Field dependence of Fiske resonances in Nb-AlOx based Josephson junctions

Fiske resonances have been measured in a rectangular Nb-Al, AlO x, Al-Nb Josephson junction as a function of the magnetic field applied parallel to a junction side. Due to the high quality factor of the junction, many resonant modes could be measured, using a special measuring technique. Each mode shows more lobes than reported before. The measured curves are in very good agreement with the high Q theory of I.O. Kulik (1967), and Q-values ranging from 40 to 450 have been obtained. The surface resistance of the Al/Nb barrier probably dominates the microwave losses in the junction barrier at 4.2 K. Losses due to the quasi-particle tunneling current can be neglected. Fiske resonances in a square junction at a field angle of ¿/4 rad were also measured. Two-dimensional resonant modes have been found. The field dependence of the (1,1) mixed mode agrees well with the theory of M.A.H. Nerenberg et al. (1976)