Characteristic Relaxation Rate Research Articles

Neutron scattering study of fluctuating and static spin correlations in the anisotropic spin glass Fe2TiO5

The anisotropic spin-glass transition, in which spin freezing is observed only along the $c$ axis in pseudobrookite ${\mathrm{Fe}}_{2}\mathrm{Ti}{\mathrm{O}}_{5}$, has long been perplexing because the ${\mathrm{Fe}}^{3+}$ moments $({d}^{5})$ are expected to be isotropic. Recently, neutron diffraction demonstrated that surfboard-shaped antiferromagnetic nanoregions coalesce above the glass transition temperature ${T}_{g}\ensuremath{\approx}55\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, and a model was proposed in which the freezing of the surfboard magnetization fluctuations leads to the anisotropic spin-glass state. Given this model, we have carried out high-resolution inelastic neutron scattering measurements of the spin-spin correlations to understand the temperature dependence of the intrasurfboard spin dynamics on neutron (picosecond) timescales. Here, we report on the temperature-dependence of the spin fluctuations measured from single-crystal ${\mathrm{Fe}}_{2}\mathrm{Ti}{\mathrm{O}}_{5}$. Strong quasi-elastic magnetic scattering, arising from intrasurfboard correlations, is observed well above ${T}_{g}$. The spin fluctuations possess a steep energy--wave vector relation and are indicative of strong exchange interactions, consistent with the large Curie-Weiss temperature. As the temperature approaches ${T}_{g}$ from above, a shift in spectral weight from inelastic to elastic scattering is observed. At various temperatures between 4 and 300 K, a characteristic relaxation rate of the fluctuations is determined. Despite the freezing of most of the spin correlations, an inelastic contribution remains even at base temperature, signifying the presence of fluctuating intrasurfboard spin correlations to at least $T/{T}_{g}\ensuremath{\approx}0.1$, consistent with an energy landscape that is a hybrid between conventional and geometrically frustrated spin glasses.

Interplay of Local Dynamic Heterogeneity, Mesoscale Aggregate Dynamics, and Transport Properties of Imidazolium Ionic Liquids

Ion dynamics and charge transport in 1-methyl-3-octylimidazolium ionic liquids with chloride, bromide, tetrafluoroborate, tricyanomethanide, hexafluorophosphate, triflate, tetrachlorodialuminate, bis(trifluoromethylsulfonyl)imide, and heptachlorodialuminate anions are investigated by broadband dielectric spectroscopy, rheology and viscometry. A detailed analysis reveals anion and temperature-dependent separation of characteristic relaxation rates extracted from various representations of the dielectric spectra. The degree of separation is interpreted as an experimental signature of significant heterogeneity of the local ion dynamics associated with the structural glass transition, viscosity, and dc ion conductivity. It is found that the degree of dynamic heterogeneity correlates closely with the strengths of slow dielectric and mechanical relaxations previously attributed to the dynamics of mesoscale solvophobic aggregates. Increasing local dynamic heterogeneity correlates with an increase in the strength of the slow, aggregate dielectric relaxation and a decrease in the strength of the slow, aggregate mechanical relaxation. Accordingly, the local dynamic heterogeneity correlates with an increase in the static dielectric permittivities and a decrease in the contribution of aggregate dynamics to the zero-shear viscosities. These results are presented within a broader framework of current understanding of dynamics in ionic liquids and the interplay of dynamics, solvophobic aggregation, and charge transport in these technologically important systems.

Dynamical Signatures of Multifunnel Energy Landscapes

Multifunctional systems, such as molecular switches, exhibit multifunnel energy landscapes associated with the alternative functional states. In this contribution the multifunnel organization is decoded from dynamical signatures in the first passage time distribution between reactants and products. Characteristic relaxation rates are revealed by analyzing the kinetics as a function of the observation time scale, which scans the underlying distribution. Extracting the corresponding dynamical signatures provides direct insight into the organization of the molecular energy landscape, which will facilitate a rational design of target functionality. Examples are illustrated for multifunnel landscapes in biomolecular systems and an atomic cluster.

Thermalization of a quantum Newton's cradle in a one-dimensional quasicondensate

We study the nonequilibrium dynamics of the quantum Newton's cradle in a one-dimensional (1D) Bose gas in the weakly-interacting quasicondensate regime. This is the opposite regime to the original quantum Newton's cradle experiment of Kinoshita et al. [Nature 440, 900 (2006)], which was realized in the strongly interacting 1D Bose gas. Using finite temperature c-field methods, we calculate the characteristic relaxation rates to the final equilibrium state. Hence, we identify the different dynamical regimes of the system in the parameter space that characterizes the strength of interatomic interactions, the initial temperature, and the magnitude of the Bragg momentum used to initiate the collisional oscillations of the cradle. In all parameter regimes, we find that the system relaxes to a final equilibrium state for which the momentum distribution is consistent with a thermal distribution. For sufficiently large initial Bragg momentum, the system can undergo hundreds of repeated collisional oscillations before reaching the final thermal equilibrium. The corresponding thermalization timescales can reach tens of seconds, which is an order of magnitude smaller than in the strongly interacting regime.

Modular theory for DC-biased electrochemical impedance response of supercapacitor

Supercapacitor electrodes consist of complex nanoporous structures in carbonaceous and non-carbonaceous materials causing alteration of electric double layer (EDL) structure and response. We develop a modular theory for DC-bias dependent electrochemical impedance spectroscopy (EIS) for EDL in the heterogeneous bimodal porous electrode, viz. arbitrary mesopores with embedded heterogeneous micropores. Theory accounts for the compact- and diffuse-EDL dynamics along with charge transfer kinetics of pseudocapacitance. The influence of applied DC-bias on various phenomenological components is accounted through a heuristic approach. This is achieved by using the potential and concentration dependent diffuse layer thickness for unsymmetrical electrolytes, charge transfer resistance, and electric field dependent dielectric constant. The generic nature of theory is further highlighted by extending it for the composite porous electrode materials. The theoretical response shows that increasing the magnitude of DC-bias enhances the characteristic ion relaxation rates therefore, increases the rate capability of supercapacitors. The electrode morphological parameters, viz. mesopore size, micropore size, micropore length, and the pore surface heterogeneity, can effectively tune the capacitance and charging-discharging rates therefore, influence the performance of supercapacitors. Finally, our theory explains the experimental EIS data for hierarchical sodium gluconate and graphdiyne porous electrodes.

Dynamic light scattering in concentrated polyacrylamide solutions

Concentrated aqueous solutions of polyacrylamide were prepared by free radical polymerization, using ammonium persulfate as initiator, with different initiator/acrylamide mass ratios. The increase in initiator/acrylamide mass ratio resulted in two effects: a slight increase in conversion and a decrease in molecular weight (characterized as a decrease in intrinsic viscosity). Calculation of dimensionless concentration indicated that all polyacrylamide solutions were in the concentrated regime, in which solubilized macromolecules are described as highly interpenetrated solubilized coils. Intensity correlation function from dynamic light scattering experiments showed random motion could be bound to slow-mode and fast-mode relaxation processes. Fast-mode characteristic relaxation rate clearly seemed to be related to an Arrhenius-type activated process, while slow-mode characteristic relaxation rate had a more complex behavior, which was described as a consequence of two concurrent effects as temperature increased: decrease of macromolecular cluster sizes, resultant from breakage of intermacromolecular interactions, and increase in macromolecular dimensions, resultant from breakage of intramacromolecular interactions.

Dielectric Relaxation Dynamics of Clay‐Containing Low‐Density polyethylene Blends and Nanocomposites

Series of clay‐containing nanocomposites have been prepared and investigated using frequency‐domain dielectric spectroscopy at different temperatures. Different matrix materials have been used: neat low‐density polyethylene (LDPE) with and without compatibilizer and co‐continuous blends of LDPE with two grades of polystyrene‐b‐poly(ethylene‐co‐butylene)‐b‐polystyrene (SEBS) copolymers. Two major relaxation modes were detected in the dielectric losses of all the nanocomposites and associated with Maxwell–Wagner–Sillars interfacial polarization and dipolar relaxation, respectively. Characteristic relaxation rates, activation energies, dielectric strength, and shape parameters of these relaxation mechanismes were calculated and discussed for the LDPE/clay nanocomposites. The addition of compatibilizer was found to slightly increase the dielectric loss of the nanocomposites while slowing the dynamics due to an improved dispersion. When combined with a high loading of nanofiller (15%), the compatibilizer addition led to low‐frequency dispersion. A new relaxation process was then observed for the nanocomposites with the blend matrix. Several speculations were made as to the origin of this phenomenon, all of which were related to the SEBS phase. POLYM. ENG. SCI., 60:968–978, 2020. © 2020 Society of Plastics Engineers

Characterization of the distribution of spin-lattice relaxation rates of lipid spin labels in fiber cell plasma membranes of eye lenses with a stretched-exponential function.

The stretched exponential function (SEF) was used to analyze and interpret saturation recovery (SR) electron paramagnetic resonance (EPR) data obtained from spin-labeled porcine eye-lens membranes. This function has two fitting parameters: the characteristic spin-lattice relaxation rate (T 1str -1) and the stretching parameter (β), which ranges between zero and one. When β = 1, the function is a single exponential. It is assumed that the SEF arises from a distribution of single exponential functions, each described by a T 1 value. Because T 1 -1s are determined primarily by the rotational diffusion of spin labels, they are a measure of membrane fluidity. Since β describes the distribution of T 1 -1s, it can be interpreted as a measure of membrane heterogeneity. The SEF was used to analyze SR data obtained from intact cortical and nuclear fiber cell plasma membranes extracted from the eye lenses of two-year old animals and spinlabeled with phospholipid- and cholesterol-analogs. The lipid environment sensed by these probe molecules was found to be less fluid and more heterogeneous in nuclear membranes than in cortical membranes. Parameters T 1str -1 and β were also used for a multivariate K-means cluster analysis of stretched-exponential data. This analysis indicates that SEF data can be assigned accurately to clusters in nuclear or cortical membranes. In future work, the SEF will be applied to analyze data from human eye lenses of donors with differing health histories.

NbRe as candidate material for fast single photon detection

The suitability of NbRe as a promising material for the design of Superconducting Single Photon Detectors is investigated in order to lower both the minimum detectable photon energy and the recovery time of the devices. Both the low values determined for the quasiparticle relaxation time, τE, and its weak temperature dependence are desirable in the design of fast single photon detectors. Both properties can be further improved by coupling NbRe with a ferromagnetic layer, as demonstrated by estimating the characteristic relaxation rates in NbRe/CuNi bilayers.

Preparation and characterization of dispersions based on chitosan and poly(styrene sulfonate)

Interpolyelectrolyte complexes (IPECs) were obtained by the mixture of solutions of chitosan and poly(sodium 4-styrene-sulfonate). The resultant dispersions were characterized using small-angle X-ray scattering (SAXS) and dynamic light scattering (DLS). Gyration radius from SAXS increased as sulfonate to aminium molar ratio, r SA , increased, while pair distance distributions, P(r), shifted to higher values of r, describing the process of particle formation as resultant from collapse of solubilized IPEC clusters in the form of solid particles, followed by nucleation of larger ones. Intensity correlation curves from DLS were used to obtain parameters related to Brownian motion of (or within) IPEC clusters. IPEC cluster dimension growth was related to characteristic relaxation rate and relaxation rate distribution width. Association of DLS-derived results to SAXS-resultant data yielded a description of nanoparticles made of solid IPEC clusters stabilized by solubilized IPEC species.

A novel approach to thickening characterization of an acrylic latex thickener

Abstract Alkali-soluble latex thickeners are extensively used in activities such as coating manufacturing, cosmetics, and textile industry. They increase apparent viscosity as a result of neutralization of carboxyl groups present in latex particles. Rheometry is the main method used to monitor the rheological activity of these substances. In this manuscript, we propose the use of dynamic light scattering (DLS) to get quantitative parameters and correlate them to apparent viscosity increase, confirming the results using more traditional methods. More specifically, we monitored by DLS the characteristic and average relaxation rates, as well as the width of the relaxation rate distributions. Critical changes of these parameters were observed during thickening; drastic changes at the same point were confirmed by rheometry, conductometry, and turbidimetry.

Dynamic and static radiation scattering in a microemulsion as a function of dispersed phase concentration

Abstract Dynamic light scattering and SAXS were used to characterize the change in dimensions of the dispersed phase in microemulsions based on C 12 E 9 non-ionic surfactant, butanol, xylene, and water, as a function of dispersed phase concentration. A KWW-based equation was fitted to data from intensity correlation functions of the microemulsions and a transition from spherical to worm-like geometry was related to a decrease in the dimensions of the randomly oscillating regions of the resultant systems as well as to the broadening of their characteristic relaxation rate distributions. More specifically, the transition was characterized by an increase in the characteristic relaxation rate as well as by a decrease in the stretching coefficient (related to an increased heterogeneity of more complex colloidal structures). Static SAXS was used to characterize the transition as occurring at the maximum ratio between the radii of gyration of the system with and without xylene, which was shown to me correlated to previously obtained data associating this transition to a maximum in the ratio of viscosities for the same systems.

Rheology and dynamic light scattering of octa-ethyleneglycol-monododecylether/chitosan solutions

In this work we used rheometry and DLS to probe relaxation phenomena in solutions of chitosan and octa-ethyleneglycol-monododecylether. The dispersions had a marked pseudoplastic behavior, which became less evident, as surfactant concentration was increased. Arrhenius plots showed that systems with surfactant presented a characteristic temperature at which apparent enthalpy of activation (varying from 3 to 40kJmol−1) changed: this change was correlated to a possible transition of colloidal aggregates to a wormlike configuration. DLS intensity correlation functions were described by KWW equation: pure chitosan solutions had relaxation rate distributions centered at a characteristic relaxation rate around 4.6×10−6μS−1; as surfactant was added, a new component, with a faster characteristic relaxation rate with a magnitude order of 10−3μs−1, appeared. It was shown that the dependence between these relaxation rates and surfactant concentration could be used to describe DLS-related relaxation phenomena as an Arrhenius-activated process, agreeing with results obtained using rheometry.

T4 Lysozyme as a Pac-Man: How Fast Can It Chew?

T4 Lysozyme as a Pac-Man: How Fast Can It Chew?

Variations of Water Uptake, Lipid Consumption, and Dynamics during the Germination of Sesamum indicum Seed: A Nuclear Magnetic Resonance Spectroscopic Investigation

Germination in sesame seeds (Sesamum indicum L.) in water and in indole-3-acetic acid (IAA) solution is investigated with magic-angle-spinning (MAS) solid state nuclear magnetic resonance (NMR) spectroscopy, supplemented by liquid state NMR spectroscopy. The spectra show good resolution and can be assigned with sufficient confidence. The characteristic spectral peaks and relaxation rates were monitored during the entire course of germination for better understanding of the biophysical and biochemical mechanisms involved in the triphasic water uptake of the seed. A highly positive correlation is found between water uptake and lipid consumption during germination. No significant variation is observed in the relaxation times for the lipid protons during the first two stages of triphasic water uptake, while evident differences are observed for water proton relaxation rates in all stages. Although the total amount of water uptake is largely not changed as a result of IAA, the addition of IAA in seed-germination medium has shown some prominent effects on the germination process, e.g, it suppresses lipid consumption and water mobility, and it reduces the longitudinal and transverse relaxation times of lipid protons and causes a more scattered range for these parameters.

Folding/unfolding kinetics of lattice proteins studied using a simple statistical mechanical model for protein folding, I: Dependence on native structures and amino acid sequences

The folding/unfolding kinetics of a three-dimensional lattice protein was studied using a simple statistical mechanical model for protein folding that we developed earlier. We calculated a characteristic relaxation rate for the free energy profile starting from a completely unfolded structure (or native structure) that is assumed to be associated with a folding rate (or an unfolding rate). The chevron plot of these rates as a function of the inverse temperature was obtained for four lattice proteins, namely, proteins a 1 , a 2 , b 1 , and b 2 , in order to investigate the dependency of the folding and unfolding rates on their native structures and amino acid sequences. Proteins a 1 and a 2 fold to the same native conformation, but their amino acid sequences differ. The same is the case for proteins b 1 and b 2 , but their native conformation is different from that of proteins a 1 and a 2 . However, the chevron plots of proteins a 1 and a 2 are very similar to each other, and those of proteins b 1 and b 2 differ considerably. Since the contact orders of proteins b 1 and b 2 are identical, the differences in their kinetics should be attributed to the amino acid sequences and consequently to the interactions between the amino acid residues. A detailed analysis revealed that long-range interactions play an important role in causing the difference in the folding rates. The chevron plots for the four proteins exhibit a chevron rollover under both strongly folding and strongly unfolding conditions. The slower relaxation time on the broad and flat free energy surfaces of the unfolding conformations is considered to be the main origin of the chevron rollover, although the free energy surfaces have features that are rather complicated to be described in detail here. Finally, in order to concretely examine the relationship between changes in the free energy profiles and the chevron plots, we illustrate some examples of single amino acid substitutions that increase the folding rate.

The effect of ions on the permittivity and dielectric relaxation of an uncrystallizableheptanol

Solvation of ions in a liquid tends to reduce the extent of intermolecular hydrogenbonding. This changes the effective dipolar orientation correlation factor,g, in most alcohols and in water, and hence the equilibrium permittivityεs, but not therelaxation time, τ. An increase in temperature has a similar effect onεs, butthe relaxation time also decreases. Here we study the dielectric relaxation spectra of two concentrationsof LiClO4 solutions in 5-methyl-2-hexanol, and compare the findings against those for thepure liquid. We also compare the effect of the ions against that observed ina similar study of 1-propanol (Power et al 2002 J. Chem. Phys. 116 4192).εs decreases by∼7% on adding1 mol% LiClO4 at 173 Kbut increases by ∼12% at 223 K, and there is similar change in the strength of the Debye relaxation process. This mayoccur partly as a result of the changing ion–ion pair equilibrium with temperature. Initial additionof LiClO4 (0.5 mol% solution) increases the dc conductivity,σdc, far more than further addition. The addition of electrolyte also causes the slowest relaxation todepart from the Debye process, but its characteristic relaxation rate does not change. Variation inσdc with τ does not follow the Debye–Stokes–Einstein relation for either the pure liquid or theLiClO4 solutions, which indicates that diffusion of ions does not follow the Brownian diffusion ofmolecules.

Transverse-field [formula omitted]SR and magnetic disorder in [formula omitted

We have carried out transverse-field ( H = 1.5 T ) μ SR experiments in UCu 4 Ni for temperatures from 300 down to 2.2 K. The muon-decay asymmetry can be fit to a stretched exponential relaxation function: G ( t ) ∝ e - ( Λ t ) K . The exponent K ∼ 2 for T > 100 K , but drops continuously below this temperature (being ∼ 1 for T ∼ 20 K and reaching ∼ 0.6 near 2 K). The characteristic relaxation rate Λ grows six-fold in the experimental temperature range (from ∼ 0.2 for T ∼ 300 K to ∼ 1.2 μ s - 1 for T ∼ 2 K ). Independently of theoretical models, the behavior of these parameters is consistent with strong magnetic disorder. Although the magnetic susceptibility of UCu 4 Ni is considerably lower than that of UCu 4 Pd , the amount of magnetic disorder in both systems is comparable. We discuss the data within the context of the Kondo disorder model.

Retarding accelerations and ratchet-like transport of the self-ordered front in a bistable system of reaction-diffusion type

Front dynamics with time delays in a reaction-diffusion equation with the piecewise linear rate function is studied by considering the response of the self-ordered front to the rapidly varying force. More specifically, the retarding accelerations of the bistable front (BF) joining two states of different stability in the system are examined. Two cases of differently shaped forcing functions are studied. In examining the ‘size’ of the retardation, namely, the lag time between the propagation velocity of the front and the driving force we approximate the forcing function by the step-like dependence of the flexible steepness. The lag time is found to be sensitive to both the rate (steepness) of the driving force and the characteristic parameters of the rate function. A significant retardation effect occurred when the steepness of the force exceeded the characteristic relaxation rate of the system; the lag time monotonically increases with increasing steepness and approaches the maximal value depending on the relaxation time of the system, when the driving force becomes extremely steep. The influence of delays on the ratchet-like transport of the self-ordered front is studied by considering the response of BF to the periodically oscillating square-pulse force. The average characteristics of the unforced dc motion of BFs are presented. The occurrence of delays, as shown, significantly reduces the unforced dc motion of BF. The ratchet-like transport is highly suppressed if the period of the ac force becomes low compared to the characteristic relaxation time of the system.

Photon-induced magnetization changes in single-molecule magnets (invited)

Microwave radiation applied to single-molecule magnets can induce large magnetization changes when the radiation is resonant with transitions between spin levels. These changes are interpreted as due to resonant heating of the sample by the microwaves. Pulsed-radiation studies show that the magnetization continues to decrease after the radiation has been turned off with a rate that is consistent with the spin’s characteristic relaxation rate. The measured rate increases with pulse duration and microwave power, indicating that greater absorbed radiation energy results in a higher sample temperature. We also performed numerical simulations that qualitatively reproduce many of the experimental results. Our results indicate that experiments aimed at measuring the magnetization dynamics between two levels resonant with the radiation must be done much faster than the ⩾20μs time scales probed in these experiments.