Abrupt Change of Josephson Plasma Frequency at the Phase Boundary of the Bragg Glass in Bi_2Sr_2CaCu_2O_{8+δ}

We report the first detailed and quantitative study of the Josephson coupling energy in the vortex liquid, Bragg glass and vortex glass phases of Bi_2Sr_2CaCu_2O_{8+\delta} by the Josephson plasma resonance. The measurements revealed distinct features in the T- and H-dependencies of the plasma frequency $\omega_{pl}$ for each of these three vortex phases. When going across either the Bragg-to-vortex glass or the Bragg-to-liquid transition line, $\omega_{pl}$ shows a dramatic change. We provide a quantitative discussion on the properties of these phase transitions, including the first order nature of the Bragg-to-vortex glass transition.