Carbon segregation on a tungsten surface

New measurements are performed and a comprehensive analysis of experimental data is made on the dependence of equilibrium carbon coverage θ on a tungsten surface on its temperature and the degree of carbon loading. It is shown that if the volume is free of carbon, the variation of θ for T≥1400 K can be approximately described by the balance between the carbon flows through the boundary with the activation energy for transition into the bulk E₁=4.6 eV and the segregation energy ΔE=1.7 eV. For tungsten loaded preliminarily with carbon to a content of ≈10^?2 at. %, the θ(T) relation cannot be described in terms of the equilibrium conditions with constant E₁ and ΔE, because these quantities depend on the degree of carbon loading; E₁ grows from 4.6 to 6.8 eV and ΔE grows from 1.7 to 2.3 eV with an increase in carbon content from 0 to 10^?2 at. %. These variations are attributed to the bonds becoming stronger in carbon-loaded tungsten with increasing carbon content.     

Electronic structure and energies of interatomic bonds in the TiSi<Subscript>2</Subscript> compound with a <Emphasis Type="Italic">C</Emphasis>49 crystal structure

Full-electron calculations of the electronic structure of the TiSi₂ compound in the structural modification C49 are performed using the augmented-plane-wave method. The total energy, the electronic band structure, and the density of states are calculated for an extended translational unit cell Ti₄Si₈, which is formed during the growth of a silicon nanowire on a p-Si substrate. Calculations are also carried out for two orthorhombic unit cells of the nonstoichiometric compositions Ti₃Si₉ and Ti₅Si₇. The energies of the interatomic bonds are determined to be E _Si-Si = 1.8 eV, E _Ti-Ti = 2.29 eV, and E _Ti-Si = 4.47 eV. The dependence of the total energy of the unit cell E _tot(V) on the unit cell volume V is obtained by optimizing the unit cell volume. The bulk modulus B ₀ = 132 GPa is determined from the Murnaghan equation of state for solids and the dependence E _tot (V). This value of the bulk modulus is used to estimate the activation energy for interstitial diffusion of silicon atoms Q _i(Si) ≈ 0.8 eV.     

Fine and hyperfine structure of <Emphasis Type="Italic">P</Emphasis>-wave levels in muonic hydrogen

Corrections of order α ⁵ and α ⁶ are calculated for muonic hydrogen in the fine-structure interval ΔE ^fs = E(2P _3/2) − E(2P _1/2) and in the hyperfine structure of the 2P _1/2-and 2P _3/2-wave energy levels. The resulting values of ΔE ^fs = 8352.08 μeV, ΔẼ ^hfs(2P _1/2) = 7819.80 μeV, and ΔẼ ^hfs(2P _3/2) = 3248.03 μeV provide reliable guidelines in performing a comparison with relevant experimental data and in more precisely extracting the experimental value of the (2P–2S) Lamb shift in the muonic-hydrogen atom. Original Russian Text ? A.P. Martynenko, 2008, published in Yadernaya Fizika, 2008, Vol. 71, No. 1, pp. 126–136.     

Influence of electromagnetic radiation on the phase composition of clustered CN<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> films

The effect of white and UV radiation on the phase composition of amorphous CN _x films are studied by X-ray diffraction analysis and visible-range spectroscopy. The films have variable-range atomic order and consist of amorphous graphite clusters (30 Å) crystalline clusters (50–100 Å) of graphite, diamond, and carbon nitride phases; and intercluster medium with long-range (1–2 Å) atomic order. It is shown that irradiation of the films by white light facilitates the growth of fine graphite clusters. Irradiation by UV light suppresses the growth of the graphite and carbon nitride phases, favoring the growth of the diamond phase (1.5%). It is demonstrated that a change in the mesoscopic phase composition of the CN _x films causes a change in the energy gap width in the visible range from E _g = 0.75 eV for the films irradiated by white light to E _g = 1.75 eV for those exposed to UV radiation.     

Critical phenomena in the β-(2×4) → α-(2×4) reconstruction transition on the (001) GaAs surface

The critical exponents of the β-(2×4) → α-(2×4) reconstruction phase transition on the (001) GaAs surface are determined experimentally. It is found that the phase transition is analogous to a van der Waals transition. The critical parameters T _c , P _c , and Θ_c have been measured experimentally. The mean field theory is applied, and three-parameter isotherms are obtained that agree with the experimental results at the following values of the parameters: E_st = 0.36 eV, ΔE = 0.18 eV, and E _i = 0.134 eV. Precision measurements of the critical exponents β and δ are carried out. Their values β = 1/8 and δ = 15 indicate that the phase transition is truly two-dimensional.     

Auger- und Coster-Kronig-Übergänge der M-Schale von Krypton

The Auger spectra of theM ₂,M ₃,M ₄,M ₅ subshells of krypton and the Coster-Kronig spectra of theM ₁,M ₂,M ₃ subshells of krypton were measured with an electrostatical spectrometer. The ionization in theM shells was caused by electron impact. The use of a gaseous target made it possible to measure the Auger lines even at energies as low as 25 eV. The absolute energies and relative intensities of a great number of transitions were determined: 22 of theM _{4, 5} spectrum, 14 of theM _{2, 3} spectrum and 2 of theM ₁ spectrum. Only in the case of theM _{2, 3} spectrum a comparison between the relative intensities, determined experimentally, and those calculated byRubenstein forZ=47 was possible. The agreement is only qualitatively. Moreover, from the Auger electron energies measured, the following binding energies were calculated:E(M ₁)=(292,1±1,0) eV,E(M ₂)=(222,1±0,6) eV,E(M ₃)=(214,6±0,6)eV,E(N ₁ N ₁)=(62,81±0,05) eV.     

Radiation widths of resonances in the <Superscript>40</Superscript>Ar(pγ)<Superscript>41</Superscript>K reaction

The excitation function for the ⁴⁰Ar(pγ)⁴¹K reaction at the accelerated proton energy E _p= 450–2700 keV is studied. Positions of 168 resonances identified as levels of the ⁴¹K nucleus are observed in this energy range. The data for the proton energy E _p < 1 MeV and E _p > 2 MeV are obtained for the first time. Relative yields of γ rays observed in the decay of the resonances are calculated from a comparison with the yield of the calibrating resonance at E _p= 1102 keV. Total radiation widths of the resonances are calculated on the basis of the data obtained.     

Anomalous thermal stability of metastable C<Subscript>20</Subscript> fullerene

The results of computer simulation of the dynamics of fullerene C₂₀ at different temperatures are presented. It is shown that, although it is metastable, this isomer is very stable with respect to the transition to a lower energy configuration and retains its chemical structure under heating to very high temperatures, T ≈ 3000 K. Its decay activation energy is found to be E _a ≈ 7 eV. Possible decay channels are studied, and the height of the minimum potential barrier to decay is determined to be U = 5.0 eV. The results obtained make it possible to understand the reasons for the anomalous stability of fullerene C₂₀ under normal conditions.     

Structure and stability of two-dimensional systems of C<Subscript>20</Subscript> fullerenes

Two-dimensional systems of C₂₀ fullerenes connected to each other by strong covalent bonds have been investigated. Several isomers differing in the type of intercluster bonds have been revealed. The lifetimes τ of the (C₂₀) _{M × M} complexes with M = 2 and 3 at T = 1800–3300 K have been directly calculated using the molecular dynamics method. It has been shown that these complexes lose their periodic cluster structure due usually to the coalescence of two or several neighboring C₂₀ fullerenes. The activation energy of this process determined by analyzing the τ(T) dependence appears to be E _a ≈ 2.5 eV in agreement with the calculations of the heights of the potential barriers preventing the coalescence. At high temperatures T > 2400 K, the decay of C₂₀ fullerenes entering into the complex is possible.     

Hyperfine structure of the <Emphasis Type="Italic">S</Emphasis> levels of the muonic helium ion

Corrections of the α⁵ and α⁶ orders to the energy spectrum of the hyperfine splitting of the 1S and 2S levels of the muonic helium ion are calculated with the inclusion of the electron vacuum polarization effects, nuclear-structure corrections, and recoil effects. The values ΔE ^hfs(1S) = ?1334.56 meV and ΔE ^hfs(2S) = ?166.62 meV obtained for hyperfine splitting values can be considered as reliable estimates for comparison with experimental data. The hyperfine structure interval Δ₁₂ = 8ΔE ^hfs(2S) ? ΔE ^hfs(1S) = 1.64 meV can be used to verify QED predictions.     

Dielectric Spectroscopy of Strongly Correlated Electronic States of Vanadium Dioxide

The thermal evolution of the conductivity of a VO₂ film and database-obtained band gap E_g of film nanocrystallites is traced in the temperature range of –196°C < T < 100°C (77 K < T < 273 K); the level position of donor impurity centers is determined to be E_d = 0.04 eV. It is shown that energy E_g decreases from 0.8 to ～0 eV with an increase in temperature in the range of 273 K < T < 300 K, which is caused by the narrowing of the energy gap due to correlation effects and considered as the temperature-extended Mott “insulator–metal” electron phase transition with the monoclinic lattice symmetry retained. The subsequent jump in the symmetry from monoclinic to tetragonal with a further increase in temperature is considered as the Peierls structural phase transition, the temperature of which is in the vicinity of 340 K and determined by the size effects, nonstoichiometry of VO₂ film nanocrystallites, and degree of their adhesion to the substrate.     

Ein experimenteller Beitrag zu der durch langsame Primärelektronen ausgelösten Sekundärelektronenemission

The energy distribution of secondary electrons emitted from a highly degassed polycristalline Pt surface was investigated as a function of low energy (V _p ) primary electrons 5<V _p <150eV. The measurements were carried out in an UHV of better than 10^?10 mm Hg. The dependence of the numberN _S ^(Ev) of secondary electrons of a fixed energyE _v (3<E _v <12eV) on the energyV _p of the primaries (=isochromates) is studied. A lot of observations can be detected:

1. I.
   The exit depth of true secondary electrons is strongly dependent of their energy.     
   
   

Nonchiral BN Haeckelite nanotubes

A new class of boron-nitrogen (BN) nanotubes composed of tetragons, pentagons, hexagons, heptagons, and octagons is considered. By analogy with carbon nanotubes of the same topological structure, these nanotubes were called Haeckelites. The geometry, energetics, and electronic properties were studied in detail for two regular mutual arrangements of the polygons. It was found that Haeckelite nanotubes are dielectrics with the energy gap E_g = 3.24–4.09 eV. As the nanotube diameter increases, the energy gap E_g decreases, approaching the value for the corresponding planar Haeckelite layer. The ground-state energy of the Haeckelite BN nanotubes is 0.3 eV/atom higher than that of well-known hexagonal BN nanotubes.     

Photoluminescence in germanium with a quasi-equilibrium dislocation structure

The dislocation-related photoluminescence of n-Ge single crystals with a quasi-equilibrium structure of 60° dislocations is investigated at a temperature of 4.2 K. It is shown that the dislocation-related photoluminescence spectra are described by a set involving from 8 to 13 Gaussian lines with a width of less than 15 meV. With due regard for the data available in the literature, the Gaussian lines with maxima at energies in the range 0.47 < E _m ≤ 0.55 eV are assigned to the emission of 90° Shockley partial dislocations involved in quasiequilibrium segments of 60° dislocations with different values of the stacking fault width Δ (Δ = Δ₀, Δ < Δ₀, and Δ > Δ₀). It is revealed that the d8 line at the energy E _m = 0.513 eV, which corresponds to the emission of straight segments with the equilibrium stacking fault width Δ₀, dominates in the photoluminescence spectra only at dislocation densities N _D < 10⁶ cm^?2. As the dislocation density N _D increases, the intensity of the d8 line decreases with the d7 line (E _m ≈ 0.507 eV) initially and the d7 and d6 lines (E _m ≈ 0.501 eV) then becoming dominant in the photoluminescence spectrum. The d7 and d6 lines are attributed to the emission of segments with stacking fault widths Δ < Δ₀. Possible factors responsible for the formation of stacking faults with particular widths Δ ≠ Δ₀ for quasi-equilibrium dislocations are discussed.     

Giant nonlinear absorption in the NiO antiferromagnet

A study of the spectrum of nonlinear two-photon and two-step absorption in NiO single crystals, carried out in the energy region ?ω₁ + ?ω₂ = 2.45–4.575 eV, showed it to have a complex shape and consist of very strong peaks (from 0.05 to 2.7 cm/MW). Within the energy interval 2.45–3.3 eV, the spectrum is due to d-d transitions in the Ni²⁺ ion. The band gap width was determined to be E _g =3.466 eV. The spectral features seen above this energy originate from interband transitions from three valence subbands to the conduction band bottom.     

“Negative” gap in the spectrum of localized states of (In<Subscript>2</Subscript>O<Subscript>3</Subscript>)<Subscript>0.9</Subscript>(SrO)<Subscript>0.1</Subscript>

The reflection R(?ω), transmission t(?ω), absorption α(?ω), and refraction n(?ω) spectra of polycrystalline In₂O₃–SrO samples with low optical transparency, which contain In₂O₃ and In₂SrO₄ crystallites with In₄SrO_{6 + δ} interlayers, are examined. In the region of small ?ω values, the reflection coefficient decreases as the resistance of samples saturated with oxygen increases. Spectral dependences n(?ω) and α(?ω) are calculated using the classical electrodynamics relations. The results are compared to the data based on the t(?ω) spectra. The calculated absorption spectra are interpreted within the model with an overlap of tails of the density of states in the valence band and in the conduction band. A “negative” gap E _gn in the density of states with a width from–0.12 to–0.47 eV is formed in highly disordered samples in this model. It is demonstrated that the high density of defects and the band of deep acceptor states of strontium in the major matrix In₂O₃ phase are crucial to tailing of the absorption edge and its shift toward lower energies. The direct gap E _gd = 1.3 eV corresponding to the In₂SrO₄ phase is determined. The energy band diagram and the contribution of tunneling, which reduces the threshold energy for interband optical transitions, are discussed.     

Multistage radiation-stimulated changes in the microhardness of silicon single crystals exposed to low-intensity β irradiation

Radiation-stimulated and postradiation changes in the microhardness of silicon single crystals exposed to irradiation with a low-intensity flux of β particles (I = 9 × 10⁵ cm^?2 s^?1, W = 0.20 + 0.93 MeV) are studied. It is established that the inversion of the radiation-induced plastic effect occurs at a characteristic irradiation time τ_c = 75 min; i.e., irradiation of silicon single crystals for a time τ < τ_c leads to nonmonotonic reversible hardening, whereas nonmonotonic reversible softening is observed under irradiation for a time τ > τ_c. It is demonstrated that there exists a correlation between the nonmonotonic dependences of the microhardness and the concentration of electrically active defects at acceptor levels with energies E _c ? 0.11 eV, E _c ? 0.13 eV, and E _c ? 0.18 eV on the irradiation time.     

Statistical mechanics of rotating systems

A microcanonical distribution function depending on the total energyE and thez-componentM of the total angular momentum of a rotating system is examined. ForM=0 the generalized microcanonical ensemble is found to give the same entropy as the usual microcanonical ensemble. The moment of inertia of a rotating gas is calculated, and the kinetic energy of rotation is given as a power series in the small parameterM ²/2I ₀E_int, whereI ₀ is the moment of inertia of the gas at rest andE _int the internal energy.     

Calibration procedure of the Δ<Emphasis Type="Italic">E</Emphasis>–<Emphasis Type="Italic">E</Emphasis> detectors for <Emphasis Type="Italic">dp</Emphasis> breakup investigation at nuclotron

Calibration procedure of ΔE ? E detectors used in dp breakup reaction measurement in the framework of DSS project is discussed. Time information from all PMTs along with amplitude information and known energy values of pp quasi elastic reaction are used to find calibration coefficients for ΔE and E detectors. Calibration coefficients are used to recover deposited particle energies. ΔE vs. E plots, energies and missing mass spectra are compared with geant4 Monte Carlo simulation. Missing mass for particular physical configuration is calculated as a test of the calibration procedure quality.     

First principles study of electronic properties,interband transitions and electron energy loss of <Emphasis Type="Italic">α</Emphasis>-graphyne

The electronic and optical properties of α-graphyne sheet are investigated by using densityfunctional theory. The results confirm that α-graphyne sheet is a zero-gap semimetal. Theoptical properties of the α-graphyne sheet such as dielectric function,refraction index, electron energy loss function, reflectivity, absorption coefficient andextinction index are calculated for both parallel and perpendicular electric fieldpolarizations. The optical spectra are strongly anisotropic along these two polarizations.For (E ∥x), absorption edge is at 0 eV, while there is noabsorption below 8 eV for (E ∥z).