Showing papers on "Relaxation (NMR) published in 2008"
TL;DR: In this paper, the authors measured carrier relaxation times in epitaxial graphene layers grown on SiC wafers and found that an initial fast relaxation transient in the 70-120fs range is followed by a slower relaxation process in the 0.4-1.7ps range.
Abstract: Using ultrafast optical pump-probe spectroscopy, we have measured carrier relaxation times in epitaxial graphene layers grown on SiC wafers. We find two distinct time scales associated with the relaxation of nonequilibrium photogenerated carriers. An initial fast relaxation transient in the 70–120fs range is followed by a slower relaxation process in the 0.4–1.7ps range. The slower relaxation time is found to be inversely proportional to the degree of crystalline disorder in the graphene layers as measured by Raman spectroscopy. We relate the measured fast and slow time constants to carrier-carrier and carrier-phonon intraband and interband scattering processes in graphene.
658 citations
TL;DR: Using the spin Hall effect, magnetization relaxation in a Ni_{81}Fe_{19}/Pt film is manipulated electrically without applying electric currents directly to the magnetic layer.
Abstract: Using the spin Hall effect, magnetization relaxation in a Ni_{81}Fe_{19}/Pt film is manipulated electrically. An electric current applied to the Pt layer exerts spin torque on the entire magnetization of the Ni81Fe19 layer via the macroscopic spin transfer induced by the spin Hall effect and modulates the magnetization relaxation in the Ni81Fe19 layer. This method allows us to tune the magnetization dynamics regardless of the film size without applying electric currents directly to the magnetic layer.
597 citations
TL;DR: By partially embedding and then removing gold nanospheres, a high surface coverage of well-defined nanodeformations on a polystyrene surface is produced, providing strong direct evidence for enhanced surface mobility relative to the bulk.
Abstract: The motion of polymer chain segments cooled below the glass transition temperature slows markedly; with sufficient cooling, segmental motion becomes completely arrested. There is debate as to whether the chain segments near the free surface, or in thin films, are affected in the same way as the bulk material. By partially embedding and then removing gold nanospheres, we produced a high surface coverage of well-defined nanodeformations on a polystyrene surface; to probe the surface dynamics, we measured the time-dependent relaxation of these surface deformations as a function of temperature from 277 to 369 kelvin. Surface relaxation was observed at all temperatures, providing strong direct evidence for enhanced surface mobility relative to the bulk. The deviation from bulk α relaxation became more pronounced as the temperature was decreased below the bulk glass transition temperature. The temperature dependence of the relaxation time was much weaker than that of the bulk α relaxation of polystyrene, and the process exhibited no discernible temperature dependence between 277 and 307 kelvin.
586 citations
TL;DR: It is found that relaxation is markedly slowed with increasing ZnSe shell thickness, which could prove useful for more efficient photovoltaic or infrared devices.
Abstract: Hot electrons in semiconductors lose their energy very quickly (within picoseconds) to lattice vibrations. Slowing this energy loss could prove useful for more efficient photovoltaic or infrared devices. With their well-separated electronic states, quantum dots should display slow relaxation, but other mechanisms have made it difficult to observe. We report slow intraband relaxation (>1 nanosecond) in colloidal quantum dots. The small cadmium selenide (CdSe) dots, with an intraband energy separation of approximately 0.25 electron volts, are capped by an epitaxial zinc selenide (ZnSe) shell. The shell is terminated by a CdSe passivating layer to remove electron traps and is covered by ligands of low infrared absorbance (alkane thiols) at the intraband energy. We found that relaxation is markedly slowed with increasing ZnSe shell thickness.
477 citations
TL;DR: In this article, the authors show computer simulations that reveal the universal correlation of the structural relaxation time (as well as the viscosity η) and the rattling amplitude from glassy to low-viscosity states.
Abstract: If liquids, polymers, bio-materials, metals and molten salts can avoid crystallization during cooling or compression, they freeze into a microscopically disordered solid-like state, a glass1,2. On approaching the glass transition, particles become trapped in transient cages—in which they rattle on picosecond timescales—formed by their nearest neighbours; the particles spend increasing amounts of time in their cages as the average escape time, or structural relaxation time τα, increases from a few picoseconds to thousands of seconds through the transition. Owing to the huge difference between relaxation and vibrational timescales, theoretical3,4,5,6,7,8,9 studies addressing the underlying rattling process have challenged our understanding of the structural relaxation. Numerical10,11,12,13 and experimental studies on liquids14 and glasses8,15,16,17,18,19 support the theories, but not without controversies20 (for a review see ref. 21). Here we show computer simulations that, when compared with experiments, reveal the universal correlation of the structural relaxation time (as well as the viscosity η) and the rattling amplitude from glassy to low-viscosity states. According to the emerging picture the glass softens when the rattling amplitude exceeds a critical value, in agreement with the Lindemann criterion for the melting of crystalline solids22 and the free-volume model23.
295 citations
TL;DR: An NMR method for quantifying millisecond time scale dynamics that involve transitions between different molecular conformations is described, and it is shown that the methodology facilitates detection of intermediates and other excited states that are populated at low levels that cannot be observed directly in spectra, so long as they exchange with the observable ground state of the protein on the milliseconds time scale.
Abstract: Biological function depends on molecular dynamics that lead to excursions from highly populated ground states to much less populated excited states. The low populations and the transient formation of such excited states render them invisible to the conventional methods of structural biology. Thus, while detailed pictures of ground-state structures of biomolecules have emerged over the years, largely through X-ray diffraction and solution nuclear magnetic resonance (NMR) spectroscopy studies, much less structural data has been accumulated on the conformational properties of the invisible excited states that are necessary to fully explain function. NMR spectroscopy is a powerful tool for studying conformational dynamics because it is sensitive to dynamics over a wide range of time scales, extending from picoseconds to seconds and because information is, in principle, available at nearly every position in the molecule. Here an NMR method for quantifying millisecond time scale dynamics that involve transitions between different molecular conformations is described. The basic experimental approach, termed relaxation dispersion NMR spectroscopy, is outlined to provide the reader with an intuitive feel for the technology. A variety of different experiments that probe conformational exchange at different sites in proteins are described, including a brief summary of data-fitting procedures to extract both the kinetic and thermodynamic properties of the exchange process and the structural features of the invisible excited states along the exchange pathway. It is shown that the methodology facilitates detection of intermediates and other excited states that are populated at low levels, 0.5% or higher, that cannot be observed directly in spectra, so long as they exchange with the observable ground state of the protein on the millisecond time scale. The power of the methodology is illustrated by a detailed application to the study of protein folding of the small modular SH3 domain. The kinetics and thermodynamics that describe the folding of this domain have been characterized through the effects of temperature, pressure, side-chain deuteration, and mutation, and the structural features of a low-populated folding intermediate have been assessed. Despite the fact that many previous studies have shown that SH3 domains fold via a two-state mechanism, the NMR methods presented unequivocally establish the presence of an on-pathway folding intermediate. The unique capabilities of NMR relaxation dispersion follow from the fact that large numbers of residues can be probed individually in a single experiment. By contrast, many other forms of spectroscopy monitor properties that are averaged over all residues in the molecule or that make use of only one or two reporters. The NMR methodology is not limited to protein folding, and applications to enzymatic catalysis, binding, and molecular recognition are beginning to emerge.
250 citations
TL;DR: Single crystal magnetic studies combined with a theoretical analysis show that cancellation of the magnetic moments in the trinuclear Dy3+ cluster resulting in a nonmagnetic ground doublet originates from the noncollinearity of the single-ion easy axes of magnetization of the Dy3- ions.
Abstract: Single crystal magnetic studies combined with a theoretical analysis show that cancellation of the magnetic moments in the trinuclear Dy3+ cluster [Dy{3}(mu{3}-OH)2L3Cl(H2O){5}]Cl{3}, resulting in a nonmagnetic ground doublet, originates from the noncollinearity of the single-ion easy axes of magnetization of the Dy3+ ions that lie in the plane of the triangle at 120 degrees one from each other. This gives rise to a peculiar chiral nature of the ground nonmagnetic doublet and to slow relaxation of the magnetization with abrupt accelerations at the crossings of the discrete energy levels.
248 citations
TL;DR: In this article, the authors measured carrier relaxation times in epitaxial graphene layers grown on SiC wafers and found that an initial fast relaxation transient in the 70-120 fs range is followed by a slower relaxation process in the 0.4-1.7 ps range.
Abstract: Using ultrafast optical pump-probe spectroscopy, we have measured carrier relaxation times in epitaxial graphene layers grown on SiC wafers. We find two distinct time scales associated with the relaxation of nonequilibrium photogenerated carriers. An initial fast relaxation transient in the 70-120 fs range is followed by a slower relaxation process in the 0.4-1.7 ps range. The slower relaxation time is found to be inversely proportional to the degree of crystalline disorder in the graphene layers as measured by Raman spectroscopy. We relate the measured fast and slow time constants to carrier-carrier and carrier-phonon intraband and interband scattering processes in graphene.
242 citations
TL;DR: Two planar tetranuclear dysprosium(III) complexes, which exhibit an anion-dependent magnetic slow relaxation behavior, have been synthesized by in situ condensation of o-vanillin and 2-aminoethanol.
Abstract: Two planar tetranuclear dysprosium(III) complexes, [Dy4(μ3-OH)2(hmmpH)2(hmmp)2(Cl)4]·3MeCN·MeOH (1) and [Dy4(μ3-OH)2(hmmpH)2(hmmp)2(N3)4]·4MeOH (2) {hmmpH2 = 2-[(2-hydroxyethylimino)methyl]-6-methoxyphenol}, which exhibit an anion-dependent magnetic slow relaxation behavior, have been synthesized by in situ condensation of o-vanillin and 2-aminoethanol. The higher energy barrier observed in 2 could be the result of a more favorable crystal field and/or orientations of single-ion easy axes of magnetization of the DyIII ions.
234 citations
TL;DR: In this paper, the dielectric constants of water and heavy water were determined from 270 to 362 K and decomposed into four components: slow relaxation, fast relaxation, intermolecular stretching vibration, and intermolescular libration.
233 citations
TL;DR: An approach based on relaxation dispersion NMR spectroscopy in which structures of invisible, excited states are obtained from chemical shifts and residual anisotropic magnetic interactions opens up the possibility for detailed structural studies of invisible protein conformers at a level of detail that has heretofore been restricted to applications involving visible ground states of proteins.
Abstract: Molecular function is often predicated on excursions between ground states and higher energy conformers that can play important roles in ligand binding, molecular recognition, enzyme catalysis, and protein folding. The tools of structural biology enable a detailed characterization of ground state structure and dynamics; however, studies of excited state conformations are more difficult because they are of low population and may exist only transiently. Here we describe an approach based on relaxation dispersion NMR spectroscopy in which structures of invisible, excited states are obtained from chemical shifts and residual anisotropic magnetic interactions. To establish the utility of the approach, we studied an exchanging protein (Abp1p SH3 domain)–ligand (Ark1p peptide) system, in which the peptide is added in only small amounts so that the ligand-bound form is invisible. From a collection of 15N, 1HN, 13Cα, and 13CO chemical shifts, along with 1HN-15N, 1Hα-13Cα, and 1HN-13CO residual dipolar couplings and 13CO residual chemical shift anisotropies, all pertaining to the invisible, bound conformer, the structure of the bound state is determined. The structure so obtained is cross-validated by comparison with 1HN-15N residual dipolar couplings recorded in a second alignment medium. The methodology described opens up the possibility for detailed structural studies of invisible protein conformers at a level of detail that has heretofore been restricted to applications involving visible ground states of proteins.
TL;DR: The solvation of carbohydrates in N, N'-dialkylimidazolium ionic liquids (ILs) was investigated to confirm that the process is governed by the interactions between the IL anion and the carbohydrate, and indicate no change in the solvation mechanism regardless of the structure of the anion.
Abstract: The solvation of carbohydrates in N,N′-dialkylimidazolium ionic liquids (ILs) was investigated by means of 13C and 35/37Cl NMR relaxation and 1H pulsed field gradient stimulated echo (PFG-STE) diffusion measurements. Solutions of model sugars in 1-n-butyl-3-methylimidazolium chloride ([C4mim]Cl), 1-allyl-3-methylimidazolium chloride ([C═C2mim]Cl), and 1-ethyl-3-methylimidazolium acetate ([C2mim][OAc]) were studied to evaluate the effects of cation and anion structure on the solvation mechanism. In all cases, the changes in the relaxation times of carbon nuclei of the IL cations as a function of carbohydrate concentration are small and consistent with the variation in solution viscosities. Conversely, the 35/37Cl and 13C relaxation rates of chloride ions and acetate ion carbons, respectively, have a strong dependency on sugar content. For [C2mim][OAc], the correlation times estimated from 13C relaxation data for both ions reveal that, as the carbohydrate concentration increases, the reorientation rate of t...
TL;DR: The water orientational relaxation in the 4 nm diameter nanopools of the two types of reverse micelles is almost identical, which indicates that confinement by an interface to form a nanoscopically confined water pool is a primary factor governing the dynamics of nanoscopic water rather than the presence of charged groups at the interface.
Abstract: Hydrogen bond dynamics of water in highly concentrated NaBr salt solutions and reverse micelles are studied using ultrafast 2D-IR vibrational echo spectroscopy and polarization-selective IR pump−probe experiments performed on the OD hydroxyl stretch of dilute HOD in H2O. The vibrational echo experiments measure spectral diffusion, and the pump−probe experiments measure orientational relaxation. Both experimental observables are directly related to the structural dynamics of water's hydrogen bond network. The measurements performed on NaBr solutions as a function of concentration show that the hydrogen bond dynamics slow as the NaBr concentration increases. The most pronounced change is in the longest time scale dynamics which are related to the global rearrangement of the hydrogen bond structure. Complete hydrogen bond network randomization slows by a factor of ∼3 in ∼6 M NaBr solution compared to that in bulk water. The hydrogen bond dynamics of water in nanoscopically confined environments are studied b...
TL;DR: The accuracy of the extracted chemical shifts opens up the possibility of obtaining quantitative structural information of invisible states of the sort that is now available from chemical shifts recorded on ground states of proteins.
Abstract: Carr-Purcell-Meiboom-Gill relaxation dispersion NMR spectroscopy has evolved into a powerful approach for the study of low populated, invisible conformations of biological molecules. One of the powerful features of the experiment is that chemical shift differences between the exchanging conformers can be obtained, providing structural information about invisible excited states. Through the development of new labeling approaches and NMR experiments it is now possible to measure backbone 13C(alpha) and 13CO relaxation dispersion profiles in proteins without complications from 13C-13C couplings. Such measurements are presented here, along with those that probe exchange using 15N and 1HN nuclei. A key experimental design has been the choice of an exchanging system where excited-state chemical shifts were known from independent measurement. Thus it is possible to evaluate quantitatively the accuracy of chemical shift differences obtained in dispersion experiments and to establish that in general very accurate values can be obtained. The experimental work is supplemented by computations that suggest that similarly accurate shifts can be measured in many cases for systems with exchange rates and populations that fall within the range of those that can be quantified by relaxation dispersion. The accuracy of the extracted chemical shifts opens up the possibility of obtaining quantitative structural information of invisible states of the sort that is now available from chemical shifts recorded on ground states of proteins.
TL;DR: This study sets the stage for robust theoretical work on Abeta (1-40) and Abeta(1-42), for which collection of detailed NMR data on the monomer will be more challenging because of aggregation and fibril formation on experimental timescales at physiological conditions.
Abstract: We combine molecular dynamics simulations and new high-field NMR experiments to describe the solution structure of the Abeta(21-30) peptide fragment that may be relevant for understanding structural mechanisms related to Alzheimer's disease. By using two different empirical force-field combinations, we provide predictions of the three-bond scalar coupling constants ((3)J(H(N)H(alpha))), chemical-shift values, (13)C relaxation parameters, and rotating-frame nuclear Overhauser effect spectroscopy (ROESY) crosspeaks that can then be compared directly to the same observables measured in the corresponding NMR experiment of Abeta(21-30). We find robust prediction of the (13)C relaxation parameters and medium-range ROESY crosspeaks by using new generation TIP4P-Ew water and Amber ff99SB protein force fields, in which the NMR validates that the simulation yields both a structurally and dynamically correct ensemble over the entire Abeta(21-30) peptide. Analysis of the simulated ensemble shows that all medium-range ROE restraints are not satisfied simultaneously and demonstrates the structural diversity of the Abeta(21-30) conformations more completely than when determined from the experimental medium-range ROE restraints alone. We find that the structural ensemble of the Abeta(21-30) peptide involves a majority population (approximately 60%) of unstructured conformers, lacking any secondary structure or persistent hydrogen-bonding networks. However, the remaining minority population contains a substantial percentage of conformers with a beta-turn centered at Val24 and Gly25, as well as evidence of the Asp23 to Lys28 salt bridge important to the fibril structure. This study sets the stage for robust theoretical work on Abeta(1-40) and Abeta(1-42), for which collection of detailed NMR data on the monomer will be more challenging because of aggregation and fibril formation on experimental timescales at physiological conditions. In addition, we believe that the interplay of modern molecular simulation and high-quality NMR experiments has reached a fruitful stage for characterizing structural ensembles of disordered peptides and proteins in general.
TL;DR: The strong tendency of ibuprofen to form hydrogen bonded aggregates such as dimers and trimers either cyclic or linear which seems to control in particular the molecular mobility of ib uprofen was confirmed by IR spectroscopy, electrospray ionization mass spectrometry, and MD simulations.
Abstract: The molecular mobility of amorphous ibuprofen has been investigated by broadband dielectric relaxation spectroscopy (DRS) covering a temperature range of more than 200 K. Four different relaxation processes, labeled as R, � , γ, and D, were detected and characterized, and a complete relaxation map was given for the first time. The γ-process has activation energy Ea ) 31 kJ ·mol -1 , typical for local mobility. The weak � -relaxation, observed in the glassy state as well as in the supercooled state was identified as the genuine Johari-Goldstein process. The temperature dependence of the relaxation time of the R-process (dynamic glass transition) does not obey a single VFTH law. Instead two VFTH regimes are observed separated by a crossover temperature, TB ) 265 K. From the low temperature VFTH regime, a Tg diel (τ )100 s) ) 226 K was estimated, and a fragility or steepness index m ) 93, was calculated showing that ibuprofen is a fragile glass former. The D-process has a Debye-like relaxation function but the temperature dependence of relaxation time also follows the VFTH behavior, with a Vogel temperature and a pre-exponential factor which seem to indicate that its dynamics is governed by the R-process. It has similar features as the Debye-type process observed in a variety of associating liquids, related to hydrogen bonding dynamics. The strong tendency of ibuprofen to form hydrogen bonded aggregates such as dimers and trimers either cyclic or linear which seems to control in particular the molecular mobility of ibuprofen was confirmed by IR spectroscopy, electrospray ionization mass spectrometry, and MD simulations.
TL;DR: An amphiphilic gadolinium (III) chelate (GdL) was synthesized from commercially available stearic acid and a negative contrast could be well observed after injection of a suspension of functionalized nanotubes into the muscle of the leg of the mouse.
Abstract: An amphiphilic gadolinium (III) chelate (GdL) was synthesized from commercially available stearic acid. Aqueous solutions of the complex at different concentrations (from 1 mM to 1 μM) were prepared and adsorbed on multiwalled carbon nanotubes. The resulting suspensions were stable for several days and have been characterized with regard to magnetic resonance imaging (MRI) contrast agent applications. Longitudinal water proton relaxivities, r1, have been measured at 20, 300, and 500 MHz. The r1 values show a strong dependence on the GdL concentration, particularly at low field. The relaxivities decrease with increasing field as it is predicted by the Solomon−Bloembergen−Morgan theory. Transverse water proton relaxation times, T2, have also been measured and are practically independent of both the frequency and the GdL concentration. An in vivo feasibility MRI study has been performed at 300 MHz in mice. A negative contrast could be well observed after injection of a suspension of functionalized nanotubes ...
TL;DR: In this article, the authors characterized the dynamic nuclear polarization (DNP) at 3.35 T and 1.2 K for pyruvic acid and the paramagnetic agent OX063Me and reported that the addition of small quantities of Gd3+ affects the DNP favorably.
Abstract: Dynamic nuclear polarization (DNP) at 3.35 T and 1.2 K is characterized for [1-13C]pyruvic acid and the paramagnetic agent OX063Me. This paramagnetic agent belongs to a class of organic radicals (trityls) with unique properties for DNP. [1-13C]Pyruvic acid is a molecule of particular relevance for in vivo metabolic studies, and was chosen for that reason. The studies in this work support the conclusion that DNP is through the thermal mixing effect. Critical parameters for DNP are studied: the electron spin resonance spectrum and relaxation times, the nuclear relaxation time and the microwave frequency dependence of the DNP enhancement. In particular, it is reported that the addition of small quantities of Gd3+ affects the DNP favorably. Of the studied parameters only the trityl electron longitudinal relaxation time is significantly affected by the presence of Gd3+. The current models for DNP by the thermal mixing effect, the high-temperature Provotorov equations and the low-temperature Borghini model, are unable to quantitatively explain the observed DNP.
TL;DR: The results set an important precedent for extremely rapid, formally forbidden (DeltaS = 2) nonradiative relaxation as well as defining the time scale for intramolecular optical switching between two electronic states possessing vastly different spectroscopic, geometric, and magnetic properties.
Abstract: A combination of femtosecond electronic absorption and stimulated Raman spectroscopies has been employed to determine the kinetics associated with low-spin to high-spin conversion following charge-transfer excitation of a FeII spin-crossover system in solution. A time constant of τ = 190 ± 50 fs for the formation of the 5T2 ligand-field state was assigned based on the establishment of two isosbestic points in the ultraviolet in conjunction with changes in ligand stretching frequencies and Raman scattering amplitudes; additional dynamics observed in both the electronic and vibrational spectra further indicate that vibrational relaxation in the high-spin state occurs with a time constant of ca. 10 ps. The results set an important precedent for extremely rapid, formally forbidden (ΔS = 2) nonradiative relaxation as well as defining the time scale for intramolecular optical switching between two electronic states possessing vastly different spectroscopic, geometric, and magnetic properties.
TL;DR: The periodic shell structure and surface reconstruction of metallic FePt nanoparticles with icosahedral structure has been quantitatively studied by high-resolution transmission electron microscopy with focal series reconstruction with sub-angstrom resolution and finds the lattice spacing of (111) planes in the surface region to be size dependent.
Abstract: The periodic shell structure and surface reconstruction of metallic FePt nanoparticles with icosahedral structure has been quantitatively studied by high-resolution transmission electron microscopy with focal series reconstruction with sub-angstrom resolution. The icosahedral FePt nanoparticles fabricated by the gas phase condensation technique in vacuum have been found to be surprisingly oxidation resistant and stable under electron beam irradiation. We find the lattice spacing of (111) planes in the surface region to be size dependent and to expand by as much as 9% with respect to the bulk value of Fe52Pt48. Controlled removal of the (111) surface layers in situ results in a similar outward relaxation of the new surface layer. This unusually large layerwise outward relaxation is discussed in terms of preferential Pt segregation to the surface forming a Pt enriched shell around a Fe-rich Fe/Pt core.
TL;DR: In this article, the formation of nanocomposites and changes in the structural properties of the materials were investigated by X-ray diffraction (XRD) analysis and complex impedance analysis.
Abstract: Structural, thermal and electrical behavior of polymer-clay nanocomposite electrolytes consisting of polymer (polyethylene oxide (PEO)) and NaI as salt with different concentrations of organically modified Na + montmorillonite (DMMT) filler have been investigated. The formation of nanocomposites and changes in the structural properties of the materials were investigated by X-ray diffraction (XRD) analysis. Complex impedance analysis shows the existence of bulk and material-electrode interface properties of the composites. The relative dielectric constant (er) decreases with increase in frequency in the low frequency region whereas frequency independent behavior is observed in the high frequency region. The electrical modulus representation shows a loss feature in the imaginary component. The relaxation associated with this feature shows a stretched exponential decay. Studies of frequency dependence of dielectric and modulus formalism suggest that the ionic and polymer segmental motion are strongly coupled manifeasting as peak in the modulus (M") spectra with no corresponding feature in dielectric spectra. The frequency dependence of ac (alternating current) conductivity obeys Jon- scher power law feature in the high frequency region, where as the low frequency dispersion indicating the presence of electrode polarization effect in the materials.
TL;DR: In this paper, the authors derived closed-form solutions to the relaxation oscillation (resonance) frequency and damping term, as well as the low-frequency damping terms, and discussed design rules for maximizing resonance frequency and broadband performance.
Abstract: The modulation response of injection-locked lasers has been carefully analyzed, theoretically and experimentally, with a focus on the strong optical injection regime. We derive closed-form solutions to the relaxation oscillation (resonance) frequency and damping term, as well as the low-frequency damping term, and discuss design rules for maximizing resonance frequency and broadband performance. A phasor model is described in order to better explain the enhancement of the resonance frequency. Experimental curves match closely to theory. Record resonance frequency of 72 GHz and broadband results are shown.
TL;DR: In this paper, the properties of nanotubes obtained from boron α-sheet were investigated using density functional theory, which confirmed their high stability and identified mechanical stiffness.
Abstract: We investigate the properties of nanotubes obtained from recently described boron α-sheet, using density functional theory. Computations confirm their high stability and identify mechanical stiffne...
TL;DR: In this paper, the Langevin spin dynamics and fluctuation-dissipation theorem were combined to perform microcanonical ensemble simulations of adiabatic spin-lattice relaxation of periodic arrays of 180° domain walls.
Abstract: combined application of the Langevin spin dynamics and the fluctuation-dissipation theorem. We investigate several applications of the method, performing microcanonical ensemble simulations of adiabatic spin-lattice relaxation of periodic arrays of 180° domain walls, and isothermal-isobaric ensemble dynamical simulations of thermally equilibrated homogeneous systems at various temperatures. The predicted isothermal magnetization curve agrees well with the experimental data for a broad range of temperatures. The equilibrium as well as time-correlation functions of spin orientations exhibit the presence of short-range magnetic order above the Curie temperature. Furthermore, short-range order spin fluctuations are shown to contribute to the thermal expansion of the material. Our analysis illustrates the significant part played by the spin degrees of freedom in the dynamics of motion of atoms in magnetic iron and iron-based alloys. It also shows that the spin-lattice dynamics algorithm developed in this paper offers a viable way of performing large-scale dynamical atomistic simulations of magnetic materials.
TL;DR: In this article, it was shown that at the glass transition temperature, the size of the domains and the configurational entropy per unit volume are anticorrelated, as originally predicted by the Adam-Gibbs theory.
Abstract: When a liquid is cooled below its melting temperature, if crystallization is avoided, it forms a glass. This phenomenon, called glass transition, is characterized by a marked increase of viscosity, about 14 orders of magnitude, in a narrow temperature interval. The microscopic mechanism behind the glass transition is still poorly understood. However, recently, great advances have been made in the identification of cooperative rearranging regions, or dynamical heterogeneities, i.e., domains of the liquid whose relaxation is highly correlated. The growth of the size of these domains is now believed to be the driving mechanism for the increase of the viscosity. Recently a tool to quantify the size of these domains has been proposed. We apply this tool to a wide class of materials to investigate the correlation between the size of the heterogeneities and their configurational entropy, i.e., the number of states accessible to a correlated domain. We find that the relaxation time of a given system, apart from a material dependent prefactor, is a universal function of the configurational entropy of a correlated domain. As a consequence, we find that, at the glass transition temperature, the size of the domains and the configurational entropy per unit volume are anticorrelated, as originally predicted by the Adam-Gibbs theory. Finally, we use our data to extract some exponents defined in the framework of the random first-order theory, a recent quantitative theory of the glass transition.
TL;DR: In this paper, the carrier relaxation process during photoconduction in quasi-one-dimensional (Q1D) ZnO nanowires (NWs) was investigated using photocurrent transient measurements.
Abstract: We have investigated the carrier relaxation process during photoconduction in quasi-one-dimensional (Q1D) ZnO nanowires (NWs) of diameters 29–36nm on different substrates using photocurrent transient measurements. Ultraviolet (UV) sensitive NWs show around three to four orders of change in the photo-to-dark current ratio. Under steady UV illumination, the photocarrier relaxation occurs through two-electron process—carrier loss due to the trapping by the surface states and recombination at the deep defect states. The results demonstrate that the carrier relaxation during photoconduction in Q1D NWs of diameter comparable to the Debye length is also dominated by the surface states.
TL;DR: In this paper, the authors investigated the low-order equal-time statistics of a barotropic flow on a rotating sphere and showed that the flow is driven by linear relaxation toward an unstable zonal jet.
Abstract: Low-order equal-time statistics of a barotropic flow on a rotating sphere are investigated. The flow is driven by linear relaxation toward an unstable zonal jet. For relatively short relaxation times, the flow is dominated by critical-layer waves. For sufficiently long relaxation times, the flow is turbulent. Statistics obtained from a second-order cumulant expansion are compared to those accumulated in direct numerical simulations, revealing the strengths and limitations of the expansion for different relaxation times.
TL;DR: The dynamics of molecular liquids and polymers exhibit various "transitions" associated with characteristic changes in properties, such as the onset of intermolecular cooperativity with consequent non-Arrhenius and non-Debye behavior.
Abstract: The dynamics of molecular liquids and polymers exhibit various “transitions”, associated with characteristic changes in properties. With decreasing temperature or increasing pressure, these transitions include (i) the onset of intermolecular cooperativity with consequent non-Arrhenius and non-Debye behavior; (ii) the dynamic crossover at which derivatives of the relaxation time and strength exhibit breaks; (iii) vitrification, corresponding to cessation of translational and rotational motions; and (iv) for anisotropic molecules the development of liquid crystallinity. At each of these transitions of a liquid, the structural or reorientational relaxation time is constant, independent of thermodynamic conditions; that is, while the temperature of the transition depends on pressure, the relaxation does not.
TL;DR: Broadband dielectric and terahertz spectroscopy are combined with pulsed field gradient nuclear magnetic resonance (PFG-NMR) to explore charge transport and translational diffusion in the 1-butyl-3-methylimidazolium tetrafluoroborate ionic liquid.
Abstract: Broadband dielectric and terahertz spectroscopy (10−2–10+12Hz) are combined with pulsed field gradient nuclear magnetic resonance (PFG-NMR) to explore charge transport and translational diffusion in the 1-butyl-3-methylimidazolium tetrafluoroborate ionic liquid. The dielectric spectra are interpreted as superposition of high-frequency relaxation processes associated with dipolar librations and a conductivity contribution. The latter originates from hopping of charge carriers on a random spatially varying potential landscape and quantitatively fits the observed frequency and temperature dependence of the spectra. A further analysis delivers the hopping rate and enables one to deduce—using the Einstein–Smoluchowski equation—the translational diffusion coefficient of the charge carriers in quantitative agreement with PFG-NMR measurements. By that, the mobility is determined and separated from the charge carrier density; for the former, a Vogel–Fulcher–Tammann and for the latter, an Arrhenius temperature depe...
TL;DR: The decay of the electronic excitation to its ground state is simulated in the (6,4) semiconducting CNT with surface hopping in the Kohn-Sham representation, providing a unique time-domain atomistic description of fluorescence quenching.
Abstract: As shown experimentally, strong nonradiative decay channels exist in carbon nanotubes (CNT) and are responsible for low fluorescence yields. The decay of the electronic excitation to its ground state is simulated in the (6,4) semiconducting CNT with surface hopping in the Kohn-Sham representation, providing a unique time-domain atomistic description of fluorescence quenching. The decay in the ideal CNT is estimated to occur on a 150 ps time scale and is only weakly dependent on temperature. Vibrationally induced decoherence strongly influences the electronic relaxation. Defects decrease the excited state lifetime to tens of picoseconds, rationalizing the multiple decay time scales seen in experiments.