Relationship Between Critical Current And Flux-Flow Resistivity In The Mixed State Of Ba(Fe1-Xcox)(2)As-2

We studied the temperature andmagnetic field dependence of vortex dissipation and critical current in themixed state of unconventional superconducting alloys Ba(Fe1-xCox)(2)As-2(0.044 <= x <= 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 rho ff : rho ff/rho n = (1 - j(c)/j(0))(-1), where rho(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 rho ff : it increases with decreasing magnetic field. In contrast, E-0 is negative for the conventional superconductor Nb since, as expected, rho ff decreases with decreasing magnetic field. Furthermore, in the underdoped and overdoped single crystals of Ba(Fe1-xCox)(2)As-2, the parameter E-0 remains temperature independent, while it decreases with increasing temperature for the single crystals around optimal doping (0.060 <= x <= 0.072). This result points to the coexistence of superconductivity with some other phase around optimal doping.