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MM: Fachverband Metall- und Materialphysik

MM 26: Electronic properties III

MM 26.3: Vortrag

Mittwoch, 28. März 2007, 17:15–17:30, H6

Ionicity of Fe_1−xCo_xSi single crystals from far-infrared spectroscopic ellipsometry — •Dirk Menzel¹, Pavlo Popovich², Alexander Boris², Matthias Neef³, Klaus Doll^2,3, 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 Fe_1−xCo_xSi 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.