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

TL;DR

This study uncovers the glass-like dynamics in organic charge-transfer salts using fluctuation spectroscopy and ab initio calculations, revealing universal features and a Vogel-Fulcher-Tammann relaxation behavior related to ethylene group conformational dynamics.

Contribution

It provides the first detailed analysis of glassy dynamics in these materials, linking experimental fluctuation data with theoretical models to explain their origin.

Findings

Universal maximum in resistance noise spectral density at glass transition.

Activation energy of ethylene endgroup dynamics matches thermodynamic and NMR data.

Observation of Vogel-Fulcher-Tammann law indicating fragile glass behavior.

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.