Berlin 2012 – scientific programme

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O: Fachverband Oberflächenphysik

O 39: Focussed session: Frontiers of electronic structure theory: Strong correlations from first principles III (jointly with TT)

O 39.9: Talk

Wednesday, March 28, 2012, 12:30–12:45, HE 101

Signatures of Electronic Correlations in the Narrow Gap Semiconductor FeSi — •Jan M. Tomczak, Kristjan Haule, and Gabriel Kotliar — Department of Physics and Astronomy, Rutgers University, Piscataway, New Jersey 08854, USA

Correlated semiconductors have been studied intensively over the years, because they exhibit an unusual metallization process which is poorly understood. At low temperatures FeSi behaves as an ordinary semiconductor, while at high temperatures the system is a bad metal with a Curie like susceptibility. Analogies with heavy fermion Kondo insulators and mixed valence compounds, and an anomalous electron-phonon coupling have been invoked to account for this behavior, but lacking quantitative methodologies applied to this problem, a consensus remained elusive to date. Here, we use realistic many-body calculation methods to elucidate the metallization mechanism of FeSi. Our methodology accounts for all substantial anomalies observed in FeSi : lack of conservation of spectral weight in optics, Curie susceptibility and, in particular, an anomalous thermoelectric power. Having quantitatively validated our methodology for this system, we propose a new scenario for FeSi : Unlike in conventional semiconductors the metallization in correlated insulators such as FeSi is induced by the emergence of non-quasiparticle incoherent states in the gap. This temperature induced coherence-incoherence crossover is accompanied by a massive reorganization of the spin excitation spectrum. Besides the fundamental interest, our work is relevant to the design of thermoelectric materials based on correlated insulators.