Grain boundaries as weak links: The case of MgB2 with reference to YNi2B2C

Abstract

The grain boundaries ~GB’s! in the intermetallic superconductor MgB2 , interestingly, do not show suppression of supercurrent density. This unexpected behavior has been investigated by a scanning tunneling microscopy/spectroscopy technique at atomic resolution. The GB in MgB2 is seen as an amorphous region extending from ;50 to 200 Å and has a metallic character. This observation supports proximity coupling between the grains, which explains why supercurrent density does not degrade in this material. The results for another intermetallic superconductor YNi2B2C having GB’s ~average width ;30 Å! that are quasi-insulating in nature have also been presented and compared with the former.