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) pairing symmetry is expected to cause the cancellation of first-order Josephson tunneling through a YBCO-Pb tunnel junction grown on an a-b face; any residual tunneling is thus second order. As a result, microwaves at frequency f are predicted to induce steps on the current-voltage characteristic at voltages that are multiples of 1/2(hf/2e). Experimentally, steps are observed only at multiples of hf/2e, suggesting that s-wave pairings present in YBCO; however, the simultaneous presence of d-wave pairing is by no means ruled out.