Magnetic-field-induced crossover of vortex-line coupling inSmFeAsO0.85single crystal

Abstract

We take the resistivity $\left(\rho \right)$ and the current-voltage $\left(I\text{-}V\right)$ characteristics of a ${\text{SmFeAsO}}_{0.85}$ single crystal as a function of temperature $\left(T\right)$ for various magnetic fields up to 18 T along the $c$ axis. The tail region of $\rho \left(T\right)$ well fits the three-dimensional (3D) vortex-glass critical behavior. The critical exponents for the vortex-glass scaling of ${\left(d\text{ln}\rho /dT\right)}^{-1}\text{-}T$, $I\text{-}V$ curves, and the thermal activation energies of vortices in magnetic fields show a crossover for magnetic fields around 3 T. The crossover behavior results from the change in the strength of the vortex pinning and the entanglement of vortex lines in the stacked superconducting structure of ${\text{SmFeAsO}}_{0.85}$ material with the $c$-axis vortex-line coupling lying in between highly anisotropic Bi-based cuprates with decoupled two-dimensional vortices and less-anisotropic ${\text{YBa}}_2{\text{Cu}}_3{\text{O}}_{7-\delta }$ with coupled 3D vortices. DOI: http://dx.doi.org/10.1103/PhysRevB.82.104523 © 2010 The American Physical Society