The absorption spectrum of beryllium

Energy terms, dipole oscillator strengths and photoionization cross-sections from the ground state are calculated. Autoionizing state transition energies and line widths for the ¹P⁰ resonances in the continuum are also obtained. The configuration interaction method for initial and final states is used, and atomic orbitals are generated through angular-momentum-dependent, scaled Thomas-Fermi-Dirac potentials.     

Infrared absorption spectrum of beryllium acceptors in silicon

Beryllium, when introduced into silicon by thermal diffusion, gives rise to several acceptor impurity centers. The infrared absorption spectrum of beryllium in silicon has been measured with a Fourier-transform spectrometer. The absorption spectrum observed clearly shows five different beryllium acceptor centers in silicon.     

Lateral diffusion and percolation in membranes

An algorithm based on Voronoi tessellation and percolation theory is presented to study the diffusion of model membrane components (solutes) in the plasma membrane. The membrane is modeled as a two-dimensional space with integral membrane proteins as static obstacles. The Voronoi diagram consists of vertices, which are equidistant from three matrix obstacles, joined by edges. An edge between two vertices is said to be connected if solute particles can pass directly between the two regions. The percolation threshold, pc, determined using this passage criterion is pc approximately equal to 0.53. This is smaller than if the connectivity of edges were assigned randomly, in which case the percolation threshold pr=2/3, where p is the fraction of connected edges. Molecular dynamics simulations show that diffusion is determined by percolation of clusters of edges.     

Reverse-selective diffusion in nanocomposite membranes

The permeability of certain polymer membranes with impenetrable nanoinclusions increases with the particle volume fraction [T. C. Merkel, Science 296, 519 (2002)10.1126/science.1069580]. The discovery contradicts qualitative expectations based on Maxwell's classical theory of conduction or diffusion in composites with homogeneous phases. This Letter presents a theory based on an hypothesis that polymer chains are repelled from the inclusions during membrane casting. The accompanying increase in free volume, and hence solute diffusivity, yields bulk transport properties that are in good agreement with experiments.     

Modeling of processes for nonmonochromatic radiation in a ring interferometer with nonlinearity, delay, and diffusion

A generalized mathematical model is suggested and the method of slowly varying variables is used. It is shown that the approximation of large radiative losses in an interferometer is not equivalent to the single-pass approximation. It is suggested to construct bifurcation and instability charts for a clear understanding of the influence of physical factors on the dynamics. It is shown that the loss of monochromaticity significantly complicates the structure of these charts and reduces the regions of stable radiation passage. It is established that the radiation delay in the interferometer gives rise to the bifurcation of period doubling at a smaller nonlinearity parameter. Tomsk State University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 29–35, February, 2000.     

Infrared lattice absorption spectra of zinc selenide doped with beryllium

Infrared localized mode sidebands due to the combined excitation of the localized mode and the host lattice modes are observed for ZnSe:Be. Two bands are obtained in the acoustic band mode region. The band at 105 cm^-1 is assigned to a resonant mode and the band at 72 cm^-1 is attributed to the impurity activated TA(X) phonon.     

Non-equilibrium point defect phenomena influencing beryllium and zinc diffusion in GaAs and related compounds

Beryllium and zinc are the main p-type dopants used for the fabrication of devices based on GaAs or related III-V materials. Both elements are substitutionally dissolved on the group III sublattice and diffuse via the kick-out mechanism which involves group III self-interstitials. Non-equilibrium concentrations of these self-interstitials have a strong influence on the diffusivities of Be and Zn with often drastic consequences on device behavior especially if Be or Zn is used to realize narrow base regions in heterojunction bipolar transistors (HBTs). Various situations in which non-equilibrium point defects play a role for Be and Zn diffusion are discussed such as: in-diffusion of these dopants from an outside source, diffusion of grown-in dopants, self-interstitial generation by Fermi level surface pinning of highly n ⁺-doped emitter cap or subcollector layers in HBTs, or recom bination-enhanced beryllium diffusion during device operation. Finally, we will comment on the diffusion behavior of carbon, which is dissolved on the group V sublattice in GaAs, is much less sensitive to non-equilibrium point defect, and, therefore, is increasingly used to replace Be and Zn as p-type dopants.On sabbatical leave from Duke University, School of Engineering, Durham, NC 27706, USA     

Fractal diffusion processes in particle physics

A series of consistent measurements of kinematic variables for pion diffraction production processes by pions with an initial momentum of about 4 GeV/c were analyzed: π ⁺ + p → p + 2π ⁺ + π ^? and π ^? + p → p + 2π ^? + π ⁺. The Hurst method analysis discovered the presence of the memory effect for both data arrays. The distributions of the transition probability density appeared to seek some equilibrium shape, characteristic of the fractal Brownian motion (FBM). The process can be defined by the special diffusion Fokker-Planck equation (FPE). The obtained values of Hurst coefficient 0.5 < H < 1, which is a parameter of FPE, mean that the processes explored are realized in fractal generalized phase space with fractional dimension.     

Clustering of volatility in variable diffusion processes

Increments in financial markets have anomalous statistical properties including fat-tailed distributions and volatility clustering (i.e., the autocorrelation functions of return increments decay quickly but those of the squared increments decay slowly). One of the central questions in financial market analysis is whether the nature of the underlying stochastic process can be deduced from these statistical properties. We have shown previously that a class of variable diffusion processes has fat-tailed distributions. Here we show analytically that such models also exhibit volatility clustering. To our knowledge, this is the first case where clustering of volatility is proven analytically in a model.Our results are compatible with the viewpoint that variable diffusion processes are possible models for financial markets.     

Infrared absorption due to localized vibrational mode of beryllium in cadmium sulfide and selenide

The polarized infrared absorption due to the fundamental and 2nd-harmonic excitation of the localized vibration of Be in CdS and CdSe is observed. The splitting and the polarization property are in accord with C_3ν point symmetry of Be impurity. The anharmonic potential coefficients are obtained up to the 3rd order. The Raman scattering from Be localized mode is also measured in the case of CdS: Be.     

Diffusion processes in defective crystals and multistate diffusion

A stochastic theory of multistate diffusion on defective lattices is introduced via a defect-dressed continuous-time random walk propagator. The method is employed for periodic defective lattices to analyze: multistate diffusion; the effects of lattice structure on diffusion constants; and variations in quasielastic neutron scattering line-shapes due to migration mechanisms. A new interpretation of anomalous vacancy diffusion, deviating from Arrhenius behaviour, due to competition between different monovacancy migration mechanisms is given.     

Mechanisms and energetic of atomic processes in surface diffusion

Surface diffusion is a subject of basic importance for understanding mass transport phenomena in surface and nano science. In the particle aspect of surface diffusion of single atoms and simple molecules, information of interest is the detail atomic mechanisms and the activation energy of various atomic processes, and also the binding energy of atoms at different surface sites. In the absence of an external force, atoms will perform random walk without a preferred direction. When an atom is subjected to an external force, or when a chemical potential gradient exists, it will move preferentially in the direction of the force, or in the direction of decreasing chemical potential, thus the random walk becomes directional. Using atomic resolution microscopy, it is now possible to observe random walk diffusion of atoms, molecules and atomic clusters directly as well as to study the dynamic behavior of atoms as perturbed by the electronic interactions of the surface in great detail. Here, methods of studying quantitatively the particle aspect of surface diffusion and how it affects the dynamic behavior of the surface are very briefly reviewed.     

Modelling of gas absorption processes in bubblers

A physical and mathematical model is developed for heat and mass exchange processes at the absorption of gaseous components from vapor-gas mixtures in bubblers. The intensity of these processes in the foam and centrifugal bubblers depends overall on the conditions of their passing in single bubbles forming on the orifices of gas distribution grates. The constructed model, as was shown by its numerical realization for specific conditions, is confirmed by known experimental data and can be used for engineering computations of thermal and mass exchange parameters of bubblers and the optimization of their operation regimes.     

Ergodic properties of anomalous diffusion processes

In this paper we study ergodic properties of some classes of anomalous diffusion processes. Using the recently developed measure of dependence called the Correlation Cascade, we derive a generalization of the classical Khinchin theorem. This result allows us to determine ergodic properties of Lévy-driven stochastic processes. Moreover, we analyze the asymptotic behavior of two different fractional Ornstein–Uhlenbeck processes, both originating from subdiffusive dynamics. We show that only one of them is ergodic.     

Behavior of general one-dimensional diffusion processes

We develop simple rigorous techniques to estimate the behavior of general one-dimensional diffusion processes. Any one-dimensional diffusion process (with drift) can be mapped onto a symmetric diffusion through an explicit change of variable. For such processes we can estimate explicitly the diffusion exponent, the recurrence properties, and the large fluctuations. In a second part, we apply these results to different models (including the Sinaï random walk: diffusion in a random drift) and we show how the main features of the diffusion can be readily handled.     

Conditional Granger causality of diffusion processes

The statistical concept of Granger causality is defined by prediction improvement, i.e. the causing time series contains unique information about the future of the caused one. Recently we proposed extending this concept to bivariate diffusion processes by defining Granger causality for each point of the state space as the Granger causality of a process obtained by local linearisation. This provides a Granger causality map, well-defined at least in the vicinity of stable fixed points of the deterministic part of the dynamics. This extension has convenient properties, but carries several important limitations. In the current paper we show how the Granger causality of diffusion processes can be further generalized, incorporating in particular the concept of conditional causality. Moreover, we demonstrate the application potential to systems with a more complex attractor structure such as limit cycles or bistability of fixed points.     

Number of arm selection in two-dimensional diffusion processes

We introduce an algorithm to generate two-dimensional diffusion-limited star-branched aggregates (DLSA)attaching bi-functional monomers successively to a central colloidal particle with any desired number of reactive sites. The proposed algorithm produces star-shaped aggregates that grow forever and show a power law polydispersity in the chemical length of the arms near the central colloid. More interestingly, it gives rise to a number of arm selection consisting in that only a small number of arms (around five) define the final structure at relatively large distances from the central colloid, independently of the initial number of reactive sites and the size of the central colloid. We characterize the structure of the aggregates by means of the particle-particle correlation function, analyze its scaling properties and obtain the fractal dimension.