Indirect optical absorption of single crystalline β-FeSi2

We investigated optical absorption spectra near the fundamental absorption edge of β-FeSi2 single crystals by transmission measurements. The phonon structure corresponding to the emission and absorption component was clearly observed in the low-temperature absorption spectra. Assuming exciton state in the indirect allowed transition, we determined a phonon energy of 0.031±0.004eV. A value of 0.814eV was obtained for the exciton transition energy at 4K.