Two-beam photoacoustic phase measurement of the thermal diffusivity of a Gd-doped bulk YBCO superconductor

The two-beam photoacoustic phase measurement was applied to measure quantitatively the thermal diffusivity (alpha(s)) of a ceramic bulk high-T-c superconductor. Neglecting the effects of thermal dilation, and thermoelastic bending was proved valid in accordance with our composite piston model for the chosen experimental conditions. It was found that alpha(s) shows different features at the onset and offset temperatures corresponding to the normal-superconducting (NS) transition. A dip was seen at the resistivity transition onset temperature and a cusp at the offset temperature where the electrical resistance disappears. The presence of the cusp at the offset temperature is proposed to be related to weak coupling between superconducting grains. Our studies indicate that the two-beam phase measurement is a very sensitive method for superconductor characterization and NS transition detection. The experimental results also confirm the presence of a large energy gap and strong electron-phonon coupling mechanism in the YBCO superconductor. (C) 1996 American Institute of Physics.