Pseudodielectric function and critical-point energies of iron pyrite

Spectroscopic ellipsometry was used to determine the pseudodielectric function ${}{ε}{}={}{{ε}}₁{}+i{}{{ε}}₂{}$ spectrum of a natural single crystal of iron pyrite (cubic FeS${}₂$) from 0.5 to 4.5 eV with the sample at 77 K. The ${}{ε}{}$ 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 ε data. The electronic origins of the six observed CP features are identified through density functional theory calculations and momentum matrix analyses.