L. Bossoni, P. Carretta, M. Horvatić, M. Corti, A. Thaler, P. C. Canfield
Nuclear spin-lattice relaxation rate ($1/T_1$) measurements in optimally doped Ba(Fe$_{1-x}$Rh$_x$)$_2$As$_2$ superconductor evidence the onset of low-frequency dynamics associated with the vortex motions. The slowing down of the dynamics yields a peak in $1/T_1$, which has been observed to progressively shift to lower temperatures as the magnetic field is increased between 3 T and 15 T. We interpreted these results in the light of a phase transition between a liquid vortex state and a glassy vortex one; the Arrhenius law, employed to describe the vortex dynamics in the cuprates, fails in the case of the iron-pnitictides, whereas a Vogel-Fulcher law gives more meaningful results, together with the dynamical scaling approach. The glassy phase transition occurs below $T_c$, very close to the Kosterlitz-Thouless transition temperature of the vortex lattice. We further present high field measurements of spin-spin relaxation rate, and their dependence on the magnetic field orientation, also ascribed to the vortex dynamics.
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http://arxiv.org/abs/1208.5142
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