A gas-microphone photoacoustic cell which allows the temperature to vary between 85 and 340 K has been constructed for the study of the superconducting transition by the photoacoustic techniques. The cell has been constructed in the form of a Helmholtz resonator. The amplitude and phase angle of the photoacoustic signal have been measured as a function of chopping frequency using the carbon black as a sample. The frequency response of the cell has been measured with air and helium gas as medium gas at room temperature and 93 K. The results of the frequency response have been discussed and compared with the results calculated by the extended Helmholtz resonator (EHR) model[Appl. Pays. $\underline{25}$, 221(1981)].
By using the photoacoustic cell, the photoacoustic measurements for polycrystalline samples of orthorhombic $GdBa_2Cu_3O_{7-x}$ $DyBa_2Cu_3O_{7-x}$, and $YBa_2Cu_3O_{7-x}$ have been carried out in the temperature ranges between 85 and 100 K. An anomalous change of the photoacoustic signal due to the specific heat anomaly of the superconductor at transition temperature has been observed. The temperature at which the photoacoustic signal intensity anomalously changes has been found to correspond to the offset temperature of superconducting transition in resistivity measurement. The changes of the photoacoustic signal have been analyzed using Rosencwaig and Gersho's theory [J. Appl. Phys. $\underline{47}$, 64(1976)]. The relative values of the specific heat for the superconducting samples have been obtained near the transition temperature from the photoacoustic signal by using the thermal conductivity data and showed a good agreement with the specific heat data measured by a calorimetric method by others. The superconducting transition of the samples was, from the photoacoustic measurements, found to be a second-order transition with no latent heat.
The solid-liquid transition for gallium and the ferroelectric transitions for KDP (potassium dihydrogen phosphate) and TGS (triglycine sulphate) have been also investigated by the photoacoustic technique.
