Jens Brandenburg

Origin of the glass-like dynamics in molecular metals κ-(BEDT-TTF)2X: implications from fluctuation spectroscopy and ab initio calculations

We have studied the low-frequency dynamics of the charge carriers in different organic charge-transfer salts κ-(BEDT-TTF)2X 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 has been minimally 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 process 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 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.

1/fnoise in the quasi-two-dimensional organic superconductorβ′′-(ET)2SF5CH2CF2SO3

We performed resistance noise spectroscopy measurements of a bulk single crystal the quasi-two-dimensional organic superconductor {beta}{double_prime}-(ET){sub 2}SF{sub 5}CH{sub 2}CF{sub 2}SO{sub 3} and found generic 1/f{sup a}-type spectra. The temperature dependence of the nearly 1/f spectra are well described by a generalized random fluctuation model. Our data suggest that the number of fluctuators and/or their coupling to the electrical resistance depends on the temperature. The phenomenological model for the electronic fluctuations explains a pronounced peak structure in the low-frequency noise at around 100 K in terms of the thermally-activated conformational degrees of freedom of the ET molecules ethylene endgroups.