High Critical Current Density 4 MA/cm2in Co-Doped BaFe2As2Epitaxial Films Grown on (La,Sr)(Al,Ta)O3Substrates without Buffer Layers

High critical current densities J c of 4 MA/cm 2 at 4 K were obtained in Co-doped BaFe 2 As 2 (BaFe 2 As 2 :Co) epitaxial films grown directly on (La,Sr)(Al,Ta)O 3 substrates by pulsed laser deposition. Use of a highly pure target and improvement of film homogeneity were the critical factors to achieve the high J c . The improved BaFe 2 As 2 :Co epitaxial films contained almost no Fe impurity and showed high crystallinity (crystallite tilt angle Δω= 0.5° and twist angle Δφ= 0.5°) and a sharp superconducting transition with a width of 1.1 K. It is considered that these improvements resulted in the enhanced superconducting properties comparable to those of single crystals.