Grand Séminaire d'Institut

Vendredi 20 Décembre 2019 à 11h00.

Glass-like phonon thermal transports in complex disordered systems via clathrates of type-I –Overview-

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A l'occasion du workshop Phonon-lifetime les 19 et 20 décembre, Stéphane Pailhes de L'équipe (Nano)matériaux pour l'énergie invite le professeur T. Nakayama, Hokkaido University.
Abstract Phonon thermal transport is a crucial issue in the search of efficient thermoelectric (TE) materials. This is because efficient TE effects are achieved by materials possessing “glass-like” low thermal conductivity κph [1]. However, it needs a caution when using the term “glass-like”, which should be specified as: (i) κph∝T2 below 1K. (ii) The plateau region at 1 KT10K. (iii) The T–linear rise at 10 KT100 K. These features are independent of a kind of glasses which are disordered at the molecular scale embracing many-body problem, while homogeneous at the macroscopic scale. The underlying mechanism governing these observations is one of unsolved problems remained over 50 years in condensed matter physics [2]. Clathrates of type-I encapsulating “off-center” guest ions in cages exhibit “identical” characteristics of κph to those of structural glasses. These clathrates have well-defined atomic structures in contrast with those of actual glasses. This enables us to clear up the underlying mechanism of κph from purely theoretical point of view [3-5]. This talk is composed of the following parts: 1). The crucial difference of κph between conventional disordered and glass systems. 2). The definition of glass-like κph. 3). κph provides rich information on phonon life-times in addition to phonon states such as extended and weakly/strongly localized. 4). Efficacy of “abstract model” for complex disordered systems. 5). Interpretation of the underlying mechanism of glass-like κph of off-center type-I clathrates. 6). Finally, on “what have we learned about “physics” from this subject?
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[1]. T. Takabatake et al., Rev. Mod. Phys 86, 669(2014). [2]. T. Nakayama, Rep. Prog. Phys. 65, 1195(2002). [3]. Y. Liu et al., Phys. Rev. B 93, 214305(2016). [4]. Q. Xi et al., Phys. Rev. B 96, 064306 (2017). [5]. Q. Xi et al., Phys. Rev. B 97, 224308(2018).''