Possible Dynamic States in Inductively Coupled Intrinsic Josephson Junctions of Layered High-T_c Superconductors

Based on computer simulations and theoretical analysis, a new dynamic state is found in inductively coupled intrinsic Josephson junctions in the absence of an external magnetic field. In this state, the plasma oscillation is uniform along the c axis with the fundamental frequency satisfying the ac Josephson relation. There are (2m+1)π phase kinks around the junction center, with m being an integer, periodic and thus nonuniform in the c direction. In the IV characteristics, the state manifests itself as current steps occurring at all cavity modes. Inside the current steps, the plasma oscillation becomes strong, which generates several harmonics in frequency spectra at a given voltage. The recent experiments on terahertz radiations from the mesa of a Bi₂Sr₂CaCu₂O_8+δ single crystal can be explained in terms of this state.