Relationship between critical current and flux-flow resistivity in the mixed state of Ba(Fe1−xCox)2As2

Abstract

We studied the temperature and magnetic field dependence of vortex dissipation and critical current in the mixed state of unconventional superconducting alloys $\mathrm{Ba}{({\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x})}_{2}{\mathrm{As}}_{2}$($0.044\ensuremath{\le}x\ensuremath{\le}0.100$) through current-voltage measurements. Our results reveal that all the electric field $E$ vs current density $j$ curves in the Ohmic regime merge to one point (${j}_{0},{E}_{0}$) and that there is a simple relationship between the critical current density ${j}_{c}$ and flux-flow resistivity ${\ensuremath{\rho}}_{\mathrm{ff}}$: ${\ensuremath{\rho}}_{\mathrm{ff}}/{\ensuremath{\rho}}_{\mathrm{n}}={(1\ensuremath{-}{j}_{c}/{j}_{0})}^{\ensuremath{-}1}$, where ${\ensuremath{\rho}}_{\mathrm{n}}={E}_{0}/{j}_{0}$ is the normal-state resistivity just above the superconducting transition. In addition, ${E}_{0}$ is positive for all five dopings, reflecting the abnormal behavior of the flux-flow resistivity ${\ensuremath{\rho}}_{\mathrm{ff}}$: it increases with decreasing magnetic field. In contrast, ${E}_{0}$ is negative for the conventional superconductor Nb since, as expected, ${\ensuremath{\rho}}_{\mathrm{ff}}$ decreases with decreasing magnetic field. Furthermore, in the underdoped and overdoped single crystals of $\mathrm{Ba}{({\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x})}_{2}{\mathrm{As}}_{2}$, the parameter ${E}_{0}$ remains temperature independent, while it decreases with increasing temperature for the single crystals around optimal doping ($0.060\ensuremath{\le}x\ensuremath{\le}0.072$). This result points to the coexistence of superconductivity with some other phase around optimal doping.