Temperature dependence of the dielectric function and the interband critical points in orthorhombic GeS

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Abstract
The dielectric function of orthorhombic GeS has been measured ellipsometrically in the 1.66–5.6-eV photon-energy region as a function of temperature between 84 and 500 K. The second derivatives with respect to frequency of the real and imaginary parts of the dielectric function, obtained numerically from the measured spectra with the electric field vector scrE parallel to the a and b axes, show distinct interband critical points that are rather different for the two polarizations. The line shapes of these structures have been fitted with standard analytic expressions for various types of critical points. The temperature dependence of the critical-point parameters obtained for the two principal polarizations is compared and analyzed in terms of average frequencies of phonons participating in the corresponding electron-phonon interaction.