Symmetry-forbidden laser-induced voltages inYBa2Cu3

We report the occurrence of potentials in the absence of applied currents up to several volts between 1-mm-spaced probes in the normal state of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ films when illuminated by a laser with intensities up to \ensuremath{\sim}30 mJ/${\mathrm{cm}}^{2}$. For intensities \ensuremath{\le}2 mJ/${\mathrm{cm}}^{2}$ these potentials disappear in the superconducting state but return, time delayed, at higher intensities. The mechanism is unknown, and the results are in contradiction to expectations based on symmetry arguments.