Symmetry-forbidden laser-induced voltages in YBa2Cu3O7

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 YBa2Cu3O7 films when illuminated by a laser with intensities up to ∼30 mJ/cm2. For intensities ≤2 mJ/cm2 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