Results 1 - 10 of 44123
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[en] The study of the electronic relaxation by means of Moessbauer spectroscopy is reviewed. After a general discussion on the Moessbauer lineshape problem, the relaxation effects are considered within the stochastic and superoperators theories. Simulated and experimental relaxation spectra are also presented. (author)
[en] Although fragility of glass forming liquids is traditionally related to cooperativity in molecular motion, the connection between those parameters remains unclear. In this paper we present the estimates of cooperativity (heterogeneity) length scale obtained from the boson peak spectra. We demonstrate that agrees well with the dynamic heterogeneity length scale for the structural relaxation estimated by 4- dimensional NMR, justifying the use of . Presented analysis of large number of materials reveals no clear correlation between and fragility. However, there is a strong correlation between the cooperativity volume 3 and the activation volume measured at Tg. This observation suggests that only the volume (pressure) dependence of structural relaxation time correlates directly with the cooperativity size. However, the pure thermal (energetic) contribution to the structural relaxation, the so-called isochoric fragility, exhibits no correlation to the heterogeneity length scale, or the amount of structural units in 3. The presented results call for a revision of traditional view on the role of cooperativity/heterogeneity in structural relaxation of glass forming systems.
[en] The basic principles that must be known to interpret NMR images, are exposed. After an analysis of the fundamental parameters (T1 and T2 relaxation time particularly), the main elements necessary to analyse a Spin Echo image are presented
[fr]Nous exposons les principes de base indispensables a connaitre pour interpreter un document en IRM (Imagerie par Resonance Magnetique). Apres avoir analyse les parametres fondamentaux (temps de relaxation T1 et T2 en particulier), nous presentons les principales notions qui permettent de lire une image en Echo de Spin
[en] We solve the dynamics of quantum spin chains when the initial condition is prepared as a spatially inhomogeneous state of the magnetization. Particularly long-lived states are found for which the relaxation time diverges and the whole relaxation process slows down as a consequence of constructive interference at degenerate stationary points. These states may be of interest for performing quantum computation
[en] The authors present their preliminary experience in the characterization of atherosclerotic lesions and thrombi according to the parameters employed in MRI, namely longitudinal (T1) transversal (T2) relaxation times and proton density. (author). 8 refs.; 3 figs
[en] In this book it is explained how images are performed in nuclear magnetic resonance tomography. Therefore detailed discussions are deveted to the physical background, the possibilities for influencing of the images by the proper choice of pulse sequences, the spatial localization of the NMR signals and the clinical applications. (author). 323 refs.; 138 figs.; 2 tabs
[en] The decay of nuclear spin-spin energy has been studied in the mixed state of vanadium and anomalously rapid relaxation rates are found as compared to the rates for spin-lattice relaxation of Zeeman energy. The experiment was performed by adiabatically demagnetizing the spins in the rotating frame at a field larger than H/subc/2 and then cycling the field to bring the sample into the mixed state for a variable time. The residual dipolar energy is detected, once the field is raised, by adiabatically remagnetizing the sample on resonance. I show that the relaxation observed, after the vortices are pinned, is due to a cross relaxation of a spin energy associated with the magnetic field gradients in the mixed state and the dipolar energy which is in semiequilibrium with the quadrupole energy. This process is mediated by a current of magnetization, proportional to the diffusion coefficient D, which is driven by the field gradients and uses dipolar energy as a heat sink. Using a field distribution in the mixed state calculated by Marcus, I find D = 2.8 +- 0.9 x 10-12 cm2 sec-1 from the measurements of the relaxation rate of dipolar energy and of the quadrupole system heat capacity. This measurement of D is the first for a metal or for nuclei with I > 1/2 and is twice the value predicted by the moment-moment calculation of Redfield and Yu. In the presence of large field gradien-ts, dynamic quenching of the diffusion is observed
[en] The reported relaxation time for several typical glass-forming liquids was analyzed by using a kinetic model for liquids which invoked a new kind of atomic cooperativity-thermodynamic cooperativity. The broadly studied 'cooperative length' was recognized as the kinetic cooperativity. Both cooperativities were conveniently quantified from the measured relaxation data. A single-exponential activation behavior was uncovered behind the super-Arrhenius relaxations for the liquids investigated. Hence the mesostructure of these liquids and the atomic mechanism of the glass transition became clearer.
[en] The stress relaxation behaviors of magnetorheological (MR) gels under stepwise shear loading are systematically investigated. The particle-enhanced effect, the magneto-induced effect, and the temperature-enhanced effect on the stress relaxation of MR gels (MRG) are discussed. For further analysis of the magneto-induced stress relaxation mechanism in MRG, a phenomenological model is established to describe the stress relaxation behavior of the matrix and the magnetic particle chains. All characteristic parameters introduced in the model, i.e. relaxation time, instantaneous modulus, and stable modulus, have well-defined physical meanings and are fitted based on the experimental results. The influence of each parameter on the macroscopic response is discussed and it is found that the relaxation stress induced by the magneto-mechanical coupling effect plays an important role in the stress relaxation process of MRG. (paper)