Photoacoustic wave propagating from normal into superconductive phases in Pb single crystals

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Photoacoustic (PA) wave has been examined in a superconductor of the first kind, Pb single crystal. The PA wave is induced by optical excitation of electronic state and propagates from normal into superconductive phases below T C. It is clearly shown by wavelet analysis that the measured PA wave includes two different components. The high-frequency component is MHz-ultrasonic and the relative low-frequency one is induced by thermal wave. The latter is observed in a similar manner irrespective of T C. On the other hand, the MHz-frequency component is obviously enhanced below T C. The behavior is reproduced by the change of attenuation of longitudinal ultrasonic wave and is consistent with BCS theory.