‘‘Double superconducting transition’’ in YBCO thin films € Orjan Festin a, * , Peter Svedlindh a , Zdravko Ivanov b a Department of Materials Science, Uppsala University, P.O. Box 534, SE-751 21 Uppsala, Sweden b Department of Physics, Chalmers University of Technology, SE-412 96 G€ oteborg, Sweden Abstract Two 150 nm thick YBCO films have been investigated using flux noise and complex conductance measurements. In both experiments thermally excited vortices are studied. The first film shows a single transition, with T c  88:2 K and DT c  0:3 K, while the second film shows a two-step transition with T c1  90:1 K and DT c  0:1 K, and T c2  89:8K and DT c  0:15 K. The experimental results obtained for the double transition sample in combination with computer simulation data in Medvedyeva et al. (in this volume) indicate that this sample can be described as two weakly coupled homogeneous superconducting layers, one on top of the other, with different critical temperatures. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: High-T c superconductors; Double transition; Complex conductance; Flux noise; Vortex fluctuations The phenomenon of a split transition in high-T c superconductors was first observed in specific heat measurements on sintered pellets of YBCO by Inderhees et al. [1], and later in resistivity mea- surements on sintered pellets [2]. Since then, nu- merous investigations have been performed on both sintered samples and single crystals, using specific heat, resistivity and ac-susceptibility mea- surements, complemented by detailed crystallo- graphic studies [3]. A split transition has also been observed in heavy fermion superconductors [4] and more recently similar properties were found in high-quality MgB 2 films [5]. Several explanations are at hand for this phe- nomenon: grain boundaries creating a network of Josephson junctions between the superconducting grains in the sample, regions in the sample with differences with respect to oxygen concentration and/or stoichiometry, all causing a split of the superconducting transition. In this paper, we report on flux noise and com- plex conductance measurements [6] on two super- conducting films, one of which shows a two-step superconducting transition. The two samples are square shaped (5  5 mm 2 ) 150 nm thick YBCO-123 films grown epitaxially on SrTiO 3 substrates by pulsed laser deposition. According to X-ray h–2h and /-scans the films are c-axis oriented and only ½100 YBCO k½100 SrTiO 3 in-plane orientation is observed. One sam- ple is a bare YBCO film (U6), while the other film (PR75) has three capping layers, PBCO/SrTiO 3 / PBCO, each 50 nm thick. The film surfaces have been examined using AFM and SEM. U6 shows a rough surface with pin holes and copper oxide outgrowths, giving a mean surface roughness of the Physica C 369 (2002) 295–299 www.elsevier.com/locate/physc * Corresponding author. Fax: +46-18-50-01-31. E-mail address: orjan.festin@angstrom.uu.se (O ¨ . Festin). 0921-4534/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII:S0921-4534(01)01262-X