We report a new class of c-axis Josephson tunneling experiments in which a conventional superconductor (Pb) is deposited across a single twin boundary of a YBa 2 Cu 3 O 72d (YBCO) crystal.We measure the critical current as a function of the magnitude and angle of a magnetic field applied in the plane of the junction. In all samples, we observe a clear experimental signature of an order parameter phase shift across the twin boundary. These results provide strong evidence for mixed d-and s-wave pairing in YBCO, with a reversal in the sign of the s-wave component across the twin boundary. [S0031-9007(97)04213-0] PACS numbers: 74.50. + r, 74.72.BkThe symmetry of the order parameter in high temperature superconductors has been a subject of theoretical and experimental debate since their original discovery [1,2]. Phase-sensitive measurements on YBa 2 Cu 3 O 72d ( YBCO) involving currents flowing within the CuO 2 planes, such as corner-junction SQUID experiments [3-5], cornerjunction flux modulation experiments [6,7], and tricrystal ring experiments [8], have indicated an order parameter D͑k͒ with primarily d x 2 2y 2 symmetry under rotations in the plane ͓D͑k͒ ϳ cos k x a 2 cos k y a͔. The observation, however, of Josephson tunneling perpendicular to the CuO 2 planes between heavily twinned YBCO and a conventional s-wave superconductor [9-13] demonstrated a significant s-wave component. In this Letter, we report results from a new c-axis Josephson tunneling experiment which resolves the apparent conflict between the two groups of experiments and makes a compelling case for mixed order parameter symmetry in YBCO, with a dominant d-wave component and a significant s-wave component.In the experiment, shown conceptually in Fig. 1(a), a c-axis Josephson tunnel junction straddles a single YBCO twin boundary. Because Pb is an s-wave superconductor, the Pb counterelectrode couples only to the s-wave component of the YBCO order parameter; the net phase difference with any d-wave component integrates to zero. If YBCO were a conventional s-wave superconductor, the critical current I c ͑B͒ would exhibit a Fraunhoferlike dependence on magnetic field strength B, with a maximum at B 0, regardless of the angle of B in the plane of the junction. If, on the other hand, YBCO were predominantly d wave, any s-wave component to the order parameter would change sign across the twin boundary [14], yielding a dramatically different angular dependence. For magnetic fields perpendicular to the boundary, I c ͑B͒ would be the same as for a conventional junction, with a maximum at B 0. Magnetic fields parallel to the boundary, however, should produce a local minimum in I c at B 0, as in the d-wave corner junction experiment [2,6].The experiments were performed at the University of California, San Diego (UCSD) and the University of California, Berkeley (UCB) on single crystals of YBa 2 Cu 3 O 72d grown at UCSD and the University of FIG. 1. (a) Schematic diagram of a Pb ͞YBCO junction (hatched region) grown across a twin boundary (dashed line). For t...

In heavily twinned crystals or films of YBa 2 Cu 3 O 72x (YBCO), d x 2 2y 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 1 2 ͑hf͞2e͒. Experimentally, steps are observed only at multiples of hf͞2e, suggesting that s-wave pairing is present in YBCO; however, the simultaneous presence of d-wave pairing is by no means ruled out. PACS numbers: 74.50.+r, 74.72.Bk The symmetry of the order parameter in the high transition temperature ͑T c ͒ oxide superconductors is the subject of ongoing debate. One school of thought proposes a d x 2 2y 2 symmetry (for example, [1,2]) while another proposes an anisotropic s-wave symmetry (for example, [3,4]). A series of experiments [5-8] has been performed on Josephson junctions made on the a-b faces of YBa 2 Cu 3 O 7-x (YBCO) crystals or films, with conventional low-T c superconductors as the counterelectrodes. These measurements provide evidence for a phase shift of p between the order parameters measured by transport of pairs across weak links along two orthogonal directions, building a strong case for d-wave pairing. Sun et al. [9], however, performed extensive measurements of tunneling along the c axis of YBCO and the alloys Y 12x Pr x Ba 2 Cu 3 O 7-d , using an insulating barrier and Pb as the counterelectrode. At temperatures near 1 K the current-voltage ͑I-V ͒ characteristics exhibited a Josephson supercurrent at zero voltage, and very small single-particle tunneling currents below the Pb gap. The application of a magnetic field parallel to the plane of the junction produced a well-defined Fraunhofer pattern in the critical current, and yielded values of the penetration depth l ab ͑T ͒ with a temperature dependence consistent with those obtained from earlier microwave measurements [10,11].In a single crystal with tetragonal symmetry and purely d x 2 2y 2 pairing, the first-order pair tunneling currents cancel along the c-axis direction. Any pair tunneling would have to be second order, and the current thus very weak. In an untwinned single crystal of YBCO, however, the orthorhombic crystal symmetry leads to asymmetric d-wave pairing so that the first-order c-axis pair tunneling is not expected to vanish. The situation is different yet again in a heavily twinned single crystal or thin film. Here, the phase of the order parameter is maintained throughout the sample, but the amplitude variation along the a and b axes is expected to ensure that the net first-order tunneling cur-rent in the c-axis direction is zero. We note that any tunneling transitions in the a-b directions would contribute only to second-order supercurrents; again, first-order components cancel by symmetry. Since the c-axis tunneling experiments demonstrating the existence of a nonzero Josephson supercurrent [9] were performed m...