Pseudodielectric function and critical-point energies of iron pyrite

Spectroscopic ellipsometry was used to determine the pseudodielectric function = + i spectrum of a natural single crystal of iron pyrite (cubic FeS2) from 0.5 to 4.5 eV with the sample at 77 K. The spectrum exhibits several pronounced optical features associated with the interband critical points (CPs). Accurate CP energies are obtained by fitting standard line shapes to second energy derivatives of the data. The electronic origins of the six observed CP features are identified through density functional theory calculations and momentum matrix analyses.