Vortex phase transitions in Bi₂Sr₂CaCu ₂O_8+x probed by nonlinear AC magnetic response

Local nonlinear AC magnetic measurements were employed in the study of the vortex phase diagram in Bi₂Sr₂CaCu₂O _8+x. The melting and the Bragg-Glass to Vortex-Glass (solid-solid) transitions are manifested by a sharp peak and a sharp drop in the third harmonic response, respectively. The peak at the solid-liquid transition signifies the hysteretic nature of the first-order melting transition. The disappearance of the nonlinear response in the vicinity of the solid-solid phase transition line suggests the existence of an intermediate soft lattice vortex state in-between the ordered and disordered vortex phases.