Involvement of structural dynamics in the charge-glass formation in molecular metals

Abstract

We present a combined study of thermal expansion and resistance fluctuation spectroscopy measurements exploring the static and dynamic aspects of the charge-glass formation in the quasi-two-dimensional organic conductors $\theta$-(BEDT-TTF)${}_2$$M{M}^{\prime }$(SCN)${}_4$ with $M$ = Cs and $M^{\prime }$ = Co,Zn. In these materials, the emergence of a novel charge-glass state so far has been interpreted in purely electronic terms by considering the strong frustration of the Coulomb interactions on a triangular lattice. Contrary to this view, we provide comprehensive evidence for the involvement of a \textit{structural} glass-like transition at $T_{\text{g}}\sim 90-100$K. This glassy transition can be assigned to the freezing of structural conformations of the ethylene endgroups in the donor molecule with an activation energy of $E_{\mathrm{a}}\approx 0.32$eV, and the concomitant slowing down of the charge carrier dynamics is well described by a model of non-exponential kinetics. These findings discolse an important aspect of the phase diagram and renders the current understanding of the charge-glass state in the whole family of $\theta$-(BEDT-TTF)${}_{2}M{M}^{\prime }$(SCN)${}_4$ incomplete. Our results suggest that the entanglement of slow structural and charge-cluster dynamics due to the intimate coupling of lattice and electronic degrees of freedom determine the charge-glass formation under geometric frustration.

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Cited by in corpus (3)

• Microstructuring YbRh2Si2 for resistance and noise measurements down to ultra-low temperatures
• Combined experimental and theoretical studies on glasslike transitions in the frustrated molecular conductors $\theta$-(BEDT-TTF)${}_{2}M{M}^{\prime }$(SCN)${}_4$
• Comparison of the charge-crystal and charge-glass state in geometrically frustrated organic conductors studied by fluctuation spectroscopy