Electronic bands and optical conductivity of the Dzyaloshinsky-Moriya multiferroic Ba2CuGe2 O7

We have measured the reflectivity in the ab plane and along the c axis of a single crystal of the Dzyaloshinsky-Moriya multiferroic Ba2CuGe2O7, from 20 to 300 K and from 700 to 24 000 cm-1. The resulting optical conductivity σ(ω) has been compared with the results of calculations of its electronic band structure based on density functional theory (DFT). A broad midinfrared band, which in the ab plane strongly hardens for decreasing temperature, is consistent with d-d transitions of the Cu atom. A further electronic absorption, whose edge falls in the near infrared and has a strong O-p character, unusually softens with decreasing temperature by more than 0.15 eV, in both polarizations. Calculations show that the behavior with temperature of those electronic bands can be related to distortions of the CuO4 tetrahedra.