Vortex structure and cavity modes in stacked double Nb/AlO_x/Nb Josephson junctions

We have studied the static I-V characteristics and cavity modes in stacked double Nb/AlO_x/Nb Josephson junctions. In junction stacks consisting of two junctions with identical critical currents I_c, the I_c vs H characteristics have been observed to deviate from the usual Fraunhofer pattern in the small junction limit; the data are consistent with a model involving a structural phase transformation to a triangular vortex lattice with increasing H. In a finite voltage state interlayer coupling leads to splitting of the Swihart mode, which manifests itself as Fiske steps with different voltage spacings. The results provide clear evidence that the two junctions in the stack do phase lock.