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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

Differential conductance spectra are obtained from nanoscale junctions on the heavy-fermion superconductor CeCoIn5 along three major crystallographic orientations. Consistency and reproducibility of characteristic features among the junctions ensure their spectroscopic nature. All junctions show a similar conductance asymmetry and Andreev reflectionlike conductance with a reduced signal ( approximately 10%-13%), both commonly observed in heavy-fermion superconductor junctions. Analysis using the extended Blonder-Tinkham-Klapwijk model indicates that our data provide the first spectroscopic evidence for d_{x;{2}-y;{2}} symmetry. To quantify our conductance spectra, we propose a model by considering the general phenomenology in heavy fermions, the two-fluid behavior, and an energy-dependent density of states. Our model fits to the experimental data remarkably well and should invigorate further investigations.

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