Optical Properties of Magnetic Semiconductors TlFeS2 and TlFeSe2

The spectral dependences of the optical conductivity and reflection coefficients of TlFeS₂ and TlFeSe₂ crystals were studied experimentally using spectral ellipsometry and theoretically from first principles using density functional theory (DFT). Based on ellipsometric data in the energy range of 0.7–6.5 eV, the widths of the indirect band gap E_g, reflection coefficients R, Urbach energy E_U, skin depth, and other parameters were determined. Analysis of the refractive index spectrum using the single-effective-oscillator model yielded estimates of the single oscillator energy E_so and the dispersion energy E_d. The experimental data were also used to determine the plasma frequency and the ratio of the charge carrier concentration to the effective mass. Furthermore, the nonlinear refractive indices and the first- and third-order nonlinear susceptibilities were calculated.