  Within the RSJ model, we performed a theoretical analysis of phase-locking in
elementary strongly coupled Josephson junction cells. For this purpose, we
developed a systematic method allowing the investigation of phase-locking in
cells with small but non-vanishing loop inductance.The voltages across the
junctions are found to be locked with very small phase difference for almost
all values of external flux. However, the general behavior of phase-locking is
found to be just contrary to that according to weak coupling. In case of strong
coupling there is nearly no influence of external magnetic flux on the phases,
but the locking-frequency becomes flux-dependent. The influence of parameter
splitting is considered as well as the effect of small capacitive shunting of
the junctions. Strongly coupled cells show synchronization even for large
parameter splitting. Finally, a study of the behavior under external microwave
radiation shows that the frequency locking-range becomes strongly
flux-dependent, whereas the locking frequency itself turns out to be
flux-independent.
