Origin of the glass-like dynamics in molecular metals $\pmb{\kappa}$-(BEDT-TTF)$_{\textbf{2}}$X: implications from fluctuation spectroscopy and $\pmb{\textit{ab initio}}$ calculations

J. Müller; B. Hartmann; R. Rommel; J. Brandenburg; S. M. Winter; J. A. Schlueter

doi:10.1088/1367-2630/17/8/083057

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arXiv · 1506.05191

Origin of the glass-like dynamics in molecular metals $\pmb{\kappa}$-(BEDT-TTF)$_{\textbf{2}}$X: implications from fluctuation spectroscopy and $\pmb{\textit{ab initio}}$ calculations

J. Müller·B. Hartmann·R. Rommel·J. Brandenburg·S. M. Winter·J. A. Schlueter

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Abstract

We have studied the low-frequency dynamics of the charge carriers in different organic charge-transfer salts $\kappa$-(BEDT-TTF)$_2$X with polymeric anions X by using resistance noise spectroscopy. Our aim is to investigate the structural, glass-like transition caused by the conformational degrees of freedom of the BEDT-TTF molecules' terminal ethylene groups. Although of fundamental importance for studies of the electronic ground-state properties, the phenomenology of the glassy dynamics is only scarcely investigated and its origin is not understood. Our systematic studies of fluctuation spectroscopy of various different compounds reveal a universal, pronounced maximum in the resistance noise power spectral density related to the glass transition. The energy scale of this precess can be identified with the activation energy of the glass-like ethylene endgroup structural dynamics as determined from thermodynamic and NMR measurements. For the first time for this class of 'plastic crystals', we report a typical glassy property of the relaxation time, namely a Vogel-Fulcher-Tammann law, and are able to determine the degree of fragility of the glassy system. Supporting $\textit{ab initio}$ calculations provide an explanation for the origin and phenomenology of the glassy dynamics in different systems in terms of a simple two-level model, where the relevant energy scales are determined by the coupling of the ethylene endgroups to the anions.

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J. Müller, B. Hartmann, R. Rommel, J. Brandenburg, S. M. Winter, J. A. Schlueter. 2015-06-17. Origin of the glass-like dynamics in molecular metals $\pmb{\kappa}$-(BEDT-TTF)$_{\textbf{2}}$X: implications from fluctuation spectroscopy and $\pmb{\textit{ab initio}}$ calculations. https://doi.org/10.1088/1367-2630/17/8/083057

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