Pseudodielectric Function and Critical-Point Energies of Iron Pyrite

Spectroscopic ellipsometry was used to determine the pseudodielectric function e=e1 + ie2 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 e 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 e data. The electronic origins of the six observed CP features are identified through density functional theory calculations and momentum matrix analyses.