Subharmonic energy-gap structures and 1/f noise properties in Nb/Al+AlOx/Nb tunnel junctions

We have investigated subharmonic energy-gap structures (SGSs) and 1/f noise spectra in Nb/Al + AlOx/Nb tunnel junctions as a function of bias voltage. The clear SGSs were observed in dV/dI vs. V curves at 4.2 K. The quasipart icle characteristic was explained by the Octavio, Tinkham, Blonder and Klap wijk theory based on multiple-Andreev reflection model. To estimate the 1/f noise parameter alpha /N-c in the Nb/Al + AlOx/Nb junctions, we used Hooge 's empirical theory. The alpha /N-c of these junctions biased above superco nducting energy-gap did not depend on bias voltage, while below the gap vol tage it was larger than that above the gap voltage. Moreover, some peak str uctures related to the SGSs were observed in alpha /N-c vs. V characteristi cs biased below the gap voltage. This enhanced 1/f noise might be caused by MAR current. (C) 2001 Elsevier Science B.V. All rights reserved.