Excitation of Josephson plasma and vortex oscillation modes in ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_{8+\mathrm{\delta }}$ in parallel magnetic fields

The Josephson plasma resonance of ${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_2$ ${\mathrm{CaCu}}_2$ ${\mathrm{O}}_{8+\mathrm{\delta }}$ in parallel magnetic fields has been measured by angular alignment with ∼0.02 degree precision to reveal the phase collective modes in the Josephson vortex lattice. When the angle θ between the field and the ${\mathrm{CuO}}_2$ plane is tilted from the c axis, the resonance field scales as H sin θ in the range θ≳4°. At lower angles, the resonance field shows anomalous decreases with decreasing θ. A disappearance of the resonance has been unexpectedly found for angles within θ∼0.2°, 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.