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DF: Fachverband Dielektrische Festkörper

DF 2: Optical and nonlinear optical properties, photonic

DF 2.4: Vortrag

Montag, 31. März 2014, 11:30–11:50, GER 37

Boson peak in overdoped manganites La_1−xCa_xMnO₃ — •F. Fischgrabe¹, E. Zhukova^2,3,5, B Gorshunov^2,3,5, V. I. Torgashev⁴, L. S. Kadyrov^2,3, E. A. Motovilova^2,3,5, T. Zhang⁶, R. Kremer⁷, U.S. Pracht⁵, S. Zapf⁵, V. V. Moshnyaga¹, and M. Dressel⁵ — ¹I. Physikalisches Institut, Universität Göttingen, Germany — ²A.M. Prokhorov General Physics Institute, Russian Academy of Sciences, Russia — ³Moscow Institute of Physics and Technology (State University), Russia — ⁴Faculty of Physics, Southern Federal University, Russia — ⁵1. Physikalisches Institut, Universitat Stuttgart, Germany — ⁶Key Laboratory of Materials Physics, Institute of Solid State Physics, Chinese Academy of Sciences, PRC — ⁷Max-Planck-Institut für Festkörperforschung, Germany

In the charge-ordered phase of strongly doped manganites
La_1−xCa_xMnO₃ (x ≥ 0.5) absorption lines appear in the terahertz spectral range for commensurate x values right below the charge-ordering temperature. They are connected to acoustic phonons that become optically active by folding of the Brillouin zone. At lower temperatures a strongly asymmetric extra absorption band develops at frequencies corresponding to the position of the lowest-energy van Hove singularity in the reduced Brillouin zone. The band is assigned to the boson peak, i.e., to the excess of lattice vibrational states over the standard Debye contribution. The folded phonons and the boson peak do not show up for incommensurate calcium contents when no distinct Brillouin zone folding exists. Polycrystals are compared with freestanding films to determine if there are differences in the IR response.