Voltage±current characteristics of c-axis oriented YBa 2 Cu 3 O 7d ®lms deposited by dc sputtering q A. De Santis a , G. Grimaldi a , U. Gambardella a, * ,1 , S. Pace a , A.M. Cucolo a , M.C. Cucolo a , V. Boa b , G. Celentano b , F. Fabbri b a INFM Research Unit, Department of Physics, University of Salerno, VS Allende, 84081 SA Baronissi, Italy b ENEA Frascati, v. E. Fermi 65, 00044 Frascati, Italy Received 5 April 2000; received in revised form 31 May 2000; accepted 8 June 2000 Abstract We analyze the voltage±current characteristics of high quality c-axis oriented YBa 2 Cu 3 O 7d ®lms with T c0  91 K. The ®lms were dc sputtered in a pure oxygen atmosphere on SrTiO 3 (1 0 0) substrates and then patterned in a shape of a long strip. The voltage±current characteristics are modeled with single or multiple Gaussian distributions of depinning currents. We derive the distribution features as a function of the applied magnetic ®eld at dierent tempera- tures. Ó 2000 Elsevier Science B.V. All rights reserved. PACS: 74.76.Bz; 74.60.Ge; 74.60.Jg Keywords: HT c ®lm; Critical current; Pinning 1. Introduction The experimental behavior of the voltage±cur- rent (V±I ) curves recorded across a supercon- ducting strip is usually analyzed to investigate the ¯ux motion which gives rise to the bending of the V±I curve near the critical current value. Beside the thermal activation, this bending has also been ascribed to statistical ¯uctuations of some pa- rameter which modify the critical current value: variation of the vortex±vortex interaction [1], grain-to-grain dispersion of the grain boundary angles [2], local critical current distribution ex- pressed by an integrated Weibull function [3]. Moreover, the low voltage portion of the V±I curve is usually analyzed to investigate the so- called vortex glass±liquid transition. This transi- tion features a crossover from downturning to upturning in the log±log plot of the V±I charac- teristics, and a critical exponent which collapse the curves to an universal curve [4]. Recently, Brown showed that these features also occurs in V±I characteristics computed by assuming a simple Gaussian distribution in the depinning currents of the superconducting YBa 2 Cu 3 O 7d (YBCO) [5]. In this work, following Warnes and Larbalestier [6], Edelman and Larbalestier [7], we derive the Gaussian distributions for the depinning currents Physica C 340 (2000) 225±229 www.elsevier.nl/locate/physc q Work partially supported by MURST/CONFIN98 pro- gram. * Corresponding author. Tel.: +39-089-96-5308; fax: +39-09- 953804. E-mail address: gamba@sa.infn.it (U. Gambardella). 1 Permanent address: INFN Frascati National Laboratory, v. E. Fermi 40, 00044 Frascati, Italy. 0921-4534/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII:S0921-4534(00)01336-8