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

Investigations of MgB2 and Fe-based superconductors in recent years have revealed many unusual effects of multiband superconductivity, but manifestations of anisotropic multiband effects in the critical current density J(c) have not been addressed experimentally, mostly because of the difficulties in measuring J(c) along the c axis. To investigate the effect of very different intrinsic anisotropies of sigma and p electron bands in MgB2 on current transport, we grew epitaxial films with a tilted c axis (theta similar to 19.5 degrees), which enabled us to measure the components of J(c) along both the ab plane and the c axis using magneto-optical and transport techniques. These measurements were combined with scanning and transmission electron microscopy, which revealed terraced steps on the surface of the tilted-c-axis films. The measured field and temperature dependencies of the anisotropic J(c) (H) show that J(c),(L) parallel to the terraced steps is higher than J(c),(T) perpendicular to the terraced steps, and Jc of thinner films (50 nm) obtained from transport experiments at 0.1 T reaches similar to 10% of the depairing current density J(d) in the ab plane, while magneto-optical imaging revealed much higher J(c) at lower fields. To analyze the experimental data we developed a model of anisotropic vortex pinning which accounts for the observed behavior of J(c) in the tilted-c-axis films and suggests that the apparent anisotropy of J(c) is affected by current pairbreaking effects in the weaker pi band. Our results indicate that the out-of-plane current transport mediated by the pi band could set the ultimate limit of J(c) in MgB2 polycrystals.