Microwave response in the vortex state of Bi₂Sr₂CaCu₂O_8+δ

We measured the surface resistance (30-60 GHz) for different crystallographic orientations in the vortex state of Bi₂Sr₂CaCu₂O_8+δ. A sharp magnetoabsorption resonance is observed below Tc when AC electric fields and magnetic fields are applied parallel to the c-axis (E_AC // B // c). We argue that the observed resonance arises from collective Josephson plasma oscillations that appear when the plasma energy is much smaller than the superconducting gap energy. The plasma mode is shown to be strongly affected by the vortex states in the CuO₂-plane.