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DS: Fachverband Dünne Schichten

DS 28: Layer Properties: Electrical, Optical, and Mechanical Properties

DS 28.10: Vortrag

Donnerstag, 14. März 2013, 12:00–12:15, H8

Coherent phonon pump-probe spectroscopy on off-stoichiometric Co-doped NiMnGa films — •Martin Schubert¹, Jan Mayer¹, Mike Hettich¹, Hanjo Schäfer¹, Aleksej Laptev¹, Moritz Merklein¹, Chuan He¹, Martin Großmann¹, Oliver Ristow¹, Yuansu Luo², Vitalyi Gusev³, Jure Demsar¹, Mikhail Fonin¹, Konrad Samwer¹, and Thomas Dekorsy¹ — ¹Department of Physics, University of Konstanz, Germany — ²Physikalisches Institut, Georg-August-Universität Göttingen, Germany — ³Université du Maine, Le Mans, France

The shape memory alloy NiMnGa is characterized by a reversible and diffusionless structural transition from a high-temperature austenitic phase to a low-temperature martensitic phase, which can be induced by stress or temperature. We investigated the phase transition of off-stoichiometric NiMnGa thin films with and without Co-doping by asynchronous optical sampling (ASOPS), an ultra-fast optical pump-probe measurement technique. A wide range of different compositions is investigated by both cooling and/or heating the sample depending on the transition temperature. For all samples that were investigated, a distinct softening of the dominant acoustic phonon mode can be observed when the temperature of the sample approaches the phase transition temperature. Structural changes due to heating are not sufficient to explain the shift in frequencies. The softening is therefore linked to a strong electron-phonon interaction similar to that in a charge density wave (CDW) system. CDW haven't been observed in off-stoichiometric films but they have been measured in stoichiometric NiMnGa films.