Optical properties of strontium monochalcogenides from first principles

Optical properties of strontium monochalcogenide compounds SrX ($\mathrm{X}=\mathrm{Se}$, Se, and Te) in NaCl crystal structure are calculated using the band structure results obtained through the full potential linearized augmented plane wave method. The exchange-correlation potential is treated by the generalized gradient approximation (GGA) within the scheme of J. P. Perdew, K. Burke, and M. Ernzerhof [Phys. Rev. Lett. 77, 3865 (1996)]. Also we have used Engel and Vosko GGA formalism [E. Engel and S. H. Vosko, Phys. Rev. B. 47, 13164 (1993)] to improve the band gap results. The real and imaginary parts of the dielectric function $\epsilon \left(\omega \right)$, the optical absorption coefficient $I\left(\omega \right)$, the reflectivity $R\left(\omega \right)$ and the energy loss function are calculated. The calculated results show good agreement with the available experimental results, particularly in the low energy region of the spectra. Furthermore, the interband transitions responsible for the structures in the spectra are specified. It is shown that the chalcogen $p$ states and Sr $4d$ states play the major role in optical transitions as initial and final states, respectively. The effect of the spin-orbit coupling on the optical properties is also investigated and found to be quite small, especially in the low energy region.