Change in velocity of high frequency transverse sound at the superconducting transition temperature

The change in velocity of transverse sound at the transition temperature consists of two parts (1) a static part (usually estimated to be 1–10 ppm arising from the difference in the elastic constants between the superconducting and normal states, and (2) a purely dynamical effect due to Meissner Screening of the transverse electromagnetic fields at the transition temperature. It is shown that under suitable conditions, as first suggested by Ozaki and Mikoshiba, the second mechanism can give rise to much larger changes in the velocity of high frequency sound. In fact for Al at a sound wave frequency of 106 c s and ql = 10 (here q is the wave number of the sound wave and l is the electron mean free path) the relative change in velocity can be as much as 1 part per thousand. The results of a detailed microscopic calculation of the relative change in velocity (v s −v n ) v n are presented for weak coupling type I superconductors.