Liquid vortex phase and strongc-axis pinning in low anisotropy BaCoxFe2 −xAs2pnictide films

We present linear and nonlinear electronic transport measurements as a function of magnetic field strength and orientation at different temperatures for BaCo x Fe 2 − x As 2 biaxially oriented films grown on (La, Sr)(Al, Ta)O 3 single crystals and determine the critical current density, melting line and upper critical field ( J c , H m and H c2 ). Unlike observations on single crystals of the same composition with similar T c ∼ 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 J c peak as a function of field orientation centered near the c -axis orientation with J c > 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.