Universality and unconventional enhancement of flux-flow resistivity in Ba(Fe${}_{1-x}$Co${}_x$)${}_2$As${}_2$

Measurements of the current-voltage characteristics were performed on Ba(Fe${}_{1-x}$Co${}_x$)${}_2$As${}_2$ single crystals with doping level $0.044 \leq x \leq 0.1$\mathrm{.}Anunconventionalincreaseintheflux-flowresistivity$\rho_{\rm ff}$ with decreasing magnetic field was observed across this doping range. Such an abnormal field dependence of flux-flow resistivity is in contrast with the linear field dependence of ${\rho }_{\mathrm{f}\mathrm{f}}$ in conventional type-II superconductors, but is similar to the behavior recently observed in the heavy-fermion superconductor CeCoIn${}_5$. A significantly enhanced $\rho_{\rm ff}$ 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 ${\rho }_{\mathrm{f}\mathrm{f}}$ scales with normalized field and temperature. The doping level dependence of these parameters strongly suggests that the abnormal upturn flux-flow resisitivity is likely related to the enhancement of spin fluctuations around the vortex cores of the optimally doped samples.