Importance of thermal disorder and electronic occupation for temperature dependence of optical conductivity in FeSi and MnSi

The spectral weight (SW) for optical transitions in FeSi and MnSi are calculated as a function of temperature by means of linear muffin-tin orbital–local density approximation band calculations. The main effects, caused by structural disorder and electronic Fermi-Dirac distribution, act oppositely on the T dependence of the SW, while the variation of the magnetic moment in MnSi has only a minor effect. The calculations agree with the experimental findings of an increasing SW in FeSi and a decreasing SW in MnSi as function of T. The results can be understood from the change of the band structure with disorder.