Superconducting gaps of nanocrystalline MgB2

The superconducting gap structure of nanocrystalline MgB2 is investigated by heat capacity measurements. It is shown that the larger sigma-gap decreases as a consequence of defects and impurities introduced by the specifics of the high-energy ball milling synthesis process. This explains the reduction of the critical temperature. The smaller pi-gap slightly increases as compared to bulk MgB2 indicating an enhanced interband scattering in the nanocrystalline samples. (c) 2007 Elsevier B.V. All rights reserved