ORIGIN OF INTRINSIC JOSEPHSON COUPLING IN THE CUPRATES AND ITS RELATION TO ORDER-PARAMETER SYMMETRY - AN INCOHERENT HOPPING MODEL

Experiments on the cuprate superconductors demonstrate that these mate rials may be viewed as a stack of Josephson junctions along the direct ion normal to the CuO2 planes (the c-axis). In this paper, we present a model which describes this intrinsic Josephson coupling in terms of incoherent quasiparticle hopping along the c-axis arising from wave-fu nction overlap, impurity-assisted hopping, and boson-assisted hopping. We use this model to compute the magnitude and temperature T dependen ce of the resulting Josephson critical current j(c)(T) for s- and d-wa ve superconductors. Contrary to other approaches, d-wave pairing in th is model is compatible with an intrinsic Josephson effect at all hole concentrations and leads to j(c)(T) proportional to T at low T. By par ameterizing our theory with c-axis resistivity data from YBa2Cu3O7-del ta (YBCO), we estimate j(c)(T) for optimally doped and underdoped memb ers of this family. j(c)(T) can be measured either directly or indirec tly through microwave penetration depth experiments, and current measu rements on Bi2Sr2CaCu2O8 and La2-xSrxCuO4 are found to be consistent w ith s-wave pairing and the dominance of assisted hopping processes. Th e situation in YBCO is still unclear, but our estimates suggest that f urther experiments on this compound would be of great help in elucidat ing the validity of our model in general and the pairing symmetry in p articular.