Characterization of YBCO bicrystal grain-boundary Josephson junctions on NdGaO 3 substrate in terms of conventional superconductivity models C. Camerlingo a , M.P. Lissitski a,b, * , C. Nappi a , M. Russo a a Istituto di Cibernetica del CNR, ‘‘E. Caianello’’, Via Campi Flegrei 34, Comprensorio ‘‘A. Olivetti’’, Building 70, 80078 Pozzuoli, Napoli, Italy b INFM, Unit  a di Napoli, Complesso Universitario di Monte Sant’ Angelo, Via Cintia, 80126 Napoli, Italy Abstract We have fabricated high quality grain-boundary bicrystal YBCO Josephson junctions on the NdGaO 3 substrates. At the temperature T ¼ 77 K, the I –V characteristic can be explained by means of the conventional overdamped resistively shunted junction model comprising thermal noise. The deviations of the magnetic dependence of the Josephson critical current I c from the usual Fraunhofer diffraction pattern can be described by a combination of a peaked-at-the-edges distribution of the critical current density and of the rounding of the I –V characteristic due to thermal fluctuations. The same measurements were done also at T ¼ 4:2 K, at low thermal noise conditions. The experimental magnetic field dependence I c vs H was described by using a peaked-at-the-edges asymmetric profile, where the asymmetry depends on the applied magnetic field. The relation between our phenomenological model and advanced microscopical theory of grain-boundary bicrystal YBCO junction based on alternating ‘0’ and ‘p’ contacts is discussed. Ó 2002 Elsevier Science B.V. All rights reserved. Keywords: Bicrystal grain-boundary junction; Josephson effect; Resistively shunted junction model; Josephson critical current 1. Introduction At present the physical mechanism underlying the Josephson effect in high T c bicrystal grain- boundary (BGB) junctions remains a open ques- tion [1]. Nevertheless the conventional low T c models for tunnel Josephson junctions (i.e. resis- tively shunted junction (RSJ) model [2]) seem to be appropriate to describe the electrical behavior of BGB junctions. A less clear situation is concerned with the magnetic field dependence of the Josephson critical current I c , mainly because the high T c supercon- ducting materials easily trap magnetic fluxons which cause distortions of the magnetic field de- pendence of the I c . In addition, important effects should arise from the unconventional d-wave symmetry of the order parameter [1]. As we report here, the use of NdGaO 3 sub- strates allow to fabricate high quality YBCO BGB junctions. Those junctions permit to obtain the Physica C 372–376 (2002) 91–94 www.elsevier.com/locate/physc * Corresponding author. Fax: +39-081-8042604. E-mail address: lisitskii@fisica.cib.na.cnr.it (M.P. Lissitski). 0921-4534/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII:S0921-4534(02)00815-8