Absence of a True Vortex-Glass Phase above the Bragg Glass Transition Line in Bi-2212

摘要

In magnetic measurements on Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ (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 $\mu$ becomes negative, together with the positivecurvature of $logE$ vs. $ logj$ and the scaling based on the 2D vortex glasstheory or plastic creep theory, we conclude that the vortex motion above thesecond peak is plastic when $j\to 0$ and there is no vortex glass phase atfinite temperatures in Bi-2212. The peak of S is then explained as thecrossover between different meta-stable vortex states.