We report an experimental investigation of laser-induced voltages across point-contact junctions of Pt tips and superconducting Bi compounds. It has been observed that the laser-induced voltages of all 2:2:2:3 granular samples and 2:2:1:2 single-crystal samples are converted with increasing temperature from positive to negative sign, passing through a zero point. The signs for 2:2:1:2 granular samples and n-type ${\mathrm{Nd}}_{1.85}$${\mathrm{Ce}}_{0.15}$${\mathrm{CuO}}_{4+\mathit{y}}$ sample are always positive. The temperature corresponding to the zero point is approximately the superconducting transition temperature (midpoint) of the measured sample.

• Received 2 April 1992

DOI:https://doi.org/10.1103/PhysRevB.46.5773