Longitudinal Josephson plasma excitation in vortex state of Bi₂Sr₂CaCu₂O_s+δ

The Josephson plasma absorption at a microwave frequency of 45 GHz in a cavity (E_osc//c-axis) has been studied in high quality single crystalline Bi₂Sr₂CaCu₂O_s+δ as functions of magnetic field, temperature, doping level, and angle between magnetic field and crystallographic axes. The observed resonance line shape, which is rather symmetric, was analyzed in detail by solving the coupled equations of the Maxwell equation and the Josephson current as well as the charge equations. As a result the observed absorption is unambiguously assigned as the longitudinal plasma mode. The temperature dependence of the absorption agrees with the prediction based on the d-wave symmetry rather than the isotropic s-wave one. The resonance line can be scaled by the perpendicular component of the field, indicating that the resonance position is determined by the number of vortices parallel to the c-axis.