Proximity-Effect and Tunneling in YBa 2 Cu 3 O 7 /Metal Layered Structures
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PROXIMITY-EFFECT AND TUNNELING IN YBa 2 Cu 3 0 7 /METAL LAYERED STRUCTURES L. H. Greene%, W. L. Feldmann', J. B. Barner*, L. A. Farrow%, P. F. Miceli%, R. Ramesh%, B. J. Wilkens%, B. G. Bagley', M. Giroud** and J.M. Rowell*** Bellcore, 331 Newman Springs Rd., Red Bank, NJ, 07701, USA, CRTBT-CNRS, Grenoble, FR,. Conductus, Inc., Sunnyvale, CA, 94086, USA. ABSTRACT Superconducting thin films of YBa 2 Cu 3 0 7 are prepared in-situ by on-axis, sputter deposition from a single, composite target. Our planar magnetron target composition of Y:Ba:Cu=1.08:1.76:4.5 sputtered onto MgO at T-7500C in a 600mTorr Ar-0 2 atmosphere yields reproducible superconducting films having Tc(R=O)>80K and stoichiometry 1:2:3, that are shiny and of near epitaxial, crystalline quality. In order to ensure clean interfaces, YBa 2 Cu 30 7/normal metal bilayers (to form SNS' Josephson junctions) and YBa 2 Cu 3 O7/normal metal/insulating barrier trilayers (to form SNIS' proximity tunnel junctions) are grown completely in-situ. (The S'= Pb counter electrode is evaporated ex-situ.) A supercurrent and Shapiro steps are observed in microwave irradiated SNS' (N = Ag) small area (5x10- 5 cm 2) junctions. In SNIS' tunnel junctions, high-quality Pb tunneling is observed.
GOALS AND APPROACH Our goal is to fabricate reproducible, planar tunnel junctionst, and to perform proximity-effect studies on high-temperature superconductors (HTSC) with normal metals and conventional superconductors. These goals are difficult to attain because of the intrinsic short coherence length of the high-Tc materials and because these materials tend to form non-superconducting surface layers. Our basic approach, therefore, is to cover the high-Tc material with a clean, normal metal which will not degrade the high-Tc material and whose coherence length is significantly larger. In Fig. 1 we show a schematic of the order parameter, or quantum-mechanical expectation value of the superconducting wave function, as a function of the distance x. As seen in Fig. 1(a), this order parameter is constant in the bulk material, but falls dramatically at the HTSC surface. However, by placing a normal metal in close proximity to the HTSC surface, as shown in Fig. 1(b), the order parameter can extend to a greater distance (determined by the coherence length in the normal layer) and a superconducting gap can be resolved by proximity electron tunneling spectroscopy (PETS) through an insulating barrier on the normal metal surface. Such an approach has been found to be useful in studies with other short coherence-length superconductors. 2 A structure without the insulating barrier, i.e., placing a superconducting counter-electrode directly on the normal metal layer, is of both scientific and technological interest. First, in such an SNS' junction the observation of a Josephson effect between HTSC and conventional3 superconductors would have ramifications with respect to proposed mechanisms. Second, reproducible Josephson junctions fabricated using a high-Tc material would allow for the possibility of
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