Impurity effects on superconductivity and anisotropy in MgB2

We have studied the effects of C- and Mn-substitution on physical properties of MgB2, using single crystals. Optical reflectivity measurements have successfully separated the changes in plasma frequency and scattering rate in σ- and π-band. The former indicates that electrons are doped by C-substitution but the two dimensional σ-band Fermi surfaces survive at least up to 10% doping. Increase in carrier scattering rate with C-substitution is much stronger in the σ-bands than in the π-bands, which leads to the decrease in the in-plane coherence length as well as the anisotropy of Hc2. Raman scattering experiment showed that Mn impurity acts as a strong magnetic pair-breaker without changing the electron-phonon coupling.