Physica C 180 (1991) 227-234 North-Holland PflYSICA ANISOTROPIC SNS JOSEPHSON JUNCTIONS USING EPITAXIAL YBaCuO FILMS Hiroshi AKOH, Satoshi KOHJIRO, Carlo CAMERLINGO*, Ryokan YUASA**, and Susurnu TAKADA Electrotechnical Laboratory, 1-1-4 Umezono, Tsukuba, Ibaraki 305, JAPAN We present the anisotropic Josephson effect in the SNS structures consisting of YBaCuO/Au/Nb film sandwiches, using epitaxial YBaCuO films. SNS junctions with a-b plane orientated YBaCuO films on exhibit the Josephson effect, while no Josephson currents are observed in the junctions with c-axis orientated YBaCuO films. In addition, a-b plane orientated junctions show anisotropic Fraunhofer patterns in the magnetic field dependence of the critical current, depending on the direction of applied magnetic fields. These anisotropic Josephson effects originate from the anisotropy in the coherence length and the magnetic penetration depth of YBaCuO films. 1. INTRODUCTION A Josephson tunnel junction, a so called SIS (superconductor - insulator - superconductor) Josephson junction is one of the most promising devices to realize a high performance computer since it has an ultra-high switching speed with the very low power dissipation. Recently, a Josephson computer with LSI circuits using Nb/A1Ox/Nb Josephson tunnel junctions has been already demonstrated and completely operated. 1 On the other hand, SIS Josephson junctions using all high-Tc oxide superconductors, which are expected to have a high operating temperature and a high gap voltage, have not been realized yet so far, in spite of the enormous efforts made. The main difficulties arise from the extremely small value of the coherence length ~ with a few tenth of nm, resulting in the critical technological constraints for the fabrication of SIS structures. The tunnel characteristics are in fact determined by the density of states within a distance of ~ inside the junction electrode. An accurate control of the stoichiometry, the oxygen content and the lattice perfection is thus required at the surface of the junction electrode within a depth of a few lattice constants. An SNS (superconductor - normal metal - superconductor) junction 2 is one of the promising devices to fabricate SIS Josephson junctions, since the junction clearly shows the Josephson effect due to the proximity effect, which gives us the information of the surface electronic properties of high-Tc superconductors, such as a short and anisotropic coherence length, the surface super- conductivity, the density of states and so forth. In addition, SNS junctions have the advantage of the easier fabrication, compared to SIS junctions in which the insulating barrier is required to have the same order of ~. In SNS junctions, however, junction properties are dominantly determined by the coherence length of normal layer. For instance, the Au has the coherence length of-100 nm due to the proximity effect, so that the Au barrier can be thicker than the insulating barrier in SIS junctions. In SNS Josephson junctions as same as SIS junctions, the crystalline orientation of the high- Tc superconducting film is very important, since the high-Tc superconductors have a very short and anisotropic ~. The surface value of energy gap in *Permanent address: Istituto di Cibemetica, Consiglio Nazionale delle Ricerche, Via Toiano, 6-80072, Arco Felice (Napoli), ITALY. **On leave from SANYO Tsukuba Research Center, 2-1 Koyadai, Tsukuba, Ibaraki 305, JAPAN. 0921-4534/91/$03.50 © 1991 - Elsevier Science Publishers B.V. All rights reserved.