Non-exponential optical modification of the electrical properties in oxygen-deficient YBa2Cu3Ox

We present studies of the time dependent electrical resistivity of YBa2Cu3Ox during visible light excitation (photodoping). Illumination with a green semiconductor laser changes the resistivity in a non-exponential way, which is well described by a stretched-exponential function with parameters beta (dispersion parameter) and tau (time constant). beta showed an unusual behaviour: at temperatures below 220 K beta was about 0.3 to 0.4, but increased near room temperature to 0.5 to 0.6. For comparison with results obtained by red-laser excitation we discuss the dependence of the resistivity on the cumulative photon dose, thereby introducing a "photon dose constant" N-e which corresponds to tau. The results indicate that green-light excitation is somewhat more effective than red-light excitation at low temperatures.