Heavy carriers, non-drude optical conductivity and transfer of spectral weight in MnSi

The optical properties of the weak magnetic metal MnSi were determined using reflectance at 80 degrees (2-800 meV) and ellipsometry (0.8-4.5 eV). At low frequencies in the magnetic phase we observe a departure of the optical conductivity from Drude behavior: m*(omega)/m is strongly frequency dependent and 1/tau(omega) is approximately linear in frequency. In fact, we show that sigma(omega)/sigma(0) = (1 + i omega/Gamma)(-0-5). Moreover, in the magnetic phase, the plasma frequency shifts to the red indicating that spectral weight is transferred to high frequencies. This is opposite to the effect recently seen in other magnetic compounds. (C) 2003 Elsevier B.V. All rights reserved.