ON THE MACROSCOPIC AND MICROSCOPIC ORIGIN OF THE PHOTOVOLTAIC EFFECT IN YBCO THIN-FILMS

The study of a photovoltaic component of the net voltage arising in il luminated YBCO thin films with different morphology was carried out. T he anisotropy of the photovoltages in the film plane correlates with t he anisotropy of such physical characteristics of the samples as resis tivity and microwave absorption and implies the existence of a macrosc opic preferential direction even in the film deposited on an isotropic MgO substrate. Disappearance of the photovoltaic signal below T-c in the films exhibiting an appreciable residual resistivity is an unambig uous indication of its relation with extended defects inside the super conducting YBCO phase. A model for the microscopic asymmetry of the fl ow of the photocurrent generated at the boundaries of an isolated defe ct was proposed, and the contributions coming from some types of exten ded structural defects into the effect were estimated. (C) 1997 Elsevi er Science B.V.