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

TL;DR

This study reveals an unconventional, doping-dependent increase in flux-flow resistivity in Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ superconductors, linked to spin fluctuations and differing from typical superconductor behavior.

Contribution

It uncovers a universal and anomalous flux-flow resistivity behavior in iron-based superconductors, highlighting the role of spin fluctuations near vortex cores.

Findings

Unconventional increase in flux-flow resistivity with decreasing magnetic field.

Enhanced resistivity at optimal doping (x=0.06), indicating strong vortex core dissipation.

Normalized resistivity scales with temperature and field across doping levels.

Abstract

Measurements of the current-voltage characteristics were performed on Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ single crystals with doping level $0.044 \leq x \leq 0.1$. An unconventional increase in the flux-flow resistivity $\rho_{\rm ff}$ 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 $\rho_{\rm ff}$ 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 $\rho_{\rm ff}$ 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.