Observation of microscopic CO dynamics on Cu(001) using 3He spin-echo spectroscopy

We present momentum resolved measurements of quasielastic helium atom scattering made using a new 3He spin-echo spectrometer. Our data for the dynamics of CO on Cu(001) indicates an activated jump mechanism which we analyze in detail using molecular dynamics simulations. A nearly isotropic potential energy surface is found with an average barrier height of approximately 125 meV, yielding comparable hopping rates along both the <110> and <100> directions. The measurements provide the first rigorous experimental test of state-of-the-art first-principles calculations previously made on this system.     

Diffusion in crystalline materials

Recently nuclear scattering of synchrotron radiation proved to be a powerful new method to study the elementary diffusion jump in crystalline solids. The scattered radiation decays faster when atoms move on the time scale of the excited-state lifetime of a Mössbauer isotope because of a loss of coherence. The acceleration of the decay rate differs for different crystal orientations relative to the beam providing information not only about the rates but also about the directions of the elementary jumps. We discuss first applications of the method.

     

Diffusion correlation effects of molybdenum and silicon in molybdenum disilicide

Diffusion data for both principal directions of silicon and molybdenum as well as germanium are briefly summarised. Analysis is performed of the defect formation energies (available from previous ab initio calculations and experimental measurements) for diffusion mechanisms via home and foreign sublattices. The home sublattice mechanism is shown to be the preferred one for both silicon and molybdenum. Tracer correlation factors for silicon and molybdenum diffusion via sublattice vacancies in the respective sublattices of the tetragonal C11_b structure of molybdenum disilicide are calculated by a direct Monte Carlo simulation technique. Correlation factors for Si diffusion on its sublattice are compared with literature values that were calculated using a more complicated Monte Carlo method based on the matrix approach. It is shown that there is no need for this complicated approach and that the direct Monte Carlo simulation technique gives highly accurate correlation factors. Correlation factors and anisotropy ratios of vacancy-mediated diffusion in both sublattices are deduced and compared with experimental data. Tracer correlation in the tetragonal direction is shown to contribute 0.40?eV (i.e. over 55%) of the migration energy of the corresponding Si diffusivity. Two possible jump rates for Si diffusion are separately estimated. Mo diffusion correlation factors are calculated using the direct Monte Carlo technique. A comparison with experiment is made and the ratio of two possible jump rates is also estimated.     

Non-Markovian quantum jumps

Open quantum systems that interact with structured reservoirs exhibit non-Markovian dynamics. We present a quantum jump method for treating the dynamics of such systems. This approach is a generalization of the standard Monte Carlo wave function (MCWF) method for Markovian dynamics. The MCWF method identifies decay rates with jump probabilities and fails for non-Markovian systems where the time-dependent rates become temporarily negative. Our non-Markovian quantum jump approach circumvents this problem and provides an efficient unraveling of the ensemble dynamics.     

漫反射各向异性散射介质内的温度热流场

本文采用射线踪迹结合节点分析法和谱带模型,研究了漫反射不透明边界下吸收、发射、各向异性散射介质内的热辐射传递过程。考虑介质辐射能的入射和散射方向,导出漫反射、不透明边界、各向异性散射介质的辐射传递系数。在辐射平衡的情况下,考察了表面发射率和散射反照率对介质内辐射热流和温度场的影响。研究表明,介质不透明边界处存在温度跃迁现象,而且,内界面发射率越大,相应界面温度跃迁越小。     

液气相变界面间断关系浅议

对池沸腾传热现象局部传递过程的细致分析(例如微重力池沸腾传热研究文献中关于热毛细效应作用及其成因的各种相互冲突的观点),涉及汽液相变界面两侧的间断关系.相变(蒸发或凝结)过程的非平衡性导致相界面两侧物理量对经典平衡态热力学中的相界面关系的偏离,分子动理论比拟模型、统计率模型和非平衡热力学模型均给出了相关描述,本文对此进行了详细评述,指出了各模型的优缺点,并对进一步的研究方向进行了讨论.     

Molecular-Dynamics Simulation of the Effects of Methyl Rotation and Other Protein Motions on the NOE

The internal motions of a 36-residue C-terminal fragment of cellobiohydrolase were simulated by a 550 ps all-atom molecular dynamics calculation. The trajectories of methyl group torsional fluctuations were appended to simulate longer (2.0 ns) trajectories for modeling ideal methyls undergoing torsional vibrations about the methyl axis and proton jumps among the three equivalent proton positions. Relaxation rates and NOEs of these ideal methyl groups were calculated for methyl systems with different geometries and proton jump rates (0, 4, 15, and 27 ns⁻¹) using correlation functions derived from the MD trajectories. The exact results could be approximated by a three-site jump model in which the correlation time of methyl jumps was 50 ps. N-site jump models with N ≥ 6 gave results similar to those expected for a freely rotating methyl group. The relaxation rates of ideal methyl groups experiencing torsional motions were reduced, depending on geometry and proton jump rate, to 30-85% of the static methyl rates. Relaxation rates were also calculated for "real" methyl groups (and various nonmethyl interactions) using exact correlation functions derived directly from the MD trajectory of the protein. Contributions of conformational fluctuations from backbone and side-chain motions were apparent as reductions and enhancements of relaxation rates relative to a static average structure. When methyl groups of the static model were calculated with a three-site jump model, similar motional effects were observed for methyl and nonmethyl interactions. Distances back-calculated from the exact relaxation rates indicate that internal motions increase the apparent distance of nonmethyl interactions by less than 0.5 Å. For interactions involving methyl groups, internal motions result in back-calculated distances that are between r − 1.0 and r + 0.6 Å of the distance in the average structure.     

High order matched interface and boundary methods for the Helmholtz equation in media with arbitrarily curved interfaces

This work overcomes the difficulty of the previous matched interface and boundary (MIB) method in dealing with interfaces with non-constant curvatures for optical waveguide analysis. This difficulty is essentially bypassed by avoiding the use of local cylindrical coordinates in the improved MIB method. Instead, novel jump conditions are derived along global Cartesian directions for the transverse magnetic field components. Effective interface treatments are proposed to rigorously impose jump conditions across arbitrarily curved interfaces based on a simple Cartesian grid. Even though each field component satisfies the scalar Helmholtz equation, the enforcement of jump conditions couples two transverse magnetic field components, so that the resulting MIB method is a full-vectorial approach for the modal analysis of optical waveguides. The numerical performance of the proposed MIB method is investigated by considering interface problems with both constant and general curvatures. The MIB method is shown to be able to deliver a fourth order of accuracy in all cases, even when a high frequency solution is involved.     

Dilatation anisotropy upon the phase transition in the rolled Ti-49.8% Ni alloy

The temperature dependence of the thermal expansion of the Ti-49.8% Ni alloy rolled at 773 K has been measured in three mutually perpendicular directions with respect to the rolling direction. It has been found that, upon phase transition of the martensitic type, the sample is elongated stepwise in two directions, and it is contracted in the third direction. The observed effect is due to the crystallographic texture. The dilatation jump during the phase transformation is determined by the joint action of the change in lattice parameters and the process of twinning.     

Two Speed TASEP

We consider the TASEP on ? with two blocks of particles having different jump rates. We study the large time behavior of particles’ positions. It depends both on the jump rates and the region we focus on, and we determine the complete process diagram. In particular, we discover a new transition process in the region where the influence of the random and deterministic parts of the initial condition interact. Slow particles may create a shock, where the particle density is discontinuous and the distribution of a particle’s position is asymptotically singular. We determine the diffusion coefficient of the shock without using second class particles. We also analyze the case where particles are effectively blocked by a wall moving with speed equal to their intrinsic jump rate.     

Master equations for subordinated processes

We study Markov jump processes constructed by subordination of diffusion processes. The procedure can be viewed as a randomization or a coarse graining of time. We construct the master equation for the cases of finite and infinite total jump rates, and give a collection of explicitly solvable examples.     

Kernresonanz-und leitfähigkeitsmessungen zu diffusion und fehlordnung in LiBr

Electrical conductivity and nuclear magnetic relaxation rates were measured with pure and doped LiBr between 400 K and the melting point (824 K). Prevalent intrinsinc disorder was observed down to 470 K. The degree of thermal disorder is 5.10⁻⁷ at 470 K and 5.10⁻³ at the melting point. From the relaxation rates of ⁷Li, which are caused by Li-diffusion and nuclear dipole interaction, mean jump frequencies of the cations are derived. Conductivities calculated from these frequencies for a jump process via neighbouing cation vacancies are in perfect agreement with directly measured conductivities. From relaxation rates of ⁸¹Br with MgBr₂-doped crystals jump frequencies of vacancies were obtained which are again in good agreement with those derived from the conductivity data. The energies resulting from the measurements are (0.43 ± 0.03) eV for migration of cation vacancies and (1.46 ± 0.05) eV for thermal disorder. From motional narrowing of the ⁸¹Br absorption line the jump frequency of the anions is obtained, which is much smaller than for the cations. Since this motional narrowing is not influenced by any doping, it is concluded that anion transport mainly occurs via pairs of cation and anion vacancies.     

On the theory of surface diffusion: kinetic versus lattice gas approach

The present paper extends the results of a recent analytic kinetic theory of particle-on-substrate diffusion. The approach treats explicitly the molecule–surface interaction and takes into account inter-molecular interaction within the hard particle approximation. The physics influencing the diffusion pre-exponential factor and mechanisms determining the density dependence of collective diffusivity are discussed. The kinetic results are compared with those of the traditional lattice gas hopping models. Analytical expressions for jump rates in the low density limit are derived, and the density dependence of effective jump rates at finite occupancy is discussed. It is shown how the traditional hopping model oversimplifies the picture of diffusion by neglecting the collision part of the hopping process.     

The diffusion limit for reversible jump processes onZ d with ergodic random bond conductivities

We consider a reversible jump process on ? ^d whose jump rates themselves are random. We show mean square convergence of this process under diffusion scaling to a limiting Brownian motion with a certain diffusion matrix, characterizing effective conductivity.     

Internal dynamics of hydroxymethyl rotation from CH2 cross-correlated dipolar relaxation in methyl-beta-D-glucopyranoside

Conventional relaxation parameters (T1(-1), T2(-1), and NOE), obtained at different temperatures and magnetic fields, are reported for the hydroxymethyl (C6) carbon in methyl-beta-D-glucopyranoside in a D2O/DMSO cryosolvent. These data are interpreted with the Lipari-Szabo model. In addition, two-field measurements of longitudinal and spin-locked relaxation rates related to the cross-correlated carbon-proton dipole-dipole interactions for the same carbon are reported. The complete data set consisting the conventional and cross-correlated relaxation parameters is interpreted using a new "hybrid" approach, in which the Lipari-Szabo model for the auto-correlated spectral densities is combined with the two-site jump model for the cross-correlated spectral densities, with the global correlation time as a common parameter. The two-site jump rates thus obtained are in reasonable agreement with the ultrasonic relaxation measurements, and have reasonable temperature dependence.     

Orientation and Rate Dependence of Wave Propagation in Shocked Beta-SiC from Atomistic Simulations

The orientation dependence of planar wave propagation in beta-SiC is studied via the molecular dynamics （MD） method. Simulations are implemented under impact loadings in four main crystal directions, i.e., （lO0）, （llO）, 〈111〉, and 〈112}. The dispersion of stress states in different directions increases with rising impact velocity, which implies the anisotropic characteristic of shock wave propagation for beta-SiC materials. We also obtain the Hugoniot relations between the shock wave velocity and the impact velocity, and find that the shock velocity falls into a plateau above a threshold of impact velocity. The shock velocity of the plateaux is dependent on the shock directions, while 〈111} and 〈112/ can be regarded as equivalent directions as they almost reach the same plateau. A comparison between the atomic stress from MD and the stress from Rankine-Hugoniot jump conditions is also made, and it is found that they agree with each other very well.     

Phonon-assisted transition rates I. Optical-phonon-assisted hopping in solids

An extensive calculation of the optical-phonon-assisted transition rates for non-adiabatic electronic hopping motion in a solid is presented. Holstein's Molecular Crystal Model is used as a basis for study and the computation involves no restrictions on either the magnitude of the electron-lattice coupling strength, the temperature, or the difference between the electronic energies of the initial and final sites.

In the strong-coupling small-polaron régime, the jump rates, associated d.c. conductivity, a.c. conductivity, and electric-field dependence of the d.c. conductivity, for a crystal are all calculated. These transport properties manifest qualitatively distinct behaviours corresponding to whether the temperature is above or well below the optical-phonon temperature. In the low-temperature régime the energy-conserving processes which involve the absorption of the minimum amount of vibrational energy provide the dominant contribution to the thermally activated jump rates. At sufficiently high temperatures multiphonon processes dominate the transition rate; the high-temperature jump rates are also activated, albeit with a different activation energy than that which characterizes the low-temperature régime.

In the complementary weak-coupling régime, the jump rate is characterized by the dominance of those processes which involve the absorption or emission of the minimum number of phonons consistent with the requirements of energy conservation. Once again two distinct temperature domains manifest themselves: a low-temperature, thermally activated, régime and a high-temperature non-activated régime.

Finally, it is shown that, while optical-phonon-assisted transition rates are well defined for a three-dimensional model, divergences will occur for a linear chain (at ω = 0) and for a planar model (at the optical phonon frequency).

The calculations in this article bring together many of the results previously obtained for special cases into a single framework and reveal a number of new considerations. The article is the first of two, the second dealing with acoustic-phonon-assisted hopping.     

高温超导磁通跳跃过程中的磁致伸缩效应

文中基于超导磁通动力学理论,考虑电磁力与热激活对磁通运动的影响,基本模型包括由等效电阻率随超导体温度和磁场变化的磁通扩散方程,以及比热随超导体温度变化的热传导方程组成.在此基础上,用数值方法求解了这组非线性磁热耦合方程,主要研究了有磁通跳跃状发生状态时环境温度和外磁场速度对于高温超导磁致伸缩的影响.结果表明:磁通进入超...     

Hydrodynamics and Platoon Formation for a Totally Asymmetric Exclusion Model with Particlewise Disorder

We consider a one-dimensional totally asymmetric exclusion model with quenched random jump rates associated with the particles, and an equivalent interface growth process on the square lattice. We obtain rigorous limit theorems for the shape of the interface, the motion of a tagged particle, and the macroscopic density profile on the hydrodynamic scale. The theorems are valid under almost every realization of the disordered rates. Under suitable conditions on the distribution of jump rates the model displays a disorder-dominated low-density phase where spatial inhomogeneities develop below the hydrodynamic resolution. The macroscopic signature of the phase transition is a density discontinuity at the front of the rarefaction wave moving out of an initial step-function profile. Numerical simulations of the density fluctuations ahead of the front suggest slow convergence to the predictions of a deterministic particle model on the real line, which contains only random velocities but no temporal noise.     

NMR relaxation interference effects and internal dynamics in gamma-cyclodextrin

Multiple-magnetic field (9.4, 14.1 and 21.1 T) measurements of (13)C spin-lattice and spin-spin relaxation rates, the heteronuclear Overhauser enhancement and cross-correlated relaxation rates (CCRRs) in the methylene groups are reported for gamma-cyclodextrin in water/dimethylsulfoxide solution at 323 and 343K. The CCRRs are obtained from differences in the initial relaxation rates of the components of the CH(2) triplet in the (13)C spectra. The relaxation data are analyzed using the Lipari-Szabo approach and a novel modification of the two-site jump model. According to the latter model, inclusion of the dipolar (CH,CH(')) cross-correlated longitudinal and transverse relaxation is important for estimating the rate of the conformational jumps in the hydroxymethyl group. Using the dynamic information from the jump model, we have also used the differences in the initial relaxation rates for the triplet components to estimate the anisotropy of the chemical shielding tensor.