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Data from ab-oriented YBa 2 Cu 3 O 7 ͞I͞Cu tunnel junctions are presented. Self-assembled monolayers form the insulating tunnel barrier, I. The YBa 2 Cu 3 O 7 features in the tunneling conductance match those of low-leakage ab-oriented YBa 2 Cu 3 O 7 ͞Pb junctions. Results show that the zero-bias conductance peak is an Andreev bound state (ABS) of a d-wave order parameter. In zero magnetic field, the ABS splits below ϳ7 K, consistent with the presence of a subdominant order parameter at the surface. An applied magnetic field induces further splitting that grows nonlinearly with increasing field.[S0031-9007 (97)03529-1] PACS numbers: 74.50. + r, 74.72.Bk

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It is demonstrated that the specular reflectivity of spin-polarized neutrons can be used to study vortices in a thin-film superconductor. Experiments were performed on a 6000 Å thick c-axis film of YBa 2 Cu 3 O 7Ϫx with the magnetic field applied parallel to the surface. A magnetic hysteresis loop was observed for the spinpolarized reflection and, from these data, the average density of vortices was extracted. A model is presented which relates the specular reflectivity to the one-dimensional spatial distribution of vortices in the direction perpendicular to the surface. Unlike other techniques, neutron reflectivity observes vortices in a geometry where they are parallel to the interface. ͓S0163-1829͑99͒07021-6͔

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