Quasiparticle tunneling in the c-direction in stacks of Bi2Sr2CaCu2O8+delta S-I-S junctions and the symmetry of the superconducting order parameter

A clear gap structure has been observed in the current-voltage characterist ics (CVC) of stacks of S-I-S junctions obtained in Bi2Sr2CaCu2O8+delta (BSC CO) crystals using a break junction technique. Due to the high resistance o f the stacks, the range of bias voltages could be extended well above the g ap voltage V-gn without significant overheating of the contacts. For sample s with a critical temperature T-c= 84-89 K and 2 Delta/kT(c) = 6.7 +/- 0.3, the value of V-gn corresponding to a steep rise of the quasiparticle curre nt I-qp is well-described by the relation V-gn=n(2 Delta/e) with a gap para meter Delta(4.2 K)=(25 +/- 1) meV. The presence of a sharp gap feature in c haracteristics of stacks points to an essential contribution of s-wave pair ing to the symmetry of the order parameter in BSCCO. (C) 1999 Published by Elsevier Science B.V. All rights reserved.