Some Ultrasonic Measurements in Normal and Superconducting Aluminum

Ultrasonic attenuation measurements in an aluminum single crystal near the superconducting transition temperature are described. These include both longitudinal and shear wave attenuations at frequencies of between 11 and 58 Mc/sec and at temperatures between 1.0 and 4.2°K, as well as attenuation as a function of magnetic field in the normal state. It is found that the attenuation of shear waves at the transition temperature shows a discontinuity of about 25% of the total attenuation, this fraction being relatively independent of frequency. The longitudinal wave attenuation shows a sharp but continuous drop at the transition temperature. It is shown that the electron mean free path l can be estimated from the magnetic field dependence of the attenuation and that the condition kl<1 (k = 2π/λ, λ being the ultrasonic wavelength) holds for the measurements. The possible significance of the shear wave discontinuity is discussed.