Subgap and above-gap differential resistance anomalies in superconductor-normal-metal microjunctions

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Abstract
We present the results of differential resistance (dV/dI) measurement on high-transmittance Nb-Ag (or Al) microjunctions. At low bias, dV/dI has the conventional Blonder-Tinkham-Klapwijk double-dip structure plus a sharp single dip at zero bias. This zero-bias anomaly is completely suppressed by a modification in interface. It is insensitive to magnetic field. We relate it to the electron phase-coherence effect in the proximity of superconducting gap potential Δ. Above Δ/e, dV/dI exhibits an anomalous peak, whose position is found to be proportional to Δ(T,H).