Josephson Plasma Mode in Fields Parallel to Layers of   Bi_2Sr_2CaCu_2O_{8+\delta}
  Josephson plasma resonance measurements under magnetic fields parallel to the CuO_2 layers as functions of magnetic field, temperature, and microwave frequency have been performed in Bi_2Sr_2CaCu_2O_{8+\delta} single crystals with doping range being from optimal to under-doped side. The feature of the resonance is quite unique and cannot be explained by the conventional understandings of the Josephson plasma for H \parallel c, that requires a new theory including coupling effect between Josephson vortex lattice and Josephson plasma. 
I. Kakeya, T. Wada, R. Nakamura, and K. Kaodowaki