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 temperat ure. For a YBa2Cu3O7-x Josephson junction Fiske resonances in low magn etic fields have been observed in the I(V) characteristic, thus provin g the tunneling nature of the contact. The reduced velocity of the ele ctromagnetic 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 ba rrier thickness d(1) to the relative dielectric constant of the barrie r epsilon was found to be d(1)/epsilon=0.09 nm. In the quasiparticle t unneling 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) characteristi cs seems to be in favor of s wave superconductivity.