Universality And Unconventional Enhancement Of Flux-Flow Resistivity In Ba(Fe1-Xcox)(2)As-2

Measurements of the current-voltage characteristics (I-V) were performed on Ba(Fe1-x Co-x)(2)As-2 single crystals with doping level 0.044 <= x <= 0.100. An unconventional increase in the flux-flow resistivity rho(ff) with decreasing magnetic field H was observed across this doping range. Such an abnormal field dependence of rho(ff) is in contrast with the linear rho(ff) (H) of conventional type-II superconductors, but similar to the behavior recently observed in the heavy-fermion superconductor CeCoIn5. A significantly enhanced rho(ff) was found for the x = 0.06 single crystals, implying a strong single-particle energy dissipation around the vortex cores. At different temperatures and fields and for a given doping concentration, the normalized rho(ff) scales with normalized field and temperature. The doping level dependence of the scaling parameters strongly suggests that the abnormal upturn in rho(ff) is likely related to the enhancement of spin fluctuations around the vortex cores of the samples with x approximate to 0.06.