Physica C 153-155 (1988) 1393-1394 North-Holland, Amsterdam JOSEPHSON TUNNELING IN YBCO-Nb BULK JUNCTIONS BARONE +* CRISTIANO ÷, * , o , o A. , R. A. DI CHIARA , G. PELUSO , A. RICCA , U. SCOTTI DI UCCIO , S.ZANNELLA • Dipartimento di Scienze Fisiche and INFN, P.le Tecchio 80 - 80125 NAPOLI; +Istituto di Ciberne- tica del CNR, Via Toiano, 6 ARCO FELICE NAPOLI; °ClSE - Tecnologie Innovative S.p.A., P.O. BOX 12081-20134, Via Reggio Emilia, 39 SEGRATE (MILANO) Josephson Junctions with Nb sputtered film as counterelectrode have been realized on YBCO pellets. The effect of the cleaning procedure of the YBCO surface in the fabrication process is discussed. The I-V characteristic at 4.2K shows a Josephson current. A rough estimation of the magnetic penetration depth is obtained by the external magnetic field dependence of the critical current. I . INTRODUCTION After the discovery of very high transition temperature in the metallic oxide superconduc- tors many efforts have been devoted to the determination of the relevant physical parameters of these materials such as the ener- gy gap value, the coherence lenght, the mag- netic penetration depth etc. To this purpose bulk junctions based on YBCO pellets employing a film of a conventional superconductor as counterelectrode (I-3) can be of complementary interest to the other junction structures (4). In the bulk junctions the surface of the base electrode obviously plays an important role, particularly when Josephson tunneling is invol- ved. In this communication we report the fabri- cation procedure of YBCO-Nb bulk junctions pay- ing a particular attention to the surface cleaning step. From the Josephson current modu- lation in the external magnetic field a rough estimate of the London penetration depth can be inferred. Previously reported values of ~ are spread over a wide range (225 - 2300 A) depend- ing on the measurements tecniques (5-7). 2. SAMPLE PREPARATION YBCO-Nb bulk junctions have been realized on single-phase Y Ba Cu~ 07 pellets with Tc= 92.4K (midpoint). j T~eXpellets were parti- ally covered by an insulating layer of SiO ab- out 1 pm thick, evaporated by heating from a molybdenum boat or by electron gun. Indium con- tacts were realized with the same tecnique. The YBCO surface was then cleaned at an Ar pressure 0921-4534/88/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division) of 3-4xi0 -4 Torr by a Fast Atom Bombardment (FAB) operating at a voltage V=~-7kV with an ion current density J 40~pA/cm-- for 30 min. Then a Nb film as counterelectrode ( 1 pm tick) was finally deposited by dc magnetron sputter- ing through a molybdenum mask in fast sequence without breaking the vacuum. The junction geometry was of the overlap type with LxW=Ixl mm area. The scanning electron mycroscope (SEM) picture of the surface of an YBCO sample before and after the cleaning procedure show a large scattering of the grain size around the mean value of about I0 pm. The main effect of the cleaning appears to be the removal of the smallest structures at the surface. X-ray emis- sion spectra (XES) of samples before and after the cleaning step indicates that the right stoichiometry of Y, Ba, Cu, is substantially preserved. 3. RESULTSAND DISCUSSION The junctions tested in a He4 cryostat were mounted on a copper block surrounded by a long solenoid and by a cryoperm box for magnetic shielding. A typical current-voltage charac- teristic at T=4.2 K exhibiting a zero-voltage current of about 20 pA is reported in Fig. I. The dependence of the supercurrent on the ex- ternally applied magnetic field H shows a e double junction interferential behavior (Fig. 2). This circumstance occurs also in single junctions with the Josephson current peaked at the edges of the sample (8). It is of course impossible to infer unambigously the actual current distribution from the Ic(H e) pattern.