Johari-Goldstein relaxation far below Tg: Experimental evidence for the Gardner transition in structural glasses?

Experimental evidence for the Gardner transition, theoretically predicted to arise deep in the glassy state of matter, is scarce. At this transition, the energy landscape sensed by the particles forming the glass is expected to become more complex. In the present work, we report the dielectric response of two typical glass formers with well-pronounced Johari-Goldstein beta relaxation following this response down to unprecedented low temperatures, far below the glass transition. As the Johari-Goldstein process is believed to arise from the local structure of the energy landscape, its investigation seems an ideal tool to seek evidence for the Gardner transition. Indeed, we find an unusual broadening of the beta relaxation below TG ~ 110 K for sorbitol and TG ~ 100 K for xylitol, in excess of the expected broadening arising from a distribution of energy barriers. Thus, these results provide hints at the presence of the Gardner transition in canonical structural glass formers.Comment: 6 pages, 3 figures + 2 pages, 3 figures in Supplemental Materia

α\alpha- and β\beta- Relaxation Dynamics of a fragile plastic crystal

We present a thorough dielectric investigation of the relaxation dynamics of plastic crystalline Freon112, which exhibits freezing of the orientational degrees of freedom into a glassy crystal below 90 K. Among other plastic crystals, Freon112 stands out by being relatively fragile within Angell's classification scheme and by showing an unusually strong $\beta$-relaxation. Comparing the results to those on Freon112a, having only a single molecular conformation, points to the importance of the presence of two molecular conformations in Freon112 for the explanation of its unusual properties.Comment: 17 pages, 6 figure

Optical properties of carbon grains: Influence on dynamical models of AGB stars

For amorphous carbon several laboratory extinction data are available, which show quite a wide range of differences due to the structural complexity of this material. We have calculated self-consistent dynamic models of circumstellar dust-shells around carbon-rich asymptotic giant branch stars, based on a number of these data sets. The structure and the wind properties of the dynamical models are directly influenced by the different types of amorphous carbon. In our test models the mass loss is not severely dependent on the difference in the optical properties of the dust, but the influence on the degree of condensation and the final outflow velocity is considerable. Furthermore, the spectral energy distributions and colours resulting from the different data show a much wider spread than the variations within the models due to the variability of the star. Silicon carbide was also considered in the radiative transfer calculations to test its influence on the spectral energy distribution.Comment: 12 pages, 6 figures. To appear in A&

Ionic conductivity and relaxation dynamics in plastic-crystals with nearly globular molecules

We have performed a dielectric investigation of the ionic charge transport and the relaxation dynamics in plastic-crystalline 1-cyano-adamantane (CNA) and in two mixtures of CNA with the related plastic crystals adamantane or 2-adamantanon. Ionic charge carriers were provided by adding 1% of Li salt. The molecules of these compounds have nearly globular shape and, thus, the so-called revolving-door mechanism assumed to promote ionic charge transport via molecular reorientations in other PC electrolytes, should not be active here. Indeed, a comparison of the dc resistivity and the reorientational alpha-relaxation times in the investigated PCs, reveals complete decoupling of both dynamics. Similar to other PCs, we find a significant mixing-induced enhancement of the ionic conductivity. Finally, these solid-state electrolytes reveal a second relaxation process, slower than the alpha-relaxation, which is related to ionic hopping. Due to the mentioned decoupling, it can be unequivocally detected and is not superimposed by the reorientational contributions as found for most other ionic conductors.Comment: 9 pages, 7 figure

The excess wing in the dielectric loss of glass-forming ethanol: A relaxation process

A detailed dielectric investigation of liquid, supercooled liquid, and glassy ethanol reveals a third relaxation process, in addition to the two processes already known. The relaxation time of the newly detected process exhibits strong deviations from thermally activated behavior. Most important, this process is the cause of the apparent excess wing, which was claimed to be present in the dielectric loss spectra of glass-forming ethanol. In addition, marked deviations of the spectra of ethanol from the scaling proposed by Dixon and Nagel have been detected.Comment: 8 pages including 4 figures submitted to Phys. Rev.

Ions in glass forming glycerol: Close correlation of alpha and fast beta relaxation

We provide broadband dielectric loss spectra of glass-forming glycerol with varying additions of LiCl. The measurements covering frequencies up to 10 THz extend well into the region of the fast beta process, commonly ascribed to caged molecule dynamics. Aside of the known variation of the structural alpha relaxation time and a modification of the excess wing with ion content, we also find a clear influence on the shallow loss minimum arising from the fast beta relaxation. Within the framework of mode-coupling theory, the detected significant broadening of this minimum is in reasonable accord with the found variation of the alpha-relaxation dynamics. A correlation between alpha-relaxation rate and minimum position holds for all ion concentrations and temperatures, even below the critical temperature defined by mode-coupling theory.Comment: 5 pages, 5 figure

Low temperature dielectric relaxation study of aqueous solutions of diethylsulfoxide

In the present work, dielectric spectra of mixtures of diethylsulfoxide (DESO) and water are presented, covering a concentration range of 0.2 - 0.3 molar fraction of DESO. The measurements were performed at frequencies between 1 Hz and 10 MHz and for temperatures between 150 and 300 K. It is shown that DESO/water mixtures have strong glass-forming abilities. The permittivity spectra in these mixtures reveal a single relaxation process. It can be described by the Havriliak-Negami relaxation function and its relaxation times follow the Vogel-Fulcher-Tammann law, thus showing the typical signatures of glassy dynamics. The concentration dependence of the relaxation parameters, like fragility, broadening, and glass temperature, are discussed in detail.Comment: 20 pages, 5 figure

Magnetic susceptibility of the frustrated spinels ZnCr2O4, MgCr2O4 and CdCr2O4

We analyzed the magnetic susceptibilities of several Cr spinels using two recent models for the geometrically frustrated pyrochlore lattice, the Quantum Tetrahedral Mean Field model and a Generalized Constant Coupling model. Both models can describe the experimental data for ACr2 O4 (with A = Zn, Mg, and Cd) satisfactorily, with the former yielding a somewhat better agreement with experiment for A = Zn, Mg. The obtained exchange constants for nearest and next-nearest neighbors are discussed.Comment: 4 pages, 1 figure, 1 table, conferenc