Phase coherence and Josephson plasma in Bi₂Sr₂CaCu₂O_{8 + δ}

The recently discovered Josephson plasma phenomena in layered high-T_c superconductors have opened possibilities for testing fundamental ideas of physics of high-T_c superconductivity as well as for the device applications at high frequencies above 100 GHz. The observation of Josephson plasma resonance manifests the existence of a series stack of equivalent weak Josephson junctions between CuO₂ layers in Bi₂Sr₂CaCu₂O_{8 + δ}, through which a Cooper pair can tunnel with phase coherence. This unique situation brings the superconducting plasma frequency into a microwave frequency region, simply because the number of Cooper pairs participating in plasma phenomena is greatly reduced by the restricted tunneling probability of Cooper pairs. It is known that two collective excitation modes, longitudinal and transverse modes, can exist in this case. Using a different dispersion relation between two modes we were able to separate two modes experimentally for the first time. Since the plasma resonance is the consequence of phase coherence between superconducting layers, the dynamical nature of the vortex state can in principle be studied by this technique. The results of the vortex correlation observed by the Josephson plasma resonance with and without correlated disorder is discussed.