Pair Tunneling fromc-AxisYBa2Cu3O7−xto Pb: Evidence fors-Wave Component from Microwave Induced Steps

In heavily twinned crystals or films of ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}x}$ (YBCO), ${d}_{{x}^{2}{\ensuremath{-}y}^{2}}$ 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 $\frac{1}{2}(hf/2e)$. Experimentally, steps are observed only at multiples of $hf/2e$, suggesting that s-wave pairing is present in YBCO; however, the simultaneous presence of d-wave pairing is by no means ruled out.