Energy gaps in doped MgB2

Despite early expectations the two-gap superconductivity is retained even upon the heavy carbon doping in MgB2 which leads to suppression of Tc down to 22 K. We show by the point-contact spectroscopy that this strong suppression of Tc is not due to an assumed large interband scattering which would eventually lead to the merging of the gaps. The systematic studies of the carbon doped samples with 0, 2.1, 3.8 and 10% C shows that the large gap on the ω-band is decreased in an essentially linear fashion with increasing the carbon concentrations. The changes in the small gap Δπ up to 3.8 % C are smaller but for the heavily doped sample with 10 % C and Tc = 22 K both gaps are significantly reduced, consistently with a strong suppression of Tc. Strong enhancement of the upper critical field is achieved by the carbon doping up to 3.8% C. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)