles with respect to both the value and temperature dependence of α. This agreement shows that the generation of the acoustic waves is due primarily to diffuse scattering of electrons at the surfaces of the film. To fit the theory to the experiment we had to assume that diffuse surface scattering persists below Tc as well and thus we conclude that pair scattering at the surface does not conserve momentum. In addition, our results show that α is still appreciable at temperatures well below Tc and thus transverse-phonon generation may possibly be one of the important loss mechanisms in superconducting cavities.