Evidence for Cherenkov radiation of a Josephson vortex is observed in the system of two magnetically coupled long Josephson junctions. This radiation leads to resonances which appear above the lowest characteristic velocity c Ϫ of the linear electromagnetic waves. Resonances result from interaction of a Cherenkov wave with a moving Josephson vortex and, in annular junctions, occur when the vortex makes one turn around the junction. Experimental data are in good agreement with the proposed model and simulations which provide a clear physical picture of the observed effect. ͓S0163-1829͑97͒01046-1͔

Long Josephson junctions of width down to less than 0.3 pm are fabricated using electron beam lithography. The junctions are made in niobiumaluminum-oxide trilayer technology using cross-linked PMMA for insulation. We measured the fluxon penetration field, the magnetic field period of the critical current modulation, and the Fiske step voltages of the junctions. A strong dependence of these quantities on the junction width is observed. Assuming a general-type relation between the spatial derivative of the phase and the spatial variation of the magnetic field along the plane of the junction, we derive a scaling relation between the measured quantities depending on the junction width. The derived relation is consistent with the experimental data.

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The surface roughness of sputtered Nb films was determined with high precision using x-ray specular reflectivity measurements in the 10 keV range. The roughness of Nb films increased from 0.9 nm for a 70-nm-thick film to 1.8 nm for a 210-nm-thick film. The roughness of the Nb surface strongly influences the tunnel barrier formation and the electrical properties of that barrier. For stacked tunnel junctions each thermal Al oxidation has to be adjusted as a function of the underlying Nb film thickness. Alternatively, one can use an Al or Al/AlOx underlayer in order to obtain stacks with small spread in critical currents.

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