Resonant Modes in a Stack of Intrinsic Josephson Junctions

In order to study the origin of coherent emission of intrinsic junctions in high-temperature superconductors, we investigated numerically ac power of emission at zero-field steps in IV-characteristics of a stack of two inductively coupled long Josephson junctions as well as emission from the solitary long junction. We found that zero-field steps appeared due to noise, random distribution of critical currents or due to normal edges of junctions in the stack. Due to the formation of collective excitations in the stack of coupled junctions, the zero-field step was split into two steps, which correspond to two normal modes of excitations. We investigated the dependence of heights of steps on the length of the stack and found that there existed two different threshold values of the length, so above the threshold length the zero-field step existed and below the threshold length it vanished. We calculated the dependence of ac power on the length of the stack and found the optimal value of the length for larger emission.