Novel Microwave Absorption Due to Strong Coupling between Josephson   Plasma and the Josephson Vortex Array in Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$
  We have investigated the Josephson plasma excitations in magnetic fields parallel to the $ab$-plane in Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ single crystals in a wide microwave frequency region (9.8 -- 75 GHz). It was found that there are two kind of phase-collective modes: one increases with magnetic fields in higher fields, approaching linear asymptotic dependence with $ck$, which lies well above the inherent plasma frequency $\omega_p$, while the other does not show considerable field dependence. The higher linear mode is attributed to the Josephson plasma mode propagating along the reciprocal lattice vector of the Josephson vortex, whereas the lower one can be ascribed to the oscillation mode of Josephson vortices. 
I. Kakeya, T. Wada, K. Kadowaki