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Liquid vortex phase and strong c-axis pinning in low anisotropy BaCoxFe2 − xAs2 pnictide films

We present linear and nonlinear electronic transport measurements as a function of magnetic field strength and orientation at different temperatures for BaCoxFe2 − xAs2 biaxially oriented films grown on (La, Sr)(Al, Ta)O3 single crystals and determine the critical current density, melting line and upper critical field (Jc, Hm and Hc2). Unlike observations on single crystals of the same composition with similar Tc ∼ 20 K and anisotropy γ ∼ 2, we observe here a vortex liquid phase as a broadening of the resistive transition with increasing H. We find evidence of a high density of correlated defects that generate a very large Jc peak as a function of field orientation centered near the c-axis orientation with Jc > 1 MA cm − 2 at 4 K and 1 T and reduce the dissipation in the vortex liquid state along the defect direction. These studies indicate that these correlated defects could be more effective than the correlated defects found in more anisotropic cuprate superconductors.