Above-gap differential resistance anomaly in superconductor-constrictions-superconductor contacts

Annealing experiments of high-transmittance Nb-constrictions-Nb (Nb-c-Nb) contacts have been carried out at various temperatures. The constrictions with diameters as small as 10 nm were grown in the thick MgO (or Al/sub 2/O/sub 3/, SiO/sub 2/) layer of an insulating Nb/Insulator/Nb sandwich by applying a high electric field of 0.1-2 V/nm in liquid helium. We observed an above-gap anomaly, i.e., anomalous differential resistance (dV/dI) peaks, only in highly transmissive Nb-c-Nb contacts. These peaks were greatly suppressed by annealing treatments at temperatures as low as 130/spl deg/C for 30 min in air, without degradation of normal resistance R/sub N/ after annealing. When we applied the high field of 0.4 V/nm to such junction at 4.2 K again, the dV/dI vs. V curve almost recovered the original shape before annealing.