Spontaneous frequency shift and phase retardation of coupled Josephson oscillations in cuprate superconductors

We examine coupling interactions used to synchronize macroscopic Josephson oscillations induced in intrinsic Josephson junction (IJJ) mesa stacks made of Bi2212 single crystals. Synchronized emissions of terahertz electromagnetic (EM) waves are detected under common voltage and current bias operations of two connected mesas with close individual emission frequencies, while uncoupled and bimodal emissions are frequently observed in two mesas with different individual emission frequencies. Detailed observations of the polarization of the EM wave emitted when two mesas are biased in parallel or series allow us to reveal the coupling matrix components, which include ratios of synchronized IJJs in the mesas and phase retardation between the macroscopic Josephson oscillations. This finding stimulates systematic survey on polarization of EM wave emitted from synchronized multiple mesa devices in order to realize powerful emissions from superconductors.