First-principles calculation of optical conductivity of FeSe and CoSe

The absorption spectra I_α(ω) (α = x, y, z) due to inter-band transitions are calculated for NiAs-type FeSe and CoSe by evaluating the matrix elements of the dipole-transition with use of the wave functions obtained by the LAPW band calculations. In both compounds the shape of I_α(ω) differs from that of the joint density-of-states, nd I_α(ω) are anisotropic, namely I_z(ω) is quite different from I_y(ω) = I_y(ω). The spectra of I_x(ω) obtained for FeSe and CoSe show a fairly good correspondence to the absorption spectra observed in Fe₇Se₈ and Co₇Se₈, respectively.