ELSEVIER Physica B 218 (1996) 224 227 PHYSICA[ Superconducting parameters of YBCO and BSCCO from 'tunneling' spectroscopy P. Seidel a'*, M. Grajcar a'b, A. Plecenik a'c, R. Hlubina b'a "lnstitut fiir Festkbrperphysik, Friedrieh-Schiller-Universitht Jena, Hehnholtzwe9 5, D-07743 Jena, Germany bDepartment of Solid State Physics, Comenius University, 842 15 Bratislava, SIovak Republie Clnstitute of Electrieal Engineering, SIovak Academy of Sciences, 84239 Bratislava, Slovak Republic dlnternational School.for Advanced Studies, Via Beirut 4, 1-34014 Trieste, Italy Abstract The differential conductivities of Bi2SrzCaCu2Os +x/Au and YBa2Cu307 6/Au point contacts were measured. The superconducting order parameter A and the Fermi velocity Vv of both superconductors were determined making use of the modified Blonde~Tinkham-Klapwijk theory. The coherence length and penetration depth of BizSrzCaCu20 8 *x and YBa2Cu307 ~ were calculated from A and vv, by applying a weak-coupling theory of superconductors with anisotropic single-particle dispersion and order parameter. Our results are compared to other experimental data. The question about the symmetry as well as the origin of pairing in HTS is still open, although recent experi- ments seem to favour dx2 y2 - wave rather than s-wave pairing El]. In any case, the superconducting order para- meter is expected to be highly anisotropic. Moreover, the layered nature of the cuprates implies a nearly two- dimensional character of the electron states and also the in-plane properties are strongly anisotropic due to the closeness of the Fermi level to the Van Hove singularities [2]. In this paper we concentrate on the determination of the basic superconducting parameters (order parameter A, coherence length ~, penetration depth )3 as well as the Fermi velocity Vv of BizSrzCaCuzOs+~ (BSCCO) and YBa2Cu307 6 (YBCO) from a measurement of the dif- ferential conductance of HTS/Au point contacts, assum- ing d-wave pairing in the HTS. We emphasise that the symmetry of the superconducting state is irrelevant for our discussion and all that matters is the superconduct- ing density of states. Following Bok and Force [3], we *Corresponding author. pay special attention to the case when there is a large modulation of the Fermi velocity along the Fermi surface when evaluating ~ and ). from A and vv. To this end, we apply formulas for the coherence length and the penetra- tion depth of a superconductor with anisotropic single- particle dispersion and order parameter [4]. An expression for the current I(V, A) across a normal metal/superconductor (N/S)junction for arbitrary trans- mittances of the interface was derived by Blonder, Tin- kham and Klapwijk (BTK) [5]. Recently, BTK theory has been modified by taking into account the finite scat- tering rate F of the quasi-particles [6]. For our geometry of the point contact (the Au tip is pressed into the c-oriented film) we assume that all 'tunneling' directions are equally probable. Thus, for a superconductor with an anisotropic gap we can write I(V) = ~ l(V,A((p))d(p, (1) where I(V,A((p)) was calculated in Ref. [6] and we take A(q0) = A cos 2~p for a d-wave superconductor. 0921-4526/96/$15.00 d 1996 Elsevier Science B.V. All rights reserved SSDI 0921-4526(95}00600-1