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 Ba(Fe1−xCox)2As2 (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 (j0, E0) and that there is a simple relationship between the critical current density jc and flux-flow resistivity ρ ff : ρ ff /ρn = (1 − jc/j0) −1 , where ρn = E0/j0 is the normal-state resistivity just above the superconducting transition. In addition, E0 is positive for all five dopings, reflecting the abnormal behavior of the flux-flow resistivity ρ ff : it increases with decreasing magnetic field. In contrast, E0 is negative for the conventional superconductor Nb since, as expected, ρ ff decreases with decreasing magnetic field. Furthermore, in the under-doped and over-doped single crystals of Ba(Fe1−xCox)2As2, the parameter E0 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 co-existence of superconductivity with some other phase around optimal doping.

Pressure tuned superconductivity and normal state behavior in Ba(Fe$_{0.943}$Co$_{0.057}$)$_2$As$_2$ near the antiferromagnetic boundary

“…11 For x = 0.06, it is suggested that superconductivity coexists with a spin density wave (SDW). 12 For thin films of Ba(Fe 1−x Co x ) 2 As 2 , the exponent n in the temperature dependence of the resistivity is minimum namely, close to unity at x ≈ 0.05 and x ≈ 0.07 for MgO and CaF 2 substrate, respectively, which may be associated with an antiferromagnetic QCP. 13 Furthermore, a sign change in the electronic-magnetic Gruneisen parameter is observed for x = 0.055 and x = 0.065, consistent with the expected behavior at a QCP.…”