Macroscopic quantum coherence effects in microwave emission from high-t_c superconducting films

Macroscopic quantum coherence effects have been observed in X-band emission from current-biased high-T_c films. Two groups of excess emission lines, current stable lines and voltage stable lines, were distinguished in the emission spectra by their different behaviour with changing temperature and magnetic field. The electromagnetic energy was found to be radiated by synchronized clusters composed of only a few intergranular Josephson junctions. Current stable lines are related to coupled two-junction quantum interferometers. Voltage stable lines are due to clusters composed of a few connected single junctions or to a large single junction with pronounced structural fluctuations.