FIRST-PRINCIPLES CALCULATION OF OPTICAL CONDUCTIVITY OF FESE AND COSE

The absorption spectra I-alpha(omega) (alpha = x, y, z) due to inter-b and transitions are calculated for NiAs-type FeSe and CoSe by evaluati ng the matrix elements of the dipole-transition with use of the wave f unctions obtained by the LAPW band calculations. In both compounds the shape of I-alpha(omega) differs from that of the joint density-of-sta tes, and I-alpha(omega) are anisotropic, namely I-z(omega) is quite di fferent from I-x(omega) = I-y(omega). The spectra of I-x(omega) obtain ed for FeSe and CoSe show a fairly good correspondence to the absorpti on spectra observed in Fe-7-Se-8 and Co7Se8, respectively.