JOSEPHSON EFFECT AND SINGLE-PARTICLE TUNNELING IN YBA2CU3O7-X AND YBBA2CU3O7-X SINGLE-CRYSTAL BREAK JUNCTIONS

We have studied the current-voltage characteristics of break junctions in single crystals of YBa2Cu3O7-x and YbBa2Cu3O7-x at helium temperature. For a YBa2Cu3O7-x Josephson junction Fiske resonances in low magnetic fields have been observed in the I(V) characteristic, thus proving the tunneling nature of the contact. The reduced velocity of the electromagnetic wave in the contact area has been calculated to be c/c(0) approximate to 0.018 (c is the Swihart velocity). The ratio of the barrier thickness d(1) to the relative dielectric constant of the barrier epsilon was found to be d(1)/epsilon=0.09 nm. In the quasiparticle tunneling regime, the YbBa2Cu3O7-x break junctions showed in some cases I(V) characteristics with a relatively small excess current at ''sub-gap'' voltages and a steep rise of the current at the ''gap'' voltage V-g=2 Delta/e. The ''BCS-like'' form of the observedI(V) characteristics seems to be in favor of s wave superconductivity.