共查询到20条相似文献,搜索用时 15 毫秒
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We present theoretical results on dynamic correlation functions of impure, classical, one-dimensional Heisenberg magnets with both ferromagnetic and antiferromagnetic coupling. The results are obtained in the framework of Mori's theory and in an approximation which includes correctly all moments of the relaxation function to lowest order in impurity concentration and temperature and which preserves the rotational symmetry of the system even at zero temperature. Explicit results are given for lineshifts and linewidths of quasi-spinwaves and the behaviour of correlation functions in the hydrodynamic limit is discussed. 相似文献
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R. S. Sorbello 《Zeitschrift für Physik B Condensed Matter》1975,19(1-4):303-316
The role of anisotropic scattering in transport properties is studied within the framework of the anisotropic relaxation time. The concept of the relaxation time is introduced in a rigorous way, without approximation. Its physical meaning and its connection with linear response theory is discussed. The manner in which electron-impurity scattering and electronphonon scattering influence the relaxation time is considered as well as the role of such anisotropies in the de Haas-van Alphen effect, Hall effect, deviations from Mathiessen's rule, and magnetoresistance. Examples are taken from recent experimental and theoretical work. Some common pitfalls surrounding the use of the anisotropic relaxation time are pointed out. 相似文献
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R. S. Sorbello 《Zeitschrift für Physik B Condensed Matter》1975,19(1):303-316
The role of anisotropic scattering in transport properties is studied within the framework of the anisotropic relaxation time. The concept of the relaxation time is introduced in a rigorous way, without approximation. Its physical meaning and its connection with linear response theory is discussed. The manner in which electron-impurity scattering and electronphonon scattering influence the relaxation time is considered as well as the role of such anisotropies in the de Haas-van Alphen effect, Hall effect, deviations from Mathiessen's rule, and magnetoresistance. Examples are taken from recent experimental and theoretical work. Some common pitfalls surrounding the use of the anisotropic relaxation time are pointed out. 相似文献
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We review dynamic processes in supercooled liquids and glasses as studied by dielectric spectroscopy. It is the only experimental technique which allows one to follow the tremendous slow-down of diffusive motion of particles in disordered condensed matter over more than 18 decades in frequency or time. The dielectric techniques used are treated in detail. As an introduction for non-specialists, the time and temperature evolution of the basic spectral features associated with various dynamic relaxation processes are discussed in detail. Among them are the structural relaxation, the occurrence of fast processes and the boson peak. The relevance of these features for glass formation is discussed. The present article may also serve as a review for recent experimental and theoretical studies on glass-forming liquids. 相似文献
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Four theoretical and computational approaches used at the University of Michigan to analyze NMR paramagnetic relaxation enhancement (NMR-PRE) are described. The primary objective of the theory is to describe the relationship of the NMR-PRE phenomenon to the electron spin hamiltonian and the spin energy level structure when zero field splitting interactions are significant. Four formulations of theory are discussed: (1) spin dynamics simulation; (2) the laboratory frame "constant H(S)" formulation; (3) the Molecular Frame "constant H(S)" formulation; and (4) the zfs-limit "constant H(S)" formulation. No single theoretical approach describes all important aspects of the relaxation mechanism in a fully satisfactory way. We use the four formulations in a complementary manner to provide as complete a picture of the relaxation mechanism as possible. We also discuss the integration of NMR-PRE theory and recently developed theory of electron spin relaxation which accounts for effects of the permanent zfs hamiltonian. 相似文献
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S. F. J. Cox 《Hyperfine Interactions》1984,19(1-4):953-958
Pulsed muon sources are ideally suited to experiments which require a repetitive or periodic excitation of the sample. Various possibilities are discussed. Magnetic relaxation is considered in terms of the various thermodynamic reservoirs which make up a spin system, both for nuclear magnetism and for electronic paramagnetism.A synchronous pulse sequence is presented which, in a single experiment, allows the following distinct relaxation functions of the samples host spin system to be determined: that of its Zeeman energy, that of its secular dipolar components, and that of the total dipolar energy in zero field. The timescale of the high field measurements overlaps with and substantially extends the range accessible to NMR and ESR. 相似文献
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A method for the direct measurement of spin relaxation rates of dilute magnetic systems by diffuse scattering of unpolarized neutrons is discussed. The magnetic scattering from the single magnetic impurities results from a difference measurement of doped and undoped sample. To discriminate against the large background from nonmagnetic scattering the choice of special energy windows is necessary for the detection of the magnetic scattering.Neutron scattering experiments were performed on the Kondo system CuFe for concentrations between 600 and 4800 at. ppm Fe and in the temperature range 15 K to 300 K. The measured absolute values and the temperature dependence of the spin relaxation rates, which have been measured for the first time in this experiment, are found to agree well with theoretical calculations by Götze and Schlottmann. This supports the explanation of the Kondo effect as a dynamical phenomenon and is contradictory to the static picture of a quasiparticle as proposed by Heeger. 相似文献
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The dynamics of spin 3/2 systems is analyzed using the density matrix theory of relaxation. By using the superoperator formalism, an algebraic formulation of the density matrix's evolution is obtained, in which the contributions from free relaxation and RF application are easily factored out. As an intermediate step, an exact form for the propagator of the density matrix for a spin 3/2 system, in the presence of static quadrupolar coupling, inhomogeneous static magnetic field, and relaxation is demonstrated. Using this algebraic formulation, exact expressions for the behavior of the density matrix in the classical one-, two-, and three-pulse experiments are derived. These theoretical formulas are then used to illustrate the bias introduced on the measured relaxation parameters by the presence of large spatial variations in the B0 and B1 fields. The theoretical predictions are easily evaluated through simple matrix algebra and the results agree very well with the experimental observations. This approach could prove useful for the characterization of the spatial variations of the signal intensity in multiple quantum-filtered sodium MRI experiments. 相似文献
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《Physics letters. A》1988,126(7):427-430
Recent theoretical calculations are discussed which show that the minimum energy site for hydrogen in silicon is the bond-centered site, while a secondary minimum is at the anti-bonding site, it is noted that these results are strongly dependent on the relaxation experienced by the silicon atoms neighboring the hydrogen. Several experimental results are discussed, namely, the observed model of the hydrogen-passivated acceptor, etching of the silicon surface by hydrogen, the exponential depth dependence of the near-surface hydrogen diffusion profile, and related muonium results, and from these results it is argued that, in some instances at least, the BC-site is the lowest energy site for hydrogen. 相似文献
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The selective and nonselective excitations of laser-stimulated surface processes are quantitatively discussed by the population dynamics of multi-photon-multi-phonon processes. The multi-level system is reduced to a few-level system in the Heisenberg-Markovian picture and the level populations of different normal modes of the adspecies/surface system are solved numerically. It is shown that specific normal modes of the system may be selectively excited without appreciably heating the system as a whole when the pumping rate is faster than the energy relaxation rate. In contrast to a homogeneous gas-phase system (governed by picosecond phenomena), much longer time scales of the relaxation dynamics are discussed for the present heterogeneous system. 相似文献
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以四种不同类型高聚物(半晶聚丙烯、聚甲基烯酸甲酯、交联环氧树酯和炭黑填充丁苯橡胶)的短时标应力松弛和蠕变测试实验数据作为实例,按照两套应力松弛模量和蠕变柔量普适对比态方程,从理论组合主直线上直线外推了四种高聚物的长时标力学行为和寿命,并将其外推预测值同理论计算值和实测值进行了对比. 结果证实四种高聚物的外推预测值均同理论计算值和实测值很好地符合. 又讨论了它们的尺寸稳定性、负荷承载性,失效寿命和失效形变量等对老化时间的依赖性. 最终证实该从理论组合主直线上来直线外推长时标力学行为和寿命的方法是有效可行的. 相似文献
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An experimental and theoretical study on the optically stimulated spin transport in zinc-blende semiconductors is presented. The first part of the paper describes an experiment which investigates the effect of a longitudinal electric field on the spin-polarized carriers induced by a circularly polarized light. Since the photo-generated hole spins relaxation is extremely fast, the experiment observes only the effect resulting from spin-polarized electrons accumulating at the transverse edges of the sample, as a result of left-right asymmetries in scattering for spin-up and spin-down electrons in the presence of spin–orbit (SO) interaction. It is found that the effect depends on the longitudinal electric field and doping density as well as on temperature. The results are discussed. The second part of the paper deals with a theoretical investigation using norm-conserving pseudopotential and Green function formalism to analyse the SO mechanism responsible for the light-induced Hall voltage. The findings resulting from the investigation are discussed and are compared with experimental data. 相似文献
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The stability enhancement of laser output power for the change of external cavity position in semiconductor lasers with optical feedback is observed by experiment. The relaxation oscillation frequency which plays an important role in the dynamics of the nonlinear system is also investigated as a function of the external cavity length. The period of the stability enhancement along the position of the external cavity is exactly coincident with the length corresponding to the relaxation oscillation frequency of the solitary laser. The experimental results are compared with theoretical and excellent coincidence between the two is found. 相似文献
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Alexandra Dobrinescu Alexandru Stancu 《Journal of magnetism and magnetic materials》2009,321(24):4132-4138
In this paper we present a theoretical study of size effects during relaxation in spin transition solids. The systems are described using an Ising-like model consisting of molecules having two energy levels and fictitious spin values of +1, (HS) and −1 (LS). We compare relaxation in various 2D or 3D systems (rectangular, hexagonal or cubic) and we realise an exhaustive analysis of the parameters that influence the relaxation in small size samples. The differences between homogenous and inhomogeneous systems reflected on the shapes of relaxation curves are discussed and analysed. 相似文献
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Cooperative charge relaxation within a random system of electrostatically interacting defect centers provides a mechanism for a “nearly constant dielectric loss” (NCL) response in structurally disordered ionic conductors. Pertinent models based on statistical mechanics are reviewed briefly. In addition, we present a theoretical frame for the problem of how two kinds of ionic motion, hopping migration and NCL-type local charge relaxation, are superimposed in the total ac-response. Using renewal theory, the modification of NCL-spectra due to hopping is calculated in terms of the waiting time distribution for ionic hops. In the special case of Poissonian hops with average rate λ the modified complex dielectric susceptibility in the NCL-regime is obtained by analytic continuation from the corresponding susceptibility in the absence of hops. Implications with respect to the crossover between hopping transport and NCL-behavior are discussed. 相似文献
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L. Gordillo T. Sauma Y. Zárate I. Espinoza M.G. Clerc N. Mujica 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2011,62(1):39-49
Development of technologies based on localized states depends on our ability to
manipulate and control these nonlinear structures. In order to achieve this, the
interactions between localized states and control tools should be well modelled and
understood. We present a theoretical and experimental study for handling
non-propagating hydrodynamic solitons in a vertically driven rectangular water
basin, based on the inclination of the system. Experiments show that tilting the basin
induces non-propagating solitons to drift towards an equilibrium position through a
relaxation process. Our theoretical approach is derived from the parametrically driven
damped nonlinear Schr?dinger equationwhich models the system. The basin tilting effect is
modelled by promoting the parameters that characterize the system, e.g. dissipation,
forcing and frequency detuning, as space dependent functions. A motion law for these
hydrodynamic solitons can be deduced from these assumptions. The model equation, which
includes a constant speed and a linear relaxation term, nicely reproduces the motion
observed experimentally. 相似文献