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A phase field model is used to study dendritic growth in a media with impurities. The model consists of a square lattice where a parameter $\psi$ can take values between $0$ and $1$ at each site. A site is in the solid phase for $\psi >1$, in the liquid phase for $\psi =0$, and the solid-liquid interface is expressed by $0<\psi <1$. A fraction of the sites are considered impurities that cannot be solidified, i.e.$\psi$ is fixed and taken as zero. These impurities are distributed randomly. As the probability $p$ of impure sites in the lattice increases, the growth loses its dendritic characteristic. It is shown that the perimeter of the growing solid goes from quadratic to a linear function with time. It was also found that as the probability of impurities reaches $p=0.004$, the solid undergoes a transition from anisotropic to isotropic growth.

Binding energy for an exciton (X) bound in a parabolic two-dimensional quantum dot by an acceptor impurity A^- located on the z-axis at a distance d from the dot plane, are calculated using the Hartree formalism with a recently developed numerical method (PMM) for the solution of the Schrödinger equation. As our analysis indicates there is a critical dot radius R^c such that for R < R^c the complex (A^-, X) is unstable and with an increase of the impurity distance this critical radius increases. Furthermore, there is a critical value σ^c of the mass ratio such that for σ > σ^c the complex is stable.

In the last decade, new techniques for producing impure superfluids with unique properties have been developed. This new class of systems includes superfluid helium confined to aerogel, HeII with different impurities, superfluids in Vycor glasses, and watergel. These systems exhibit very unusual properties including unexpected acoustic features. We discuss the sound properties of these systems and show that sound phenomena in impure superfluids are modified from those in pure superfluids.

We calculate the coupling between temperature and pressure oscillations for impure superfluids and show that this coupling increases significantly. This leads to the existence in impure superfluids of such unusual sound phenomena as slow "pressure" waves and fast "temperature" waves. This also decreases the threshold values for nonlinear processes as compared to pure superfluids. Sound conversion, which has been observed in pure superfluids only by high intensity waves should be observed at moderate sound amplitude in impure superfluids. Cerenkov emission of second sound by first sound (which has never been observed in superfluids) could be observed in impure superfluids. Even the nature of the sound modes in impure superfluids turns out to be changed. We have also derived for the first time the nonlinear hydrodynamic equations for superfluid helium in aerogel.

We calculated the binding energies of shallow donors and acceptors in a spherical GaAs-Ga_1-xAl_xAs quantum dot under the combined effect of isotropic hydrostatic pressure and an intense laser. We used a variational approach within the effective mass approximation. The binding energy was computed as a function of hydrostatic pressure, dot sizes and laser field amplitude. The results showed that the impurity binding energy increases with pressure and decreases with the laser field amplitude when other parameters are fixed. We also found that the pressure effects are more dramatic for donor than acceptor impurities, especially for quantum dots with small radii.

The effects of differently charged Ge impurities on the local atomic structure and lattice dynamics of $\alpha$-quartz were studied. We have determined the equilibrium structures and calculated the symmetrized local density of vibrational states for the Ge-doped $\alpha$-quartz. The frequencies of localized vibrations of $A$- and $B$-symmetries induced by Ge impurities were obtained. Besides, we have analyzed what contribution the vibrations of atoms located around the Ge impurities make to the localized symmetrized vibrations.

The purpose of this paper is to study thermal effects in a graphene sheet on substrate. The temperature evolution of the phonon branches and of the crystal lattice is obtained when also the presence of the substrate is taken into account. The numerical strategy is based on the coupling of stochastic and deterministic numerical methods; Boltzmann equation for charge transport is treated by using the Direct Simulation Monte Carlo approach, whose results are treated as source terms for the phonon Boltzmann equation which is solved deterministically. The results are useful also for applications as correct design of graphene-based prototypes, when heating effects in the presence of a substrate are relevant.

We present a possible explanation of the discrepancy between theory and experiment (observed by Chikkatur et al.) in the collisional density of impurities in a Bose–Einstein condensate of sodium atoms. The discrepancy is ascribed to the fact that the experiment was carried out in a situation of strong instability, where small variations of the initial number of impurities in the condensate may give rise to a large variation in the number of colliding atoms.

The complete high-order perturbation formulas are established by both crystal-field (CF) and charge-transfer (CT) mechanisms. The EPR g factors of MgTiO₃:Cr³⁺, SrTiO₃:Cr³⁺ and SrTiO₃:Mn⁴⁺ crystals are calculated from the formulas. The calculations of the EPR g factors are in agreement with the experimental values. The contribution rate of the CT mechanism (|Δg^T/Δg^F|) to EPR parameters, increases with the growth of the valence state for the 3dⁿ ions in the crystals. For the higher valence state 3d³ ion Mn⁴⁺ in crystals, the explanation of the EPR parameters reasonably involves both CF and CT mechanisms. The g values are also given from one-spin-orbit-parameter model and crystal-field (CF) mechanism for comparison.

In this paper, we investigate the influence of a gate electric field on the tunneling current for the contact of impurity graphene nanoribbon with a metal or quantum dots. Based on the Hamiltonian for graphene in the tight-binding approximation, the density of states is calculated, which allows us to obtain a tunneling current. We analyze the effect of the field magnitude on the detecting possibility of an impurity in the graphene nanoribbon. A sufficient change of current–voltage characteristic (CVC) of the contact is observed, with an increase in the constant electric field applied parallel to the nanoribbon plane.

In fusion devices, strongly localized intensive sources of impurities may arise unexpectedly or can be created deliberately through impurity injection. The spreading of impurities from such sources is essentially three-dimensional and nonstationary phenomenon involving physical processes of extremely different time scales. Numerical modeling of such events is still a very challenging task even by using most modern computers. To diminish the calculation time drastically a "shell" model has been elaborated that allows to reduce equations for particle, parallel momentum and energy balances of various ion species to one-dimensional equations describing the time evolution of radial profiles for several most characteristic parameters. The assumptions of the "shell" approach are verified by comparing its predictions with a numerical solution of one-dimensional time-dependent transport equations.

The properties of Ni-doped strontium titanate are studied using X-ray diffraction and XAFS spectroscopy. It is shown that regardless of the preparation conditions, the SrTi_1-xNi_xO₃ solid solution and the NiTiO₃ phase are the most stable phases which can coexist. According to the EXAFS data, in the single-phase sample of SrTi_0.97Ni_0.03O₃, the Ni atoms substitute for the Ti ones and are on-center. The distortion of the oxygen octahedra is not observed. The XANES spectra analysis shows that the oxidation state of nickel in NiTiO₃ is 2+, and in the SrTi_1-xNi_xO₃ solid solution it is close to 4+. It is shown that the strongest light absorption in doped samples is associated with the presence of tetravalent nickel in the SrTi_1-xNi_xO₃ solid solution. This doping seems the most promising one for solar energy converters that exploit the bulk photovoltaic effect.

In this paper, the nonlinear suspension mathematical model obtained from the actually measured data of a car is applied to simulate the nonlinear spring force and damping force of vehicle suspension system. A dynamic equation of quarter suspension system with one d.o.f is derived. The theories of nonlinear dynamics are applied to study the nonlinear model and to reveal its nonlinear vibration characteristics. The bifurcation behavior is analyzed by using central manifold theorem. The response under road stochastic excitation is obtained by computer simulation. Through the methods of phase trajectory, Poincaré map, time history, power spectrum diagram and Lyapunov exponent, it is revealed that the chaos occurred in the nonlinear suspension system. Furthermore, the speed feedback controller is applied to control the chaos motion of the suspension system. This method is proved to be valid and feasible by numerical simulation.

The effect on surfaces that are in plasma environment is one of the problems of plasma chemistry. A variable electric field layer (called sheath) between plasma and surface is capable of changing ion concentration which results to the reduction of surface of the objects that exist in plasma environment (cleaning of archaeological objects). A proposed improvement of plasma restoration system with the application of external electric field is expected to have better results on the conservation of archaeological objects.