Mn Substitution Effect in Two-Gap Superconductor MgB2

Mn substitution effects on normal and superconducting state of MgB 2 are reported. Although the superconducting transition temperature ( T c ) radically decreases with Mn substitution, the E 2 g phonon peak in Raman spectra shows no significant change, confirming that the T c -suppression is not due to weakening of electron–phonon coupling by band-filling but due to the pair-breaking caused by magnetic scattering. In-plane resistivity indicates that Mn substitution introduces potential scattering, which is much stronger than the case of C and Al substitution. Nevertheless, the upper critical field H c2 is never enhanced by Mn-substitution. Based on this fact together with the behavior of H c2 anisotropy ratio, it is likely that strong potential scattering is introduced mainly in the π-bands. The effect of spin flip scattering other than the T c suppression is not obvious.