The relaxation of a reconstructed metallic surface and its influence on the EFG acting on a substitutional PAC probe

The structure of the 1×2 reconstructed Au(110) surface, calculated using equivalent crystal theory, is studied. We perform point-charge calculations to estimate the electric field gradient (EFG) acting on a substitutional¹¹¹In probe deposited in that surface. The relaxation breaks the local symmetry producing an asymmetric EFG but the calculated asymmetry is lower than the experimental one. Comparing the computed orientations of the principal axes of the EFG to the experimental values we conclude that In probes prefer to substitute gold into the high coordinated sites at the bottom of the channels.     

PAC perturbation factor for spin 5/2 nuclei subject to a rapidly fluctuating EFG1

We report numerical computations of the PAC perturbation factor G₂(1) for spin 5/2 nuclei subject to a static EFG symmetric about the z-axis and an additional axially-symmetric EFG whose symmetry axis fluctuates randomly among the x, y, z directions. For sufficiently large fluctuation rates, the numerical results are described by the expression for the static interaction alone with the addition of relaxation terms. Results of applying this model to¹¹¹Cd TDPAC measurements on tetragonal ZrO₂ are described briefly. The model allows one to evaluate the probability that oxygen vacancies are trapped. the energy of association of vacancy-metal pairs, and the vacancy activation energy of motion. Supported in part by USDOE contract DE-FG06-85ER45191     

Magnetic field dependence of the superconducting fluctuation contribution to NMR-NQR relaxation

The dependence of the Maki-Thompson and of the density of states (DOS) depletion contributions from superconducting fluctuations to nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) relaxation is derived in the framework of the diagrammatic theory, applied to layered three-dimensional (3-D) high-T_c superconductors. The regularization procedure devised for the conductivity (A. I. Buzdin, A. A. Varlamov: Phys. Rev. B58, 14195, 1998) is used in order to avoid the divergence of the DOS term. The theoretical results are discussed in the light of NMR-NQR measurements in YBCO and compared with the recent theory (M. Eschrig et al.: Phys. Rev. B59, 12095, 1999), on the basis of the assumption of a purely 2-D spectrum of fluctuations.     

Scaling theory of relaxation and fluctuation near the instability point

The idea of generalized scale transformation of time is introduced to evaluate asymptotically the relaxation and fluctuation in non-equilibrium systems. This scaling theory is demonstrated to be very powerful in discussing the relaxation just from the instability point.     

Nonlinear coupling and relaxation of fluctuations and the central peak

A simple model Hamiltonian for a structural phase transition is discussed. It is shown that using the modecular field approximation one obtains a nonlinear coupling of the order parameter and entropy fluctuations. This results in a central peak in the order parameter fluctuation spectrum. The width of the central peak is proportional to the entropy relaxation rate.     

Scaling theory of fluctuation in superradiance relaxation from the complete inversion

The general scaling theory of transient phenomena near the instability point, which has been proposed by the author, is applied to investigate the fluctuation and relaxation of superradiance from the complete inversion. This study gives a foundation or justification of the treatment of Glauber and Haake.     

Approximation of the Blume’s stochastic model by asymptotic models for PAC relaxation analysis

The time-dependent perturbation factors G₂(t) of PAC are numerically calculated by diagonalizing the Blume’s supermatrix when static and fluctuating electric field-gradients are simultaneously present at the nucleus. The calculated functions are fitted with the expression of G₂(t) given by isotropic models, strictly valid for asymptotic conditions. The asymptotic relations between the spin relaxation constant λ and the jump frequency w introduced in the stochastic model are found to be a good approximation in large domains of w values.     

The relaxation peak in the internal friction of kbr and KBr-KCl solid solutions

It is considered that this peak is due to rapidly changing fluctuations in the distribution of bends in dislocation lines. The internal friction is found to vary for a time after plastic deformation or X-irradiation.I am indebted to Professor E. K. Zavadovskaya for direction in this work and to Professor A. A. Vorob'ev for discussion of the results.     

Contribution of strain to the valence EFG in a cubic metal

Unlike the earlier works, which evaluated the electrostatic potential V and its derivatives at the un-relaxed position of the host ions, we have evaluated them at the relaxed position. Besides improving consistency, this approach allows one to study the direct influence of lattice relaxation and strain on V, V_ij and hence on the valence electric field gradient q^v. For the nearest neighbours in Al host, this effect is shown to change q^v by up to 18%.     

Measuring the longitudinal NMR relaxation rates of fast relaxing nuclei using a signal eliminating relaxation filter

A new experiment for selective determination of the relaxation rates of fast relaxing NMR signals is presented. The experiment is derived from the conventional inversion recovery experiment by substituting the 180 degrees inversion pulse of this experiment with a signal eliminating relaxation filter (SERF) consisting of three 180 degrees pulses separated by two variable delays, Delta1 and Delta2. The SERF experiment allows a selective suppression of signals with relaxation rates below a given limit while monitoring the relaxation of faster relaxing signals. The experiment was tested on a sample of 20% oxidized plastocyanin from Anabaena variabilis, where the fast exchange of an electron between the reduced (diamagnetic) and the oxidized (paramagnetic) form results in a series of average signals with widely different relaxation rates. To ensure an optimum extraction of information from the experimental data, the relaxation rates were obtained from the SERF experiment by a simultaneous analysis of all the FIDs of the experiment using a fast linear prediction model method developed previously. The reliability of the relaxation rates obtained from the SERF experiment was confirmed by a comparison of the rates with the corresponding rates obtained from a conventional inversion recovery experiment.     

The effect of neutron and gamma irradiation on the dislocation relaxation peak in silver

Summary The internal-friction spectrum has been determined in extremely pure polycrystalline silver crystals after being subjected to neutron and gamma irradiation. The spectra obtained for an applied frequency of 10 MHz show that the mechanical losses due to dislocation relaxation decrease as the doses of neutron and gamma radiation increase. The effect of gamma radiation was found to be more significant than that of neutron irradiation. The relationship between damping neutron and gamma dose was found to be of the formQ _max ⁻¹ α ΛD ^η, where η is equal to −1.2 for neutron and −1.45 for gamma irradiation. The variations of the peak height and width, and temperature of the dislocation relaxation peak as functions of neutron and gamma doses are explained in terms of the pair-kink formation model.     

Thickness dependence of the surface-polariton relaxation rates in a crystal slab

A few useful formulas are derived concerning the damping properties of long-wavelength surface polaritons (SP) in a crystal slab. For example, it is shown that the SP propagation length in a metal slab with thickness 2a satisfying $\text{ω}{}_{\mathrm{p}}\text{a}\text{c}\text{< 1}$ (c is the velocity of light and ω_p the electronic plasma frequency) is increased by the factor $\text{3}\text{2}\text{(}\text{c}\text{ω}{}_{\mathrm{p}}\text{a}\text{)}{}^4$ in comparison with the corresponding value for SP in a semi-infinite metal.     

Dependence of the magnetization relaxation time of single-domain ferromagnetic particles on damping in the Brown model

Analytical expressions for the magnetization relaxation time τ of single-domain ferromagnetic particles with cubic or uniaxial anisotropy in a static transverse magnetic field are derived. The derivation is based on calculating the escape rate of a Brownian particle from a potential well; this technique is applicable at any damping and is generalized to the case of magnetic relaxation of superparamagnetic particles. The validity of the expressions obtained for τ is checked against a numerical solution of the Landau-Lifshitz-Gilbert equation over the whole range of damping (very low, intermediate, and high damping and the crossover region between low and intermediate damping).     

Experimental test of the fluctuation theorem for a driven two-level system with time-dependent rates

A single defect center in diamond periodically excited by a laser is shown to provide a simple realization for a system obeying a fluctuation theorem for nonthermal noise. The distribution of these fluctuations is distinctly non-Gaussian, which has also been verified by numerical calculation. For driving protocols symmetric under time reversal a more restricted form of the theorem holds, which is also known from entropy fluctuations caused by thermal noise.     

Product operator analysis of the influence of chemical exchange on relaxation rates

Measurements of chemical-exchange processes by NMR are widely used to obtain valuable information about molecular dynamics and structure. Here, a computational method is introduced to assess the influence of chemical exchange on spin relaxation rates. The method is based on the inclusion of a random exchange process in product operator calculations on a microscopic level. This product operator approach can be applied to estimate exchange contributions when using sophisticated pulse sequences that cannot be easily described analytically. The method applies to the full range of exchange times measurable by NMR and can incorporate interference effects between exchange and other processes such as scalar coupling. To demonstrate its utility, simulated relaxation data were compared with theoretical predictions of spin-locking and Carr-Purcell spin-echo sequences with hard and adiabatic pulses, using different time scales for a two-site chemical-exchange process. Finally, simulations were used to examine a system in which a second random process is superimposed on a simple two-site exchange process. The method was found to provide a simple and robust tool to analyze pulse sequences and equations commonly used to study exchange-induced relaxation.     

关于人眼视觉的一些计算

本文着重介绍了在工程技术中常用的一些关于视觉的计算。首先介绍模型眼,然后介绍有关相对光谱光视效率、主观亮度、瞳孔直径、网膜刺激大小、对比度及调制度、视力与分辨率的计算,最后说明人眼极限分辨角与亮度、对比度的关系,并举例说明了在夜视仪中的应用。     

Relating the Bures Measure to the Cauchy Two-Matrix Model

The Bures metric is a natural choice in measuring the distance of density operators representing states in quantum mechanics. In the past few years a random matrix ensemble and the corresponding joint probability density function of its eigenvalues was identified. Moreover, a relation with the Cauchy two-matrix model was discovered but never thoroughly investigated, leaving open in particular the following question: How are the kernels of the Pfaffian point process of the Bures random matrix ensemble related to the ones of the determinantal point process of the Cauchy two-matrix model, and moreover, how can it be possible that a Pfaffian point process derives from a determinantal point process? We give a very explicit answer to this question. The aim of our work has a quite practical origin since the calculation of the level statistics of the Bures ensemble is highly mathematically involved while we know the statistics of the Cauchy two-matrix ensemble. Therefore, we solve the whole level statistics of a density operator drawn from the Bures prior.