Excitation of Josephson plasma and vortex oscillation modes in Bi2Sr2CaCu2O8+delta in parallel magnetic fields

The Josephson plasma resonance of Bi2Sr2CaCu2O8+delta in parallel magnetic fields has been measured by angular alignment with similar to 0.02 degree precision to reveal the phase collective modes in the Josephson vortex lattice. When the angle theta between the field and the CuO2 plane is tilted from the c axis, the resonance field scales as H sin theta in the range theta greater than or similar to 4 degrees. At lower angles, the resonance held shows anomalous decreases with decreasing theta. A disappearance of the resonance has been unexpectedly found for angles within theta similar to 0.2 degrees, which may correspond to the angle where the lock-in transition takes place. The origin of the phenomenon is discussed in terms of the Josephson plasma oscillation and Josephson vortex oscillation modes.