Regensburg 2007 – scientific programme
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MM: Fachverband Metall- und Materialphysik
MM 26: Electronic properties III
MM 26.3: Talk
Wednesday, March 28, 2007, 17:15–17:30, H6
Ionicity of Fe1−xCoxSi single crystals from far-infrared spectroscopic ellipsometry — •Dirk Menzel1, Pavlo Popovich2, Alexander Boris2, Matthias Neef3, Klaus Doll2,3, and Joachim Schoenes1 — 1Institut für Physik der Kondensierten Materie, TU Braunschweig, Mendelssohnstr. 3, 38106 Braunschweig, Germany — 2Max-Planck-Institut für Festkörperforschung, Heisenbergstr. 1, 70569 Stuttgart, Germany — 3Institut für Mathematische Physik, TU Braunschweig, Mendelssohnstr. 3, 38106 Braunschweig, Germany
The nonmagnetic narrow-gap semiconductor FeSi transforms into a ferromagnetic metal by doping between 5 at.% and 80 at.% Co. High-quality tri-arc Czochralski grown Fe1−xCoxSi single crystals were investigated by spectroscopic ellipsometry in the far-infrared spectral range between 100 and 700 cm−1. Four infrared active T-modes are observed at 206, 330, 351, and 459 cm−1 for FeSi. The best fit of the dielectric function is achieved with a sum of Fano oscillators reflecting a coupling of the optical phonons with electronic excitations. Interestingly, at Co concentrations x > 0.05 the phonons remain visible although they should be screened by free carriers as expected in a metal. This is even more surprising since the Szigeti effective charge is lower than 0.1e for all the investigated Co concentrations whereas the Born effective charge is 3.0e. For pure FeSi the indirect optical band gap is 33 meV which decreases with increasing x and closes at 5 at.% Co.