ELSEVIER PhysicaC253 (1995) 63-70 PHYSICA Electrical properties of 45 ° grain boundaries of epitaxial YBaCuO, dominated by crystalline microstructure and d-wave-symmetry C.A. Copetti a, F. Riiders a, B. Oelze a, Ch. Buchal ~'*, B. Kabius b, J.W. Seo b a lnstitutfiir Schicht- und Ionentechnik, Forschungszentrum Jiilich GmbH, 52425 J~ilich~ Germany b lnstitutfiir Festki~rperforschung, Forschungszentrum J~lich GmbH, 52425 Jiilich, Germany Received 16 June 1995 Abstract We show that the microstrncture of 45 ° grain boundaries in epitaxial YBaCuO together with the d~2_y2 symmetry of the superconducting wave function accounts for the measured magnetic field modulation patterns of the critical currents. Our results resolve the conflict between contradictory observations of other authors and unfold a consistent picture of d-wave-symmetry of the YBaCuO superconducting ground state. Evidence for currentless junctions with a phase difference of xr is given. 1. Introduction At present, the symmetry of the electronic pairing state of high-temperature superconductors is under intense and controversial discussion. The evidence for unconventional symmetry has become very strong. The dx2_y2-symmetry is especially favoured by an increasing number of observations [1-5]. Nev- ei~daeless, very ingenious experiments have been pub- lished which seem to favour s-wave-symmetry [6,7]. In this publication we resolve some of the re- cently published discrepancies. We present measure- ments of the magnetic field modulation of the critical currents of 45 ° grain boundary Josephson junctions of epitaxial thin films of YBaCuO and high-resolu- tion electron micrographs of these grain boundaries. We show that, on a microscopic scale, the 45 ° grains * Corresponding author. do not meet along arbitrary grain boundary angles, Instead, the grain boundaries are faceted into seg- ments with (100) and (110) habit planes, forming a staircase of (100) and (110) facets [8,9]. As a conse- quence, the all-determining Josephson phase rela- tions have to be evaluated for each facet separately, resulting in complicated current patterns. The very strong reduction of the critical current at 45 ° grain boundaries is a direct consequence of the anisotropy of the order parameter. Furthermore, very small addi- tional in-plane angular tilts due to microtwins lead to a microscopic sequence of junction areas with con- secutive 180° phase jumps. This leads to a sequence of electrically parallel "O"- and "Tr"-junctions. Therefore, adjacent microfacets may produce Joseph- son currents of opposite direction. The observable macroscopic Josephson current is the result of sum- ming up all the microscopic currents along the grain boundary. This overall current varies in an unusual 0921-4534/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSD! 0921-4534(95)00489-0