Phonon absorption and pair breaking in superconductors at 35 GHz

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Abstract
Measurements of the absorption of longitudinal sound in aluminium at a microwave frequency of 35 GHz are reported. The phase problems associated with the very short sound wavelength have been partly overcome by using a semiconductor avalanche bolometer which detects scattered phonons rather than unscattered coherent ultrasound. A simple theory is developed to relate the bolometer signal to the ultrasonic attenuation coefficient and to its variation with temperature. The results for the pair breaking region of the superconducting state would appear to be in agreement with the BCS Bobetic theory, and the broadening of the transition to the region where quasi-particle scattering dominates is interpreted in terms of energy gap anisotropy.