Indirect Optical Absorption of Single Crystalline beta-FeSi2
  We investigated optical absorption spectra near the fundamental absorption edge of beta-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 +- 0.004 eV. A value of 0.814 eV was obtained for the exciton transition energy at 4K. 
Haruhiko Udono, Isao Kikuma, Tsuyoshi Okuno, Yasuaki Masumoto,   Hiroyuki Tajima