An unusual technique utilizing the two-dimensional junction configuration is introduced to study the pairing symmetry of a YBa 2 Cu 3 O 7Ϫy cuprate superconductor. The zero bias conductance peak due to the Andreev bound states was measured for the YBa 2 Cu 3 O 7Ϫy /I/Ag ramp-edge tunnel junctions with different crystalinterface boundary angles fabricated on the same chip. It was found to develop monotonically as the angle was increased and became a maximum at 45°. Two gaplike structures were also found at the intermediate angle of 30°. The result implies that d x2Ϫy2 -wave pairing symmetry predominates in this cuprate. RAPID COMMUNICATIONS PRB 62 R6133 ANGLE-RESOLVED ANDREEV BOUND STATES IN . . . 55, 15 282 ͑1997͒. 29 L.

Superconductors show zero resistance to electric current, and expel magnetic flux (the Meissner effect) below the transition temperature (Tc). In conventional superconductors, the 'Cooper pairs' of electrons that are responsible for superconductivity form only below Tc. In the unconventional high-Tc superconductors, however, a strong electron correlation is essential for pair formation: there is evidence that some pairs are formed above Tc in samples that have less than the optimal density of charge carriers (underdoped) and an energy gap-the 'pseudogap'-appears to be present. Moreover, excitations that look like the vortices that carry magnetic flux inside the superconducting state have been reported above Tc (refs 6, 7). Although the origin of the pseudogap remains controversial, phase fluctuations above Tc, leading to some form of local superconductivity or local pairing, seem essential. Here we report magnetic imaging (scanning SQUID microscopy) of La2-xSrxCuO4 thin films. Clear quantized vortex patterns are visible below Tc (18-19 K), and we observe inhomogeneous magnetic domains that persist up to 80 K. We interpret the data as suggesting the existence of diamagnetic regions that are precursors to the Meissner state.

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We report measurements on high-T c YBa 2 Cu 3 O 7Ϫy ͑YBCO͒ Josephson ramp-edge junctions with different ab-plane orientation electrodes relatively rotated by 45°. The ramp-edge junctions with different crystal angles against the interface boundary are fabricated on an MgO͑100͒ substrate using a CeO 2 seed-layer technique. The Shapiro steps under microwave irradiation of 9 GHz appear qualitatively different for different crystal angles, which supports the d-wave nature of a YBCO superconductor. The magnetic-field dependence of the Josephson maximum current shows a Fraunhofer-like pattern, which is consistent with the calculated result for the d-wave junctions. The temperature dependencies of the maximum Josephson current for different angle geometry fall off rapidly with increasing temperature and deviate much from the conventional s-wave Ambegaokar-Baratoff prediction. The results are in good qualitative agreement with the calculated results based on the d-wave pairing symmetry taking the presence of the zero energy state into account. The observed angle dependencies clearly indicate that the angle-dependent Josephson current really exists.

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