Showing papers on "Relaxation (NMR) published in 2004"
TL;DR: T1 relaxation times are generally higher and T2 relaxation time are generally lower at 3.0 T than at 1.5 T, but the magnitude of change varies greatly in different tissues, including liver, kidney, spleen, and muscle tissue.
Abstract: PURPOSE: To measure T1 and T2 relaxation times of normal human abdominal and pelvic tissues and lumbar vertebral bone marrow at 3.0 T. MATERIALS AND METHODS: Relaxation time was measured in six healthy volunteers with an inversion-recovery method and different inversion times and a multiple spin-echo (SE) technique with different echo times to measure T1 and T2, respectively. Six images were acquired during one breath hold with a half-Fourier acquisition single-shot fast SE sequence. Signal intensities in regions of interest were fit to theoretical curves. Measurements were performed at 1.5 and 3.0 T. Relaxation times at 1.5 T were compared with those reported in the literature by using a one-sample t test. Differences in mean relaxation time between 1.5 and 3.0 T were analyzed with a two-sample paired t test. RESULTS: Relaxation times (mean ± SD) at 3.0 T are reported for kidney cortex (T1, 1,142 msec ± 154; T2, 76 msec ± 7), kidney medulla (T1, 1,545 msec ± 142; T2, 81 msec ± 8), liver (T1, 809 msec ± 7...
750 citations
TL;DR: This work identifies the class of secondary relaxations that bears a strong connection or correlation to the primary relaxation in all the dynamic properties and proposes that only these should be called the Johari-Goldstein beta-relaxation.
Abstract: Dynamic properties, derived from dielectric relaxation spectra of glass-formers at variable temperature and pressure, are used to characterize and classify any resolved or unresolved secondary relaxation based on their different behaviors. The dynamic properties of the secondary relaxation used include: (1) the pressure and temperature dependences; (2) the separation between its relaxation time τβ and the primary relaxation time τα at any chosen τα; (3) whether τβ is approximately equal to the independent (primitive) relaxation time τ0 of the coupling model; (4) whether both τβ and τ0 have the same pressure and temperature dependences; (5) whether it is responsible for the “excess wing” of the primary relaxation observed in some glass-formers; (6) how the excess wing changes on aging, blending with another miscible glass-former, or increasing the molecular weight of the glass-former; (7) the change of temperature dependence of its dielectric strength Δeβ and τβ across the glass transition temperature Tg; ...
715 citations
TL;DR: The Dy compound has been found to give out-of-phase AC susceptibility signals, suggesting it might be a single-molecule magnet (SMM), which is the initial demonstration of hysteresis and quantum behavior in a mixed 3d/4f SMM.
Abstract: The preparation of a new family of mixed transition metal/lanthanide clusters is reported. The reaction of [Mn3O(O2CPh)6(py)2(H2O)] with Ln(NO3)3 (Ln = Nd, Gd, Dy, Ho, and Eu) in a 1:2 molar ratio in MeOH/MeCN (1:20 v/v) leads to dark crystals in 55−60% isolated yield of complexes all containing the [Mn11Ln4]45+ core. The Dy compound has been found to give out-of-phase AC susceptibility signals, suggesting it might be a single-molecule magnet (SMM). This was confirmed by the observation of magnetization hysteresis loops. An Arrhenius plot constructed from magnetization decay data gave a barrier to relaxation of 9.3 K and showed the temperature-independent relaxation at very low temperatures indicative of quantum tunneling of magnetization. This is the initial demonstration of hysteresis and quantum behavior in a mixed 3d/4f SMM.
481 citations
TL;DR: In this article, the specific absorption rate (SAR) values of aqueous suspensions of magnetite particles with different diameters varying from 7.5 to 416nm were investigated by measuring the time-dependent temperature curves in an external alternating magnetic field (80 kHz, 32.5 kA/m).
473 citations
TL;DR: In this article, the decay rate of single colloidal CdSe quantum dots is measured by selecting only those photons collected while the single quantum dot emission intensity was near its maximum.
Abstract: We present measurements of photoluminescence decay dynamics from single colloidal CdSe quantum dots. We find that the decays fluctuate in time with decay rates that correlate with time-averaged emission intensities. Moreover, the decays measured by selecting only those photons collected while the single quantum dot emission intensity was near its maximum yields single-exponential dynamics. We find that the “maximum-intensity” decays are nearly identical across different independently synthesized samples of nearly the same size. The combination of single-exponential kinetics and decays that are reproducible across samples leads us to speculate that it is the radiative lifetime that is measured and that the quantum yield of a single dot near its maximum emission intensity is close to unity. The variations in decay rates with time and their correlation with emission intensity indicate these intensity time trajectories primarily reflect fluctuations in nonradiative relaxation pathways.
467 citations
TL;DR: In this article, an alternating current (ac) magnetic susceptibility and magnetization hysteresis loop measurements have been carried out for anionic bis(phthalocyaninato)terbium and bis (phthalocyclichenato)dysprosium and it has been found that the magnetization relaxation in the Tb complex is dominated by the two-phonon Orbach process in the temperature range 25−40 K and direct or Raman process below 25 K.
Abstract: Alternating current (ac) magnetic susceptibility and magnetization hysteresis loop measurements have been carried out for anionic bis(phthalocyaninato)terbium and bis(phthalocyaninato)dysprosium. The two mononuclear lanthanide complexes show the characteristic temperature and frequency dependence in the ac susceptibility signals, reflecting their slow magnetization relaxation. From the Arrhenius analysis of the ac susceptibility data obtained for a diluted sample in a diamagnetic matrix, it has been found that the magnetization relaxation in the Tb complex is dominated by the two-phonon Orbach process in the temperature range 25−40 K and direct or Raman process below 25 K. In the Dy complex case, the Orbach process is the main relaxation process in the range 3−12 K. The Δ values in the Orbach term, corresponding to the height of the potential energy barrier to magnetic moment reversal, are in good agreement with the energy differences between the lowest and second lowest substates of the ground multiplet ...
429 citations
TL;DR: In this article, a generalized stochastic Landau-Lifshitz equation and its corresponding Fokker-Planck equation were proposed for the magnetization dynamics in the presence of spin-transfer torques.
Abstract: We propose a generalized stochastic Landau-Lifshitz equation and its corresponding Fokker-Planck equation for the magnetization dynamics in the presence of spin-transfer torques. Since the spin-transfer torque can pump a magnetic energy into the magnetic system, the equilibrium temperature of the magnetic system is ill defined. We introduce an effective temperature based on a stationary solution of the Fokker-Planck equation. In the limit of high-energy barriers, the law of thermal agitation is derived. We find that the N\'eel-Brown relaxation formula remains valid as long as we replace the temperature by an effective one that is linearly dependent on the spin torque. We carry out the numerical integration of the stochastic Landau-Lifshitz equation to support our theory. Our results agree with existing experimental data.
378 citations
TL;DR: The results support the idea that the surface contains, on average, far more reactive states than the bulk, and the average value of the dipole decreases and the highest occupied molecular orbital for each water molecule increases approaching the liquid-vapor interface.
Abstract: We present an ab initio molecular dynamics simulation of the aqueous liquid-vapor interface. Having successfully stabilized a region of bulk water in the center of a water slab, we were able to reproduce and further quantify the experimentally observed abundance of surface “acceptor-only”(19%) and “single-donor”(66%) moieties as well as substantial surface relaxation approaching the liquid-vapor interface. Examination of the orientational dynamics points to a faster relaxation in the interfacial region. Furthermore, the average value of the dipole decreases and the average value of the highest occupied molecular orbital for each water molecule increases approaching the liquid-vapor interface. Our results support the idea that the surface contains, on average, far more reactive states than the bulk.
331 citations
TL;DR: In this paper, femtosecond time-resolved photoelectron spectra (TRPES) of the DNA and RNA bases adenine, cytosine, thymine, and uracil in a molecular beam were presented.
Abstract: We present femtosecond time-resolved photoelectron spectra (TRPES) of the DNA and RNA bases adenine, cytosine, thymine, and uracil in a molecular beam. We discuss in detail the analysis of our adenine TRPES spectra. A global two-dimensional fit of the time and energy-resolved spectra allows for reliable separation of photoelectron spectra from several channels, even for overlapping bands. Ab initio calculations of Koopmans’ ionization correlations and He(I) photoelectron spectra aid the assignment of electronically excited states involved in the relaxation dynamics. Based upon our results, we propose the following mechanism for electronic relaxation dynamics in adenine: Pump wavelengths of 250, 267 and 277 nm lead to initial excitation of the bright S2(pp*) state. Close to the band origin (277 nm), the lifetime is several picoseconds. At higher vibronic levels, i.e. 250 and 267 nm excitation, rapid internal conversion (t < 50 fs) populates the lower lying S1(np*) state which has a lifetime of 750 fs. At 267 nm, we found evidence for an additional channel which is consistent with the dissociative S3(ps*) state, previously proposed as an ultrafast relaxation pathway from S2(pp*). We present preliminary results from TRPES measurements of the other DNA bases at 250 nm excitation.
318 citations
TL;DR: In this article, the phonon thermal conductivity of the Lennard-Jones argon face-centered cubic crystal is predicted between temperatures of 20 K and 80 K using the Boltzmann transport equation under the single-mode relaxation time approximation.
Abstract: The phonon thermal conductivity of the Lennard-Jones argon face-centered cubic crystal is predicted between temperatures of 20 K and 80 K using the Boltzmann transport equation under the single-mode relaxation time approximation. The temperature and frequency dependencies of the phonon dispersion and phonon relaxation times are obtained from lattice-dynamics calculations based on the results of molecular-dynamics simulations. No fitting parameters are required. The predicted thermal conductivities are in reasonable agreement with independent predictions made from the simulations using the Green-Kubo method. The assumption of an isotropic medium, as used in the Boltzmann transport equation formulation, leads to an overprediction of the Green-Kubo results at low temperatures. At higher temperatures, where anharmonic effects become increasingly important, the harmonic nature of the relaxation time calculation method leads to an underprediction of the Green-Kubo results. Assuming that the low-frequency behavior of the relaxation times can be extended over the entire frequency range, that there is no dispersion, or that the dispersion is independent of temperature, leads to significant errors in the predictions. This finding indicates that in analytical calculations, where such assumptions are often made, these errors are offset by the use of fitting parameters.
292 citations
TL;DR: In this article, a vector alkali-metal magnetometer that simultaneously and independently measures all three components of the magnetic field was described. But the magnetometer was designed for a laboratory environment without magnetic shields, and it was shown that magnetic field sensitivity on the order of 1pT∕Hz was achieved.
Abstract: We describe a vector alkali–metal magnetometer that simultaneously and independently measures all three components of the magnetic field. Using a feedback system, the total field at the location of the magnetometer is kept near zero, suppressing the broadening due to spin-exchange collisions. The resonance linewidth and signal strength of the magnetometer compare favorably with two different scalar operation modes in which spin-exchange relaxation is only partially suppressed. Magnetic field sensitivity on the order of 1pT∕Hz is demonstrated in a laboratory environment without magnetic shields.
TL;DR: It is shown that the existence of a distribution of energetically different molecules in the ground state coupled with a low rate of the excited-state relaxation processes, viz., solvation and energy transfer, are responsible for the excitation-wavelength-dependent fluororescence behavior in three room-temperature ionic liquids.
Abstract: The fluorescence behavior of several dipolar molecules has been studied in three room-temperature ionic liquids, namely, [BMIM][BF4], [EMIM][BF4], and [BMIM][PF6], as a function of the excitation wavelength. Although a large majority of these systems show normal fluorescence behavior with no excitation wavelength dependence, a few systems surprisingly exhibit fairly strong excitation-wavelength-dependent fluorescence behavior in these media. The excitation-wavelength-dependent shift of the fluorescence maximum is measured to be between 10 and 35 nm. The various fluorescence parameters of the systems have been carefully examined to determine the factors that contribute to this kind of behavior, generally not observed in conventional media. It is shown that the existence of a distribution of energetically different molecules in the ground state coupled with a low rate of the excited-state relaxation processes, viz., solvation and energy transfer, are responsible for the excitation-wavelength-dependent fluor...
TL;DR: It is shown that the Rh environment is more similar to the “gas phase” than to the solution environment and that the mechanism of the ∼30 kcal·mol-1 photon energy storage observed for Rh is not consistent with a model based exclusively on the change of the electrostatic interaction of the chromophore with the protein/counterion environment.
Abstract: We demonstrate that a "brute force" quantum chemical calculation based on an ab initio multiconfigurational second order perturbation theory approach implemented in a quantum mechanics/molecular mechanics strategy can be applied to the investigation of the excited state of the visual pigment rhodopsin (Rh) with a computational error <5 kcal.mol(-1). As a consequence, the simulation of the absorption and fluorescence of Rh and its retinal chromophore in solution allows for a nearly quantitative analysis of the factors determining the properties of the protein environment. More specifically, we demonstrate that the Rh environment is more similar to the "gas phase" than to the solution environment and that the so-called "opsin shift" originates from the inability of the solvent to effectively "shield" the chromophore from its counterion. The same strategy is used to investigate three transient structures involved in the photoisomerization of Rh under the assumption that the protein cavity does not change shape during the reaction. Accordingly, the analysis of the initially relaxed excited-state structure, the conical intersection driving the excited-state decay, and the primary isolable bathorhodopsin intermediate supports a mechanism where the photoisomerization coordinate involves a "motion" reminiscent of the so-called bicycle-pedal reaction coordinate. Most importantly, it is shown that the mechanism of the approximately 30 kcal.mol(-1) photon energy storage observed for Rh is not consistent with a model based exclusively on the change of the electrostatic interaction of the chromophore with the protein/counterion environment.
TL;DR: In this article, a spin-relaxation filter was used to separate the 1H nuclear magnetic resonance (NMR) resonances based on their relaxation times and allow simplification of the complex NMR spectra.
TL;DR: By resolving the binding energy of the excess electron in real time with femtosecond resolution, this work captured the ultrafast dynamics of the electron in the presolvated (“wet”) and hydrated states and obtained, as a function of cluster size, the subsequent relaxation times.
Abstract: We directly observed the hydration dynamics of an excess electron in the finite-sized water clusters of (H_2O)^-_n with n = 15, 20, 25, 30, and 35. We initiated the solvent motion by exciting the hydrated electron in the cluster. By resolving the binding energy of the excess electron in real time with femtosecond resolution, we captured the ultrafast dynamics of the electron in the presolvated (“wet”) and hydrated states and obtained, as a function of cluster size, the subsequent relaxation times. The solvation time (300 femtoseconds) after the internal conversion [140 femtoseconds for (H_2O)^-_35] was similar to that of bulk water, indicating the dominant role of the local water structure in the dynamics of hydration. In contrast, the relaxation in other nuclear coordinates was on a much longer time scale (2 to 10 picoseconds) and depended critically on cluster size.
TL;DR: In this article, the massively parallelized full-potential linearized augmented plane-wave bulk and film program FLEUR for first-principles calculations in the context of density functional theory was adapted to allow calculations of materials with complex magnetic structures.
Abstract: The massively parallelized full-potential linearized augmented plane-wave bulk and film program FLEUR for first-principles calculations in the context of density functional theory was adapted to allow calculations of materials with complex magnetic structures---i.e., with noncollinear spin arrangements and incommensurate spin spirals. The method developed makes no shape approximation to the charge density and works with the continuous vector magnetization density in the interstitial and vacuum region and a collinear magnetization density in the spheres. We give an account of the implementation. Important technical aspects, such as the formulation of a constrained local moment method in a full-potential method that works with a vector magnetization density to deal with specific preselected nonstationary-state spin configurations, the inclusion of the generalized gradient approximation in a noncollinear framework, and the spin-relaxation method are discussed. The significance and validity of different approximations are investigated. We present examples to the various strategies to explore the magnetic ground state, metastable states, and magnetic phase diagrams by relaxation of spin arrangements or by performing calculations for constraint spin configurations to invest the functional dependence of the total energy and magnetic moment with respect to external parameters.
TL;DR: In this paper, the electrical properties of polycrystalline samples of (Ba1−xSrx)(Zr0.05Ti0.95)O3 (x=0, 0.03,0.06 and 0.09) (BSZT) were prepared by a high-temperature solid-state reaction technique.
TL;DR: In this article, the specific loss power (SLP) of aqueous ferrofluids with carboxydextran was investigated with respect to their SLP in dependence on frequency and field amplitude of magnetic AC-fields.
TL;DR: In this paper, three basic classes of polymer dynamics models, namely the Rouse model, the tube/reptation model, and the renormalized Rouse model are outlined and discussed with respect to predictions for NMR measurands.
Abstract: The universal features of polymer dynamics are specifically represented by laws for (anomalous) segment diffusion and chain relaxation modes. Nuclear magnetic resonance (NMR)-based techniques provide direct access to these phenomena. This in particular refers to NMR relaxation and diffusion studies. Methods suitable for this purpose are described in detail. Three basic classes of polymer dynamics models, namely the Rouse model, the tube/reptation model, and the renormalized Rouse models are outlined and discussed with respect to predictions for NMR measurands. A wealth of experimental NMR data are reviewed and compared with predictions of the model theories. It is shown that characteristic features of all three types of models can be verified in great detail provided that the model premisses are suitably mimicked in the experiments. Rouse dynamics is shown to be relevant for polymer melts with molecular weights below the critical value and for solutions of diminished entanglement effect. Features specific for the renormalized Rouse model reveal themselves in the form of high- and low-mode-number limits of the spin–lattice relaxation dispersion. These results are considered to mirror the analytical structure of the Generalized Langevin Equation. Finally, anomalous-diffusion and relaxation laws characteristic for the tube/reptation model can be perfectly reproduced in experiment if the polymer chains are confined in a nanoporous, solid matrix whereas bulk melts are not in accord with these predictions. The dynamics of chains confined in artificial tubes can be treated analytically assuming a harmonic radial potential for the polymer/wall interaction. These results derived for a real tube closely render the characteristic features of the original Doi/Edwards model predicted for a fictitious tube.
TL;DR: In this article, the Glauber dynamics is studied in a single-chain magnet and a single relaxation mode of the magnetization is found, which is in perfect agreement with independent thermodynamical measurements.
Abstract: The Glauber dynamics is studied in a single-chain magnet. As predicted, a single relaxation mode of the magnetization is found. Above 2.7 K, the thermally activated relaxation time is mainly governed by the effect of magnetic correlations and the energy barrier experienced by each magnetic unit. This result is in perfect agreement with independent thermodynamical measurements. Below 2.7 K, a crossover towards a relaxation regime is observed that is interpreted as the manifestation of finite-size effects. The temperature dependences of the relaxation time and of the magnetic susceptibility reveal the importance of the boundary conditions.
TL;DR: In this paper, a technique for detecting magnetically labeled Listeria monocytogenes and measuring the binding rate between antibody-linked magnetic particles and bacteria was demonstrated, using a superconducting quantum interference device, an extremely sensitive detector of magnetic flux.
Abstract: We demonstrate a technique for detecting magnetically labeled Listeria monocytogenes and for measuring the binding rate between antibody-linked magnetic particles and bacteria. This sensitive assay quantifies specific bacteria in a sample without the need to immobilize them or wash away unbound magnetic particles. In the measurement, we add 50-nm-diameter superparamagnetic magnetite particles, coated with antibodies, to an aqueous sample containing L. monocytogenes. We apply a pulsed magnetic field to align the magnetic dipole moments and use a high-transition temperature superconducting quantum interference device, an extremely sensitive detector of magnetic flux, to measure the magnetic relaxation signal when the field is turned off. Unbound particles randomize direction by Brownian rotation too quickly to be detected. In contrast, particles bound to L. monocytogenes are effectively immobilized and relax in about 1 s by rotation of the internal dipole moment. This Neel relaxation process is detected by the superconducting quantum interference device. The measurements indicate a detection limit of (5.6 ± 1.1) × 106 L. monocytogenes in our sample volume of 20 μl. If the sample volume were reduced to 1 nl, we estimate that the detection limit could be improved to 230 ± 40 L. monocytogenes cells. Time-resolved measurements yield the binding rate between the particles and bacteria.
TL;DR: In this paper, the magnetic anisotropy of nanometer thin films and of nanosize structures is discussed, and it is shown that film strain and its relaxation give rise to film thickness dependent anisotsropy, which can be misinterpreted as a surface anisotropic.
Abstract: The magnetic anisotropy of nanometer thin films and of nanosize structures is discussed. Experimental methods for the quantitative determination of magnetic anisotropy are described. Magnetocrystalline, shape, and magnetoelastic anisotropy contributions are reviewed, and recent examples for the non-bulk-like magnetic anisotropy and of the temperature dependence of both the magnetization and magnetic anisotropy of nanoscale materials are presented. It is shown that film strain and its relaxation give rise to film thickness dependent anisotropy, which can be misinterpreted as a surface anisotropy. The decisive role of the surface anisotropy for adsorbate-induced spin-reorientation transitions (SRT) is elucidated. The application of x-ray magnetic circular dichroism (XMCD) for the determination of magnetic anisotropy of nanosize islands down to the single atom size is presented.
TL;DR: In this paper, the complex dielectric spectra of ethanol/water mixtures at 25 °C for the nine molar fractions of ethanol, XEA = 004, 008, 011, 018, 03, 05, 07, 09, and 10, was determined.
Abstract: We have determined the complex dielectric spectra of ethanol/water mixtures at 25 °C for the nine molar fractions of ethanol, XEA = 004, 008, 011, 018, 03, 05, 07, 09, and 10, in the frequency range 01 ≤ ν/GHz ≤ 89 using TDR in 01 ≤ ν/GHz ≤ 25 and waveguide interferometers in 13 ≤ ν/GHz ≤ 89 At 03 ≤ XEA ≤ 10, a three-step relaxation model turns out to be most appropriate Besides a Cole−Cole relaxation for the dominating low-frequency process (j = 1), assigned to the cooperative dynamics of the H-bond system, which exhibits a pronounced increase of its relaxation time, τ1, when going from XEA = 0 to 1, two additional Debye terms (j = 2 and 3) with the relaxation times of τ2 ≈ 10 ps and τ3 ≈ 1−2 ps are required to reproduce the high-frequency part of the spectrum In view of the well-established relaxation mechanisms of pure liquids, these high-frequency processes can be validly assigned to the motion of singly H-bonded ethanol monomers at the ends of the chain structure (j = 2) and the flipp
TL;DR: In this paper, a detailed investigation of the temperature and time dependence of the spin-chain features was carried out and it was found that some of the magnetization steps can disappear as the characteristic time of the measurement is increased.
Abstract: For the spin-chain compound ${\mathrm{Ca}}_{3}{\mathrm{Co}}_{2}{\mathrm{O}}_{6}$, the magnetization curves as a function of the magnetic field are strongly out-of-equilibrium at low temperature, and they exhibit several steps whose origins are still a matter for debate. In the present paper we report on a detailed investigation of the temperature and time dependence of these features. First, it is found that some of the magnetization steps can disappear as the characteristic time of the measurement is increased. A comparison of the influence of temperature and time points to the existence of a thermally activated process that plays an important role in determining the form of the magnetization curves. Second, direct investigations of the magnetic response as a function of time show that this thermally activated process competes with a second relaxation mechanism of a very different nature, which becomes dominant at the lowest temperatures. These results shed new light on the peculiar magnetization process of this geometrically frustrated, Ising-like spin-chain compound.
TL;DR: In this paper, a dielectric relaxation study of aqueous solutions of ribonuclease A at 298.15 K as a function of protein concentration between 0.5 and 6 wt % in the MHz/GHz frequency range was performed.
Abstract: We report on a dielectric relaxation study of aqueous solutions of ribonuclease A at 298.15 K as a function of protein concentration between 0.5 and 6 wt % in the MHz/GHz frequency range. The spectra can be decomposed into five modes of Debye type diffusive behavior. In agreement with the standard interpretation, we assign the two dominant modes at low and high frequency (β-relaxation and γ-relaxation, respectively) to protein tumbling and bulk water relaxation. We observe three further modes (δ1−δ3) between β- and γ-relaxation, in contrast to a bimodal δ-dispersion frequently reported. We attribute the high frequency part (δ3) near 40 ps to hydration water reorientation, which, in the notion of other authors, corresponds to “loosely bound water”. We argue that the existence of “tightly bound” water, often deduced from the low frequency part in the nanosecond regime (δ1), is inconsistent with a highly mobile hydration layer observed by NMR techniques and molecular dynamics (MD) simulations. On the same gr...
TL;DR: In this paper, the enthalpy relaxation from the amorphous state into the equilibrium supercooled liquid state was found to follow a stretched exponential function with the relaxation time obeying an Arrhenius law.
TL;DR: The first observation of quantum tunneling of the magnetization (QTM) in a magnetic oxide belonging to the large family of A3BB′O6 compounds is reported in this paper.
Abstract: The magnetic behavior of the Ca3Co2O6 spin chain compound is characterized by a large Ising-like character of its ferromagnetic chains, set on a triangular lattice, that are antiferromagnetically coupled. At low temperature, T < 7 K, the 3D antiferromagnetic state evolves towards a spin frozen state. In this temperature range, magnetic field driven magnetization of single crystals (H
// chains) exhibits stepped variations. The occurrence of these steps at regular intervals of the applied magnetic field, Hstep
= 1.2 T, is reminiscent of the quantum tunneling of the magnetization (QTM) of molecular based magnets. Magnetization relaxation experiments also strongly support the occurrence of this quantum phenomenon. This first observation of QTM in a magnetic oxide belonging to the large family of A3BB′O6 compounds opens new opportunities to study a quantum effect in a very different class of materials from molecular magnets.
TL;DR: In this article, the reaction of Ni(NO3)2·6H2O with H3thme (trihydroxymethylethane) in the presence of NaOMe leads to a tetranuclear Ni4 complex with a cubane-like structure.
Abstract: The reaction of Ni(NO3)2·6H2O with H3thme (trihydroxymethylethane) in the presence of NaOMe leads to a tetranuclear Ni4 complex with a cubane-like structure. Intramolecular ferromagnetic interactions [J = 10 cm−1 based on H = −J(S1·S2 + S1·S3 + S1·S4 + S2·S3 + S2·S4 + S3·S4)] lead to an S = 4 ground state stabilized by 40 cm−1 from the first S = 3 excited state. Magnetization vs. field studies at different temperatures above 2 K suggest the presence of an axial magnetic anisotropy. Single-crystal magnetization measurements show a single-molecule-magnetic behavior below T = 0.5 K with fast relaxation of the magnetization due to resonant quantum tunneling. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)
TL;DR: In this paper, the authors used self-consistent, periodic, density functional theory (DFT-GGA) calculations to determine preferred binding sites, chemisorbed structures, binding energies, vibrational frequencies, and the effect of surface relaxation for the above species at 0.25 ML surface coverage.
Abstract: The chemisorption of atomic (H, O, N, S, C), molecular (N2, CO, NO, NH3), and radical (CH3, OH, NOH) species on Ir(111) has been systematically studied. Self-consistent, periodic, density functional theory (DFT-GGA) calculations, using PW91 and RPBE functionals, have been used to determine preferred binding sites, chemisorbed structures, binding energies, vibrational frequencies, and the effect of surface relaxation for the above species at 0.25 ML surface coverage. The following order in binding energies from least to most strongly bound was determined: N2 < NH3 < NO < CH3 < CO < OH < H < NOH < O < N < S < C. A preference for 3-fold sites for the atomic adsorbates was observed, with the exception of atomic H, which prefers top sites. Molecular species showed a preference for top sites with the exception of NOH; this species preferred fcc sites. Surface relaxation had only a small effect on energetics in most cases. Calculated vibrational frequencies, in general, were in good agreement with experimental ...
TL;DR: In this paper, a two-structure (N(e) = 2) ensemble representation of anisotropic motions along the backbone bond vectors in terms of order parameters is presented.
Abstract: Backbone residual dipolar coupling (N-H, Calpha-Halpha, N-C', and Calpha-C') data collected in five different media on the B3 IgG binding domain of streptococcal protein G (GB3) have been analyzed by simultaneous refinement of the coordinates and optimization of the magnitudes and orientations of the alignment tensors using single and multiple structure representations. We show, using appropriate error analysis, that agreement between observed and calculated dipolar couplings at the level of experimental uncertainty is obtained with a two-structure (N(e) = 2) ensemble representation which represents the simplest equilibrium description of anisotropic motions. The data permit one to determine the magnitude of the anisotropic motions along the four different backbone bond vectors in terms of order parameters. The order parameters, , for the N-H bond vectors are in qualitative agreement with the generalized order parameters, S(2)NH(relaxation), derived from (15)N relaxation measurements, with a correlation coefficient of 0.84. S(2)NH(relaxation) can be regarded as the product of an anisotropic order parameter, corresponding to derived from the residual dipolar couplings, and an axially symmetric order parameter, S(2)NH(axial), corresponding to bond librations which are expected to be essentially uniform along the polypeptide chain. The current data indicate that the average value of S(2)NH(axial) is approximately 0.9. The close correspondence of and S(2)NH(relaxation) indicates that any large-scale displacements from the mean coordinate positions on time scales longer than the rotational correlation time are rare and hence do not perturb the observed dipolar couplings. Analysis of a set of 100 N(e) = 2 ensembles reveals the presence of some long-range correlated motions of N-H and Calpha-Halpha vectors involving residues far apart in the sequence but close together in space. In addition, direct evidence is obtained for ubiquitous crankshaft motions along the entire length of the polypeptide backbone manifested by the anticorrelation of the backbone torsion angles phi(i) and psi(i-1).