Absence of a true vortex-glass phase above the Bragg glass line in Bi 2Sr 2CaCu 2O 8+ δ

Abstract

In magnetic measurements on Bi 2Sr 2CaCu 2O 8+ 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 log E vs. log j 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.