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Investigations of MgB2 and Fe-based superconductors in recent years haverevealed many unusual effects of multiband superconductivity but manifestationsof anisotropic multiband effects in the critical current density Jc have notbeen addressed experimentally, mostly because of the difficulties to measure Jcalong the c-axis. To investigate the effect of very different intrinsicanisotropies of sigma and pi electron bands in MgB2 on current transport, wegrew epitaxial films with tilted c-axis (THETA ~ 19.5{\deg}), which enabled usto measure the components of Jc both along the ab-plane and the c-axis usingmagneto-optical and transport techniques. These measurements were combined withscanning and transmission electron microscopy, which revealed terraced steps onthe surface of the c-axis tilted films. The measured field and temperaturedependencies of the anisotropic Jc(H) show that Jc,L parallel to the terracedsteps is higher than Jc,T perpendicular to the terraced steps, and Jc ofthinner films (50 nm) obtained from transport experiments at 0.1 T reaches ~10%of the depairing current density Jd in the ab plane, while magneto-opticalimaging revealed much higher Jc at lower fields. To analyze the experimentaldata we developed a model of anisotropic vortex pinning which accounts for theobserved behavior of Jc in the c-axis tilted films and suggests that theapparent anisotropy of Jc is affected by current pairbreaking effects in theweaker {\pi} band. Our results indicate that the out-of-plane current transportmediated by the {\pi} band could set the ultimate limit of Jc in MgB2polycrystals.

physical review. b, condensed matter and materials physics

Roles of intrinsic anisotropy and pi-band pairbreaking effects on  critical currents in tilted c-axis MgB2 films probed by magneto-optical and  transport measurements