Absence of a true vortex-glass phase above the Bragg glass line in Bi2Sr2CaCu2O8+δ

In magnetic measurements on Bi2Sr2CaCu2O8+δ (Bi-2212) single crystals, a general peak with a dynamical feature on both S-H and S-T curves was found with S the magnetic relaxation rate. At higher fields, the characteristic exponent μ, becomes negative, together with the positive curvature of logE vs. logj and the scaling based on the 2D vortex glass (VG) theory or plastic creep theory, we conclude that the vortex motion above the second peak is plastic when j→0 and there is no VG phase at finite temperatures in Bi-2212. The peak of S is then explained as the crossover between different meta-stable vortex states