PAIR TUNNELING FROM C-AXIS YBA2CU3O7-X TO PB - EVIDENCE FOR S-WAVE COMPONENT FROM MICROWAVE-INDUCED STEPS

In heavily twinned crystals or films of YBa2Cu3O7-x (YBCO), d(x2-y2) p airing symmetry is expected to cause the cancellation of first-order J osephson tunneling through a YBCO-Pb tunnel junction grown on an a-b f ace; any residual tunneling is thus second order. As a result, microwa ves at frequency f are predicted to induce steps on the current-voltag e characteristic at voltages that are multiples of 1/2(hf/2e). Experim entally, steps are observed only at multiples of hf/2e, suggesting tha t s-wave pairings present in YBCO; however, the simultaneous presence of d-wave pairing is by no means ruled out.