Several features of the Ar<Superscript>+</Superscript>-ion irradiation of vanadium and its alloys

Changes in the surface properties of vanadium and its alloys irradiated by Ar⁺ ions with the energy 20 keV, to a dose of 10²² m⁻² at T _irr ≈ 700K have been studied. The radiation effect consists of material surface hardening, increasing the lattice parameters of the irradiated samples, and radiation erosion of the surface layers in the form of flaking. Features of radiation damage to the material’s surface irradiated by gas ions with high sputtering coefficients are discussed.     

Molecular dynamics investigation of the helical structure of smectic C*

The numerical simulation of the behavior of the molecules of the DOBAMBC liquid crystal by the molecular dynamics method allowed us to find the change of the the conformation of molecules as a function of temperature and to reveal the helical structures with various pitches in the smectic phase C*. These results explained the anomalous temperature dependences of the order parameter at the molecular level, as well as the optical second harmonic generation in the region of smectic A.     

Low-frequency Raman spectra of paradichlorobenzene thin films

The low-frequency Raman spectra of paradichlorobenzene nanofilms with a thickness of about 30 μm, 1 μm, and 400 nm are experimentally measured. With decreasing film thickness, the spectral lines shift to lower frequencies and broaden and some additional lines appear, the intensity of which increases with decreasing film thickness. The film structure is modeled using the method of molecular dynamics, and the histograms of lattice vibration spectra are calculated by the Dean method. It is found that a decrease in the film thickness leads to an increase in the lattice parameters and to a higher orientational disorder both inside the film and, especially, at the film surface. The film structure is similar to the structure of the α-paradichlorobenzene single crystal. To correctly interpret the spectra of lattice vibrations, it is necessary to take into account surface vibrations and existence of vacancies in the structure.     

Sur quelques récents résultats dans la théorie des surfaces algébriques

Sans résumé     

Luminescence of silicon films on sapphire irradiated with high-energy particles

Using low-temperature (4.2–78 K) photoluminescence, we study the processes of defect formation in silicon films on sapphire irradiated with high-energy particles (electrons, γ-quanta of⁶⁰Co). It is established that carbon atoms, as a residual process impurity, participate in the formation of luminescence centers stable up to annealing temperatures of about 550 K. For carbon-containing centers we reveal a shift in the spectral lines relative to their position in spectra of single-crystal silicon. It is proposed that this spectral shift is associated with the presence of internal stresses of about 5·10⁸ N/m² in the silicon films. Institute of Solid-State Physics and Semiconductors, National Academy of Sciences of Belarus, 17, P. Brovka Str., Minsk, 220072, Belarus. Translated from Zhurnal Prikladnoi Spektroskefii, Vol. 66, No. 3, pp. 383–386, May–June, 1999.     

Approximation of Hartree-Fock atomic form factors within the shell model

On the basis of the shell model for the electron distribution in an atom, analytic expressions and approximations are constructed for relativistic Hartree-Fock atomic form factors. The proposed approach ensures a precision that is two to three times higher than that of approximations based on existing models. This approach makes it possible to obtain simple expressions for a screening function of the Thomas-Fermi type.     

Effect of various components of slightly diluted tungsten-based alloys on the formation of defects in atomic displacement cascade regions: Field-ion microscopic analysis

New complex field-ion microscopic data for the formation of radiation-induced defects in VChV ultrapure tungsten, VA-3 commercial-grade tungsten, and four slightly diluted tungsten-based alloys (W-Hf-C, P39A; W-1.5% ThO₂, VT-15; W-5% Re, VR-5; and W-2% Fe, VZh-2) are reported. Samples were irradiated in an external unit by Ar⁺ and Ni⁺ ions of energy 35 keV. In the experiments, the ion current is kept at j=2.0 μA and the irradiation fluence equals Φt=5×10¹⁴ ions/cm². The clustering of single vacancies in samples irradiated is studied in relation to the impurity concentration and type. The distribution of vacancy clusters over the number of aggregated single vacancies is studied. These distributions are found to differ noticeably inside and outside the depletion zones. The mean lengths of focused substitutional atomic collision chains in samples with different impurity concentrations and types are measured indirectly. From these data, the efficiency of trapping intrinsic interstitials by various impurities in tungsten is estimated.     

Moments for stationary Markov chains with asymptotically zero drift

We consider a Markov chain on ℝ⁺ with asymptotically zero drift and finite second moments of jumps. We assume that the chain has invariant distribution. The paper is devoted to the existence and nonexistence of moments of invariant distribution. Our analysis is based on the technique of test functions.     

Evolution of the superconducting state of Fe-based compounds with doping

We introduce an effective low-energy pairing model for Fe-based superconductors with s- and d-wave interaction components and a small number of input parameters and use it to study the doping evolution of the symmetry and the structure of the superconducting gap. We argue that the model describes the entire variety of pairing states found so far in the Fe-based superconductors and allows one to understand the mechanism of the attraction in s(±) and d(x(2)-y(2)) channels, the competition between s- and d-wave solutions, and the origin of superconductivity in heavily doped systems, when only electron or only hole pockets are present.