Observation of strong intrinsic pinning in MgB2 films

By using three types of MgB2 superconductors, such as c-axis-oriented single-crystal films, c-axis-oriented columnar-structure films and films without c-axis orientation perpendicular to the substrate surface, we have investigated the intrinsic pinning effect in MgB2 superconductors. The strong field performance of J(c) was observed by turning the orientation of grains from the c axis to the a axis. No c-axis-oriented MgB2 films showed a noticeable increase of J(c) at high fields compared with c-axis-oriented films, whether they had columnar structures or not. Our results clearly show that MgB2 has strong intrinsic pinning caused by the large anisotropy of the superconducting energy gap in the boron layers like high-T-c cuprate superconductors with a layered structure.