Low-frequency Dynamical Properties Research Articles

Fine-tuning the Mott metal–insulator transition and critical charge carrier dynamics in molecular conductors

The unique possibilities of fine-tuning their physical properties in the vicinity of the Mott metal–insulator transition make the quasi-two-dimensional organic charge-transfer salts -(BEDT-TTF)X unprecedented model systems for studying the fundamentals of electron–electron correlations and the coupling between charge, spin and lattice degrees of freedom in reduced dimensions. The critical properties and the universality class of the Mott transition, however, are controversially debated for these materials, and information on the low-frequency dynamical properties of the correlated electrons is rather limited. By introducing fluctuation (noise) spectroscopy as a powerful new tool for studying the slow dynamics of charge carriers, in the past years we have been able to extract spectroscopic information on the coupling of charge carriers to the vibrational degrees of freedom of the crystal lattice. This is related to a glassy freezing of the BEDT-TTF molecules’ ethylene end-group (EEG) rotations at elevated temperatures, which (i) results in a small amount of (intrinsic) disorder and (ii) crucially influences the ratio of bandwidth to on-site Coulomb repulsion (W / U) and therefore the samples’ position in the phase diagram, i.e. the electronic ground state. The low-frequency resistance fluctuations show a dramatic enhancement and divergent behaviour when tuning the sample close to the critical point of the Mott transition, accompanied by a strong shift of spectral weight to low frequencies and the onset of non-Gaussian behaviour. This indicates the critical slowing down of the order-parameter (doublon density) fluctuations and suggests a collective dynamics of the correlated electrons. In order to enable detailed investigations of this hypothesis in future experiments, by exploiting the structural EEG relaxation, a ‘warming cycle’ protocol can be established that allows for fine-tuning the sample across the Mott transition and therefore precisely accessing the finite-temperature critical endpoint. We ‘calibrate’ this procedure by a comparison to pressure-tuning experiments on the same sample. This method will allow to map out the region of ergodicity breaking around the critical endpoint and its dependence on disorder.

Electronic transport in thermoelectric YbzCo4Sb12 skutterudite thin films studied by resistance noise spectroscopy

Skutterudites CoSb3 are considered interesting candidates for thermoelectric applications, because the filling of guest atoms into the cage-like structure has the potential to improve its thermoelectric properties by an increased phonon scattering, which reduces the thermal conductivity. This, however, requires that a high electrical conductivity is maintained. In this study, we performed resistivity, Hall effect, and fluctuation spectroscopy measurements on polycrystalline thin films of semiconducting YbzCo4Sb12 with 0<z<0.27. Our aim is to better understand the conventional dc electronic transport but also the low-frequency dynamical properties of the charge carriers. The electronic properties are highly sensitive to the filling factor z as well as other parameters, e.g., the Sb content. The resistivity can be described by Mott variable range hopping at low temperatures. A large 1/f noise level suggests an influence of the granularity of the polycrystalline thin films. By analyzing the 1/f-noise and two-level fluctuations, which are abundant for filled samples annealed at 500 °C, we are able to determine the energy distribution of the relevant electronic switching processes. A likely explanation for the observed low-frequency dynamics is capture/emission processes of impurities with a broad distribution within the energy gap.

Carbon Dioxide and Fruit Odor Transduction in Drosophila Olfactory Neurons. What Controls their Dynamic Properties?

We measured frequency response functions between odorants and action potentials in two types of neurons in Drosophila antennal basiconic sensilla. CO2 was used to stimulate ab1C neurons, and the fruit odor ethyl butyrate was used to stimulate ab3A neurons. We also measured frequency response functions for light-induced action potential responses from transgenic flies expressing H134R-channelrhodopsin-2 (ChR2) in the ab1C and ab3A neurons. Frequency response functions for all stimulation methods were well-fitted by a band-pass filter function with two time constants that determined the lower and upper frequency limits of the response. Low frequency time constants were the same in each type of neuron, independent of stimulus method, but varied between neuron types. High frequency time constants were significantly slower with ethyl butyrate stimulation than light or CO2 stimulation. In spite of these quantitative differences, there were strong similarities in the form and frequency ranges of all responses. Since light-activated ChR2 depolarizes neurons directly, rather than through a chemoreceptor mechanism, these data suggest that low frequency dynamic properties of Drosophila olfactory sensilla are dominated by neuron-specific ionic processes during action potential production. In contrast, high frequency dynamics are limited by processes associated with earlier steps in odor transduction, and CO2 is detected more rapidly than fruit odor.

One-dimensional semiconductor in a polar solvent: Solvation and low-frequency dynamics of an excess charge carrier

Due to solvation, excess charge carriers on 1d semiconductor nanostructures immersed in polar solvents undergo self-localization into polaronic states. Using a simplified theoretical model for small-diameter structures, we study low-frequency dynamical properties of resulting 1d adiabatic polarons. The combined microscopic dynamics of the electronic charge density and the solvent leads to macroscopic Langevin dynamics of a polaron and to the appearance of local dielectric relaxation modes. Polaron mobility is evaluated as a function of system parameters. Numerical estimates indicate that the solvated carriers can have mobilities orders of magnitude lower than the intrinsic values.

Diffusive relaxation processes and low-frequency dynamical properties in bulk and confined ethylene glycol by neutron spectroscopy

In this work, we report on a detailed study of the diffusional and vibrational dynamics of liquid ethylene glycol (EG: HO–CH2CH2–OH) in the bulk state and confined in a matrix of sol-gel porous glass with 26 Å interconnected cylindrical pores. The investigation is performed, by incoherent quasielastic and inelastic neutron scattering (IQENS and IINS), as a function of temperature. IQENS data have been definitively interpreted in terms of the α-relaxation dynamics predicted by mode coupling theory for supercooled liquids. We verified that a description by means of an Havriliak–Negami profile is consistent with the measured spectral line shape. Furthermore, IINS spectra show a clear different evolution versus T of the vibrational density of states, Z(ω), passing from the bulk samples to the confined ones, revealing, in particular, a thermal stabilization of the disordered longitudinal acoustic mode induced by interaction of EG molecules with the active silanol (Si–OH) groups of the surface.

Intra- and Interchain Relaxation Processes in Meshlike Polymer Networks

The dynamic properties of meshlike polymer networks are considered using the cubic network of multisegmental “bead and spring” Rouse chains. The normal mode treatment is used. Rigorous expressions for the mean-square displacements of elements of the Gaussian network (junctions and nonjunction beads) are obtained. The autocorrelation functions of chain vectors between neighboring nonjunction beads (subchain vectors) and of end-to-end chain vectors between neighboring junctions are calculated. The time dependence of the relaxation modulus of a network is also considered. Contributions of intra- and interchain relaxation processes to various dynamic characteristics of a Gaussian network are compared. The time behavior of dynamic quantities obtained is estimated for different scale motions. It is shown that the relaxation of regular polymer networks consisting of long multisegmental Gaussian chains can be considered as a superposition of four different types of relaxation processes. These are the relaxation of network chains with fixed junctions, the relaxation of chains with free ends, the relaxation processes in the intermediate region of the spectrum, and interchain cooperative network motions. The possibility of describing low-frequency dynamic properties of networks by a simplified coarse-grained network model used previously is proved.