Collective Dynamics of Josephson Vortices in Intrinsic Josephson Junctions :Exploration of In-phase Locked Superradiant Vortex Flow States

In order to clarify the ``superradiant'' conditions for the moving Josephson vortices to excite in-phase AC electromagnetic fields over all junctions, we perform large scale simulations of realistic dimensions for intrinsic Josephson junctions under the layer parallel magnetic field. Three clear step-like structures in the I-V curve are observed above a certain high field ($H > 1T$ in the present simulations), at which we find structural transitions in the moving flux-line lattice. The Josephson vortex flow states are accordingly classified into four regions (region I $\sim $ IV with increasing current), in each of which the power spectrum for the electric field oscillations at the sample edge are measured and typical snapshots for Josephson vortex configurations are displayed. Among the four regions, especially in the region III, an in-phase rectangular vortex lattice flow state emerges and the power spectrum shows remarkably sharp peak structure, i.e., superradiant state. Comparison of the simulation results with an eigenmode analysis for the transverse propagating Josephson plasma oscillations reveals that the resonances between Josephson vortex flow states and some of the eigenmodes are responsible for the clear flux lattice structural transitions. Furthermore, the theoretical analysis clarifies that the width of the superradiant state region in the I-V characteristics enlarges with decreasing both the superconducting and insulating layer thickness.