ELSEVIER Physica C 288 (1997) 268-276 PHYSICA @ On the macroscopic and microscopic origin of the photovoltaic effect in YBCO thin films A.I. Grachev a,*, V.Yu. Davydov a, P.M. Karavaev ", S.F. Karmanenko b, A.P. Paugurt ", I.V. Pleshakov a a A.F. loffe Physical-Technical Institute. Russian Academy of Sciences, St. Petersburg 194021, Russian Federation Electrotechnical Unicersity, St. Petersburg 197376, Russian Federation Received 14 January 1997 Abstract The study of a photovoltaic component of the net voltage arising in illuminated YBCO thin films with different morphology was carried out. The anisotropy of the photovoltages in the film plane correlates with the anisotropy of such physical characteristics of the samples as resistivity and microwave absorption and implies the existence of a macroscopic 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 unambiguous indication of its relation with extended defects inside the superconducting YBCO phase. A model for the microscopic asymmetry of the flow of the photocurrent generated at the boundaries of an isolated defect was proposed, and the contributions coming from some types of extended structural defects into the effect were estimated. © 1997 Elsevier Science B.V. Keywords: Thin films of YBa2Cu307; Intrinsic junctions; Optical response of superconductors 1. Introduction Optically induced voltages (OIV) were observed in YBa2Cu307 (YBCO) samples of different types, such as films [1-6], crystals [7,8], and ceramics [8], and every time the question about the origin of the micro- and macroscopic asymmetry which is a nec- essary condition for the macroscopic voltage genera- tion arose. For instance, the pyroelectric effect ob- served by Mihailoviz et al. [7,8] in YBCO single crystals contradicted the presumed centrosymmetry of this material. The dependence of the laser-induced " Correspondingauthor. E-mail: root@shuv.pti.spb.su voltages on the angle of light incidence observed by Kleinhammes et al. [3] and Tate et al. [9] is well consistent with the photovoltaic (PV) mechanism of the effect. However, in this case the knowledge of both the origin of relevant extended defects (intrinsic junctions) and the reasons for the asymmetry of the photocurrent generation both for an individual defect and the whole sample is required. Probably, not finding answers to these questions, the authors [3,9] did not attribute unambiguously the observed OIV to the PV effect. Our studies of OIV in YBCO thin films [10,1 1] have revealed that the observed signal is a superposi- tion of at least three components of different origins, one of them being a PV signal. This paper describes the investigations of PV voltages aimed at finding 0921-4534/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PII S0921-4534(97)01493-7