Spatial distribution profiles of defects in cadmium-mercury-tellurium after ion implantation

Defect spatial distributions are investigated after implantation of ions in Cd_xHg_1−xTe under various sets of conditions (radiation dose, type and energy of ions, ion current density, and dose absorption rate). Distribution profiles of electrically active radiation-induced defects are calculated with allowance for the generation of defect complexes of vacancion nature. Defect profiles are determined in experiments after implantation of hydrogen and iron ions at constant low ion current densities, and after implantation of copper, tungsten, and aluminum ions in the case of pulsed bombardment at high ion current densities. Secondary-ion mass spectrometry, electron-positron annihilation, Rutherford backscattering of ions, and differential Hall measurements are used to obtain distribution profiles of interstitial ions, vacancion and extended defects, and electrically active defects, respectively. The profiles of these defects are analyzed for various ion-implantation conditions. V. D. Kuznetsov Siberian Physicotechnical Institute at Tomsk State University Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 101–116, January, 1998.     

以最大原子密度定义合金相中的第一近邻团簇

本文提出利用不同壳层所包含的径向原子密度, 即单位体积内的原子个数随着径向的分布, 来方便而精确地定义团簇, 即具有最大径向原子密度的且表面呈现三角密堆结构的完整壳层为第一近邻团簇. 最后以Al-Ni-Zr合金相为例说明了该方法的合理性与适用性, 及此方法所定义的团簇与非晶形成的关系.     

Slow diffusion of light in a cold atomic cloud

We study the diffusive propagation of multiply scattered light in an optically thick cloud of cold rubidium atoms illuminated by a quasiresonant laser beam. In the vicinity of a sharp atomic resonance, the energy transport velocity of the scattered light is almost 5 orders of magnitude smaller than the vacuum speed of light, reducing strongly the diffusion constant. We verify the theoretical prediction of a frequency-independent transport time around the resonance. We also observe the effect of the residual velocity of the atoms at long times.     

Growth instability in diffusion controlled polymerization

We report the observation of a sharp transition in the structure of polypyrrole prepared via diffusion controlled polymerization in a quasi-two dimensional electrochemical cell. As the oxidation potential is increased, the fractal dimension drops sharply from values near 2 (compact structures) to 1 as the growth becomes dendritic. At higher potentials, where the polymerization becomes diffusion limited, a continuum of structures is observed as the dendrites become irregular and the fractal dimension increases to 1.74+-0.01. The mean branch angle was also measured and found to decrease from 90° at the point of dendritic growth, to 45° at high potential.     

Investigation of photon-photon angular correlations in atomic mercury

By application of the time differential coincidence technique developed in nuclear spectroscopy, for the first time the angular correlation of light quanta of atomic decays was observed. The experiments were carried out with the 7³ S ₁-6³ P ₁-6¹ S _o-cascade of mercury excited by electron impact at energies of 50 eV. The two transitions of 4358 and 2536 Å were selected by interference filters and detected by photomultipliers. Time differential coincidence spectra were measured at the two angular positions 0=90 ° and 0=180 °. It was found that the excitation of the 7³ S ₁-state by the electron beam leads to no alignment. Therefore the usual γ-γ angular correlation theory is applicable. Perturbations occur by static interaction with external magnetic fields and by free hyperfine interaction in the odd isotopes of mercury. The interaction frequencies of the free hyperfine interaction are in all cases too high to be resolved. Measurements were performed using mercury of natural abundances, with and without external magnetic fields. First of all the spin rotation in the magnetic field of the earth (690 mG) was observed, giving for theg-factor of the 6³ P ₁-state $$g(Hg, 6^3 P_1 ) = 1.35 \pm 0.10.$$ This value is in agreement with the more precise values determined by other techniques. Shielding of the earth's field gave a nearly unattenuated angular correlation, with an integral attenuation factor of $$G_2 = 0.98 \pm 0.06.$$ The value of the angular correlation coefficientA ₂. $$A_2 = - 0.188 \pm 0.009,$$ is in good agreement with the expected average value for the natural isotope mixture $$\overline {A_2^{theor} } = - 0.196.$$ A third measurement in an external magnetic field of 5.6 G showed several complete spin rotations. A nice fit was possible by using the superposition of all six participating cascades in the hyperfine level schemes of the different isotopes. The theoretical anisotropies,g _F -values, and intensities were inserted according to natural isotope abundances. From the time differential measurements also a value for the lifetime of the Hg-6³ P ₁-level was derived. The result $$\tau (Hg, 6^3 P_1 ) = (120 \pm 2) nsec$$ is in agreement with earlier measurements. Further applications of the method are discussed.     

Electron-spin-resonance instability in two-dimensional atomic hydrogen gas

We study by electron-spin-resonance spin-polarized atomic hydrogen adsorbed on the surface of superfluid helium at temperatures T(S) from 50 to 110 mK. The average dipolar field in this 2D system shifts the electron-spin-resonance peak of the adsorbed atoms relative to that of bulk atoms. The shift is directly proportional to surface density. The role of longitudinal magnetization relaxation is played by particle exchange between the 2D and the 3D phases, which diminishes exponentially with decreasing T(S). Therefore at T(S) less, similar 80 mK an excitation field of 0.1 mG disturbs the equilibrium surface density and leads to a magnetization instability observed as sawtooth shaped resonance lines.     

Anomalous photon diffusion in atomic vapors

The multiple scattering of photons in a hot, resonant, atomic vapor is investigated and shown to exhibit a Lévy flight-like behavior. Monte Carlo simulations give insights into the frequency redistribution process that originates the long steps characteristic of this class of random walk phenomena.     

Quantum diffusion of atomic species in metals

The coefficient of quantum diffusion D(T) is calculated using exact states of the localized impurity in the deformed vibrating lattice. Without resorting to the Born-Oppenheimer approximation an explicit general expression for D(T), valid for impurities of arbitrary mass and holding in the whole range of temperature, is obtained in terms of the crystal properties. Interesting new features arise from retaining some terms of the Hamiltonian which were left aside in previous theories.     

On the mechanism of atomic diffusion in condensed matter

The mechanism of an elementary diffusion act in condensed matter is analyzed by a molecular dynamics method using the example of a three-dimensional lattice with the Lennard-Jones interatomic interaction potential. It is found that, in a certain temperature range, the diffusion of atoms changes in character from individual hopping to cooperative movement of many atoms (stringlike diffusion). The collective displacement of atoms is associated with the nonlinear interatomic potential and manifests itself at specific relationships between the potential barriers and the temperature. Analogous phenomena must occur in any condensed system.     

A necessary condition for the emergence of diffusion instability in media with nonclassical diffusion

A necessary condition is derived for the emergence of diffusion instability in media in which diffusion does not obey classical Fick laws. The equations derived by Yadav and Horsthemke [Phys. Rev. E 74, 066118 (2006)] using the continuous-time random walk model are employed as equations simulating reaction—diffusion processes. The waiting-time distribution function is represented by the sum of a finite number of exponents. It is shown that passage to the diffusion limit in the time variable is an incorrect operation if it is used to analyze diffusion instability in media with a distribution function that differs from the Poisson distribution function.     

The fast-time instability of diffusion flames

A diffusion flame burns with varying intensity if the pressure is varied, and the nonlinear steady-state response is typically S-shaped; that is, multiple solutions exist for some range of pressure. The physically relevant branch can only be determined by unsteady analyses, and in this paper we discuss the stability of a large class of diffusion flames when the activation energy characterizing the reaction rate is large. Specifically, we examine the evolution from the stationary solution on a time scale so short that changes are confined to the thin flame sheet where all the reaction occurs. In this region time derivatives are added to the steady state equations, which otherwise describe a balance between diffusion and chemical reaction. The stability problem is then reduced to the determination of the spectrum of Schrödinger's equation, defined on the infinite interval, with a potential that is not of one sign on this interval. In this way certain conclusions about extinction are drawn and certain past misconceptions are clarified.     

Energies and widths of atomic core-levels in liquid mercury

High-resolution measurements of the photoinduced X-ray emission of liquid mercury were performed, using a transmission DuMond-type crystal spectrometer for transitions above 11 keV and a reflection von Hamos-type crystal spectrometer for transitions below 11 keV. The target X-ray fluorescence was produced by irradiating the sample with the Bremsstrahlung from X-ray tubes. The energies and natural linewidths of 6 K-shell, 26 L-shell and 2 M-shell X-ray transitions were determined. Using a least-squares-fit method to solve the two sets of equations derived from the observed transition energies and transition widths the binding energies of the subshells K to M₅ and O₁ and the level widths of the subshells K to N₅ and O₁ could also be determined.     

Convective instability and mass transport of diffusion layers in a Hele-Shaw geometry

We consider experimentally the instability and mass transport of flow in a Hele-Shaw geometry. In an initially stable configuration, a lighter fluid (water) is located over a heavier fluid (propylene glycol). The fluids mix via diffusion with some regions of the resulting mixture being heavier than either pure fluid. Density-driven convection occurs with downward penetrating dense fingers that transport mass much more effectively than diffusion alone. We investigate the initial instability and the quasisteady state. The convective time and velocity scales, finger width, wave number selection, and normalized mass transport are determined for 6000相似文献   

Solvent diffusion kinetics in the dilute random alloy

In this paper we address the problem of enhancement of the solvent diffusivity by the addition of very small amounts of solute. The random alloy model is employed at the dilute limit. No Monte Carlo calculations have been available for evaluating the available theoretical treatments. In the present study we report on very-high-precision results of the solvent correlation factors as obtained from Monte Carlo calculations with averaging over 6 210 7 atoms and very long runs (2000 jumps per atom). The results are expressed in terms of the usual solvent enhancement factors b 1 and b 2 . There is good agreement with the theory of Holdsworth and Elliott and excellent agreement with the theory of Moleko et al . but not with other available theories.