Symmetry-forbidden laser-induced voltages in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$

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 ∼30 mJ/${\mathrm{cm}}^2$. For intensities ≤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.