INFRARED SPECTROSCOPIC STUDY OF PHONONS COUPLED TO CHARGE EXCITATIONSIN FESI

From an investigation of the optical conductivity of FeSi single cryst als using Fourier-transform infrared spectroscopy in the frequency ran ge from 30 to 20 000 cm-l we conclude that the transverse effective ch arge of the Fe and Si ions is approximately 4e. Of the five optical ph onons that are allowed by symmetry we observe only four, three of whic h have a Fano line shape presumably resulting from an interaction of t hese modes with the electronic continuum. We show that the large oscil lator strength of the phonons results from a relatively weak coupling (lambda approximate to 0.1) of the lattice degrees of freedom to an el ectronic resonance above the semiconductor gap, which is also responsi ble for the large electronic polarizability (epsilon(infinity)approxim ate to 200) of the medium.