Magnetic-field-induced crossover of vortex-line coupling in SmFeAsO0.85 single crystal

Abstract

We take the resistivity (rho) and the current-voltage (I-V) characteristics of a SmFeAsO0.85 single crystal as a function of temperature (T) for various magnetic fields up to 18 T along the c axis. The tail region of rho(T) well fits the three-dimensional (3D) vortex-glass critical behavior. The critical exponents for the vortex-glass scaling of (d ln rho/dT)(-1)-T, I-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 SmFeAsO0.85 material with the c-axis vortex-line coupling lying in between highly anisotropic Bi-based cuprates with decoupled two-dimensional vortices and less-anisotropic YBa2Cu3O7-delta with coupled 3D vortices.