X-ray spectra and specific features of the structure of lithium-sodium cobaltite Li<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Na<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript>CoO<Subscript>2</Subscript>

The electronic structure and specific features of the structure of nonstoichiometric cobaltite Li _x Na _y CoO₂ (x = 0.42, y = 0.36) have been studied comprehensively. The calculated multiplet for the lowspin state of the Co³⁺ ion agrees with the experimental spectra. It has been established using X-ray absorption spectra measured in the total photoelectric effect yield and total fluorescence yield modes that the Li _x Na _y CoO₂ cobaltite is stoichiometric with respect to the alkali metal near its surface and is defective inside. It has been demonstrated that the charge compensation in the case of an alkali metal deficit in LixNayCoO₂ is due to holes in O 2p states.     

Electronic Structure of Amorphous SiO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> with Variable Composition

The heights of barriers for the injection of electrons and holes from silicon in SiO_x have been calculated in the tight binding approximation without any fitting parameters. The dependence of the electronic structure of silicon-enriched amorphous silicon oxide SiO_x on the degree of enrichment has been found. The calculations have been performed with the parameterization of the matrix elements of the tight binding Hamiltonian proposed in our previous work. This parameter involves a change in the localization region of valence electrons of an insulated atom at its introduction into a solid. It has been shown that the inclusion of this change makes it possible to calculate the electronic structure without fitting parameters using the parameters of individual atoms as initial data. This circumstance allows the calculation in the absolute energy scale with zero corresponding to the energy of the electron in vacuum.     

Atomic and Electronic Structures of Metal-Rich Noncentrosymmetric ZrO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>

The atomic and electronic structures of metal-rich noncentrosymmetric zirconium oxide synthesized by the ion beam sputtering of a metallic target in an oxygen atmosphere has been studied by X-ray photoelectron spectroscopy, Raman scattering, spectral ellipsometry, and quantum-chemical simulation. It has been established that ZrO_{x < 2} consists of ZrO₂, metallic Zr, and zirconium suboxides ZrO_y. The stoichiometry parameter of ZrO_y has been estimated. It has been shown that the optical properties of ZrO_{x < 2} are determined by metallic Zr. A model of fluctuation of the width of the band gap and a potential for electrons and holes in ZrO_{x < 2} based on spatial fluctuations of the chemical composition has been proposed.     

Tunneling time correction to the intersubband optical absorption in a THz laser-dressed GaAs/Al<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Ga<Subscript>1-</Subscript><Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>As quantum well

Tunneling effect on the intersubband optical absorption in a GaAs/Al _x Ga_{1- x} As quantum well under simultaneous presence of intense non-resonant laser and static electric fields is theoretically investigated. Based on the shooting method the quasi-stationary energy levels and their corresponding linewidths are obtained. By considering the joint action of the two external fields the linear absorption coefficient is calculated by means of Fermi’s golden rule and taking into account the intersubband relaxation. We found that: (i) the linewidth broadening due to the electron tunneling has an appreciable effect on the absorption spectrum; (ii) a constant relaxation time adopted in the previous studies could not be justified even for moderate electric fields, especially in the laser dressed wells. Our model predicts that the number of absorption peaks can be controlled by the external applied fields. While in the high-electric fields the excited states become unbounded due to a significant tunneling of the electrons, for high laser intensities and low/moderate electric fields the absorption spectrum has a richer structure due to the laser-generated resonant states. The possibility of tuning the resonant absorption energies by using the combined effects of the static electric field and the THz coherent radiation field can be useful in designing new optoelectronic devices.     

Anisotropy of amorphous nanogranular composites CoNbTa-SiO<Subscript><Emphasis Type="Italic">n</Emphasis></Subscript> and CoFeB-SiO<Subscript><Emphasis Type="Italic">n</Emphasis></Subscript>

A comparative investigation of the magnetic properties of amorphous nanogranular composites (Co₄₁Fe₃₉B₂₀) _x (SiO _n )_{100 − x} and (Co₈₆Nb₁₂Ta₂) _x (SiO _n )_{100 − x} has been performed in the subpercolation region at temperatures in the range 4.2–300 K. The thermomagnetic dependences in the range 4.2–300 K and the processes of magnetization reversal and remanent magnetization relaxation at liquid-helium temperatures have been studied. It has been established that the average anisotropy constants of amorphous nanograins are equal to 3.6–7.0 kJ/m³ for the (Co₄₁Fe₃₉B₂₀) _x (SiO _n )_{100 − x} composites and 5–8 kJ/m³ for the (Co₈₆Nb₁₂Ta₂) _x (SiO _n )_{100 − x} composites. The fundamental differences in the concentration dependences of the anisotropy constant K _eff and the coercive force H _C have been revealed for the two systems under investigation. It has been demonstrated that, as the concentration of the metal phase increases, the quantities K _eff and H _C increase for the (Co₈₆Nb₁₂Ta₂) _x (SiO _n )_{100 − x} composites and decrease for the (Co₄₁Fe₃₉B₂₀) _x (SiO _n )_{100 − x} composites.     

Large-scale potential fluctuations caused by SiO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> compositional inhomogeneity

The short-range order in amorphous SiO _x (0 ≤ x ≤ 2) films has been studied by high-resolution X-ray photoelectron spectroscopy. Both the random bonding and random mixture models do not describe experimental photoelectron spectra of SiO _x (x ≤ 2). An intermediate model of the SiO _x structure has been proposed. The measured photoelectron spectra of the SiO _x (x ≤ 2) valence band indicate the presence of the silicon phase and silicon oxide.     

Electrical properties of a-Ge<Subscript>x</Subscript>Se<Subscript>100-<Emphasis Type="Italic">x</Emphasis></Subscript>

In general, the conductivity in chalcogenide glasses at higher temperatures is dominated by band conduction (DC conduction). But, at lower temperatures, hopping conduction dominates over band conduction. A study at lower temperature can, eventually, provide useful information about the conduction mechanism and the defect states in the material. Therefore, the study of electrical properties of Ge_xSe_100-x in the lower temperature region (room temperature) is interesting. Temperature and frequency dependence of Ge_xSe_{100-x (x} = 15, 20 and 25) have been studied over different range of temperatures and frequencies. An agreement between experimental and theoretical results suggested that the behaviour of germanium selenium system (Ge_xSe_100-x ) have been successfully explained by correlated barrier hopping (CBH) model.     

Electronic Structure and Magnetic Phase Transition in Helicoidal Fe<Subscript>1 - <Emphasis Type="Italic">x</Emphasis></Subscript>Co<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Si Ferromagnets

LSDA + U + SO calculations of the electronic structure of helicoidal Fe_{1 - x}Co _x Si ferromagnets within the virtual crystal approximation have been supplemented with the consideration of the Dzyaloshinski-Moriya interaction and ferromagnetic fluctuations of the spin density of collective d electrons with the Hubbard interactions at Fe and Co atoms randomly distributed over sites. The magnetic-state equation in the developed model describes helicoidal ferromagnetism and its disappearance accompanied by the occurrence of a maximum of uniform magnetic susceptibility at temperature T _C and chiral fluctuations of the local magnetization at T > T _C . The reasons why the magnetic contribution to the specific heat at the magnetic phase transition changes monotonically and the volume coefficient of thermal expansion (VCTE) at low temperatures is negative and has a wide minimum near T _C have been investigated. It is shown that the VCTE changes sign when passing to the paramagnetic state (at temperature T _S ).     

Multiband tight-binding theory of disordered A<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>B<Subscript>1-</Subscript><Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>C semiconductor quantum dots

Zero-dimensional nanocrystals, as obtained by chemical synthesis, offer a broad range of applications, as their spectrum and thus their excitation gap can be tailored by variation of their size. Additionally, nanocrystals of the type A _x B_{1- x} C can be realized by alloying of two pure compound semiconductor materials AC and BC, which allows for a continuous tuning of their absorption and emission spectrum with the concentration x. We use the single-particle energies and wave functions calculated from a multiband sp ³ empirical tight-binding model in combination with the configuration interaction scheme to calculate the optical properties of Cd _x Zn_{1- x} Se nanocrystals with a spherical shape. In contrast to common mean-field approaches like the virtual crystal approximation (VCA), we treat the disorder on a microscopic level by taking into account a finite number of realizations for each size and concentration. We then compare the results for the optical properties with recent experimental data and calculate the optical bowing coefficient for further sizes.     

Decay of the homogeneous state in Fe<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>TiSe<Subscript>2</Subscript>

The stability of a homogeneous state in materials with a more than half-filled polaron band has been investigated for the Fe _x TiSe₂ system used as an example. It has been demonstrated that the factor limiting the stability of the homogeneous state of these materials is a change in the degree of filling of the conduction band due to the thermal broadening of the polaron band. This factor becomes substantial when the top of the polaron band intersects the Fermi level. It has been revealed that the decay of the homogeneous state leads to the precipitation of the intercalant. The morphology and structure of the precipitates formed in this decay in Fe _x TiSe₂ single crystals have been studied by transmission electron microscopy.     

Electronic structure of defective lithium cobaltites Li<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>CoO<Subscript>2</Subscript>

X-ray absorption, resonant X-ray emission, and X-ray photoelectron spectroscopical methods have been applied for the study of the electronic structure of defective lithium cobaltites Li _x CoO₂ (0.6≤x≤1.0). Resonant O K α X-ray emission spectra of LiCoO₂ showed localized excitonic states due to a dd transition between occupied and unoccupied Co 3d states. On the base of measurements of Co 3s X-ray photoelectron, Co 2p, and O 1s X-ray absorption spectra, it was established that in defective cobaltites the electronic holes are localized mainly in O 2p states. An evidence of phase separation in Li _x CoO₂ has been found. It was shown that the semiconductor-to-metal transition in Li _x CoO₂ (x<0.76) at about 160 K is not accompanied by changes in the Co 3d electronic configuration which remains 3d ⁶.     

Resonant photoemission and absorption spectroscopy of the Cu<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>TiSe<Subscript>2</Subscript> compound

Single crystals of the Cu _x TiSe₂ compound with x = 0.05, 0.09, and 0.33 have been grown. Resonance photoelectron Cu 3p-3d and 2d-3d spectra of the valence bands, the spectra of the core levels, and the L absorption spectra for titanium and copper have been obtained. It is shown that the degree of oxidation of titanium atoms is +4 and the state of copper atoms is close to the state of free copper ions. It is found that the spectra of the valence bands obtained under the Cu 3p and 2p resonance conditions radically differ. For the spectra in the Cu 2p excitation regime, several bands corresponding to different decay channels of the excited state are observed. According to calculations of the density of states, the 3d states of copper are filled incompletely; the occupancy of the 3d band of copper is 9.5 electrons per atom.     

a-Si/SiN<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> multilayered light absorber for solar cell

40 alternate a-Si/SiN _x multilayer are incorporated as an absorber layer in a p–i–n solar cell. The device is fabricated using hot-wire chemical vapor deposition (HWCVD) technique. The structure of the multilayer film is examined by high resolution transmission electron microscopy (HR-TEM) which shows distinct formation of alternate a-Si and SiN _x layers. The a-Si and SiN _x layers have thickness of ~3.5 and 4 nm, respectively. The photoluminescence (PL) of multilayer film shows bandgap energy of ~2.52 eV, is larger than that of the c-Si and a-Si. Dark and illuminated current–voltage (I–V) characterization of the ML films shows that these ML are photosensitive. In the present work, it is seen that the p–i–n structure with i-layer as ML quantum well (QW) structures show photovoltaic effect with relatively high open-circuit voltage (V _OC). The increment of bandgap energy in PL and high V _OC of the device is attributed to the quantum confinement effect (QCE).     

Cluster-Type Structure of Amorphous Smooth Hydrocarbon CD<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> Films (<Emphasis Type="Italic">x</Emphasis> ~ 0.5) from T-10 Tokamak

Structure of smooth hydrocarbon CD _x films with a high deuterium ratio x ~ 0.5 redeposited from T-10 tokamak D-plasma discharges (NRC Kurchatov Institute, Moscow) has been studied. For the first time, small and wide angle X-ray scattering technique using synchrotron radiation and neutron diffraction have been employed. A fractal structure of CD _x films is found to consist of mass-fractals with rough border, surface fractals (with rough surface), plane scatterers and linear chains forming a branched and highly cross-linked 3D carbon network. The found fractals, including sp² clusters, are of typical size ~1.60 nm. They include a C₁₃ fragment consisting of three interconnected aromatic rings forming a minimal fractal sp² aggregate 9 × C₁₃. These graphene-like sp² clusters are interconnected and form a 3D lattice which can be alternatively interpreted as a highly defective graphene layer with a large concentration of vacancies. The unsaturated chemical bonds are filled with D, H atoms, linear sp² C=C, C=O, and sp³ structural elements like C-C, C-H(D), C-D_2,3, C-O, O-H, COOH, C _x D(H) _y found earlier from the infrared spectra of CD _x films, which are binding linear elements of a carbon network. The amorphous structure of CD _x films has been confirmed by the results of earlier fractal structure modeling, as well as by researches with X-ray photoelectron spectroscopy which allow finding a definite similarity with the electron structure of their model analogues — polymeric a-C:H and a-C:D films with a disordered carbon network consisting of atoms in sp³ + sp² states.     

Spin-dependent electron dynamics and recombination in GaAs<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>N<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> alloys at room temperature

Conduction-electron spin polarization dynamics achieved by pulsed optical pumping at room temperature in GaAs_1−x N _x alloys with a small nitrogen content (x = 2.1, 2.7, and 3.4%) is studied both experimentally and theoretically. It is found that the photoluminescence circular polarization determined by the mean spin of free electrons reaches 40–45% and this giant value persists within 2 ns. Simultaneously, the total free-electron spin decays rapidly with the characteristic time ≈ 150 ps. The results are explained by spin-dependent capture of free conduction electrons on deep paramagnetic centers resulting in the dynamical polarization of bound electrons. A nonlinear theory of spin dynamics in the coupled system of spin-polarized free and localized carriers has been developed which describes the experimental dependencies, in particular, the electron spin quantum beats observed in a transverse magnetic field. The text was submitted by the authors in English.     

Optical spectroscopy of intermetallic compounds TbNi<Subscript>2</Subscript>Mn<Subscript> <Emphasis Type="Italic">x</Emphasis> </Subscript> (<Emphasis Type="Italic">x</Emphasis> = 0, 0.5, 1)

The optical properties of intermetallic compounds TbNi₂Mn_x (x = 0, 0.5, 1) have been investigated using the ellipsometric method in the spectral range from 0.22 to 16 μm. The specific features of the modification of the dispersions of spectral characteristics with a variation in the manganese concentration have been determined. The behavior of the frequency dependences of the optical conductivity in the interband absorption region has been discussed in terms of the available data on the electronic structure of these compounds. The concentration dependences of a number of electronic parameters have been calculated.     

Optical characteristics of nickel and boron carbonitride films in Si(100)/Ni/BC<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>N<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript> structures

The optical characteristics of nickel films deposited on Si(100) substrates by vacuum thermal evaporation have been studied. The thickness and optical constants of the films are determined using monochromatic zero ellipsometry, while the inverse problems are solved within the three-layer optical model of the samples. It is shown that thermal annealing leads to a change in the optical constants of nickel films in the heating-temperature range of 500–900°C. Boron carbonitride layers deposited on silicon substrates with a nickel sublayer are analyzed within multilayer optical models, which make it possible to determine the refractive index and absorption coefficient distributions along the thickness of the synthesized Si(100)/Ni/BC _x N _y structure.     

Decay properties of new <Emphasis Type="Italic">D</Emphasis>-mesons

We consider radiative and pionic decays of the new D _s -mesons in the framework of a phenomenologically motivated approach. Present data on ratios of the two kinds of decays can be described without explicitly using a 4-quark component. Most probably, the isospin violation in decays of different D _s -mesons is not universal, and the binding potential should be different from Coulombic. New precise measurements may provide further clarification for the nature of the D _s excited states.Received: 10 March 2004, Published online: 14 September 2004PACS: 14.40.Lb Charmed mesons - 13.20.Fc Decays of charmed mesons - 13.25.Ft Decays of charmed mesons     

Phase segregation in Na<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>CoO<Subscript>2</Subscript> for large Na contents

We have investigated a set of sodium cobaltates (Na _x CoO₂) samples with various sodium content (0.67 ≤ x ≤ 0.75) using Nuclear Quadrupole Resonance (NQR). The four different stable phases and an intermediate one have been recognized. The NQR spectra of ⁵⁹Co allowed us to clearly differentiate the pure phase samples which could be easily distinguished from multi-phase samples. Moreover, we have found that keeping samples at room temperature in contact with humid air leads to destruction of the phase purity and loss of sodium content. The high sodium content sample evolves progressively into a mixture of the detected stable phases until it reaches the x = 2/3 composition which appears to be the most stable phase in this part of phase diagram.     

Dielectron widths of the <Emphasis Type="Italic">S</Emphasis>-, <Emphasis Type="Italic">D</Emphasis>-vector bottomonium states

The dielectron widths of Y(nS)(n = 1, …, 7) and vector decay constants are calculated using the relativistic string Hamiltonian with a universal interaction. For Y(nS) (n = 1, 2, 3) the dielectron widths and their ratios are obtained in full agreement with the latest CLEO data. For Y(10580) and Y(11020) a good agreement with experiment is reached only if the 4S-3D mixing (with a mixing angle θ = 27°± 4°) and 6S-5D mixing (with θ = 40°±5°) are taken into account. The possibility to observe higher “mixed D-wave” resonances, $ \tilde \Upsilon $ \tilde \Upsilon (n ³ D ₁) with n = 3, 4, 5 is discussed. In particular, $ \tilde \Upsilon $ \tilde \Upsilon (≈11120), originating from the pure 5³ D ₁ state, can acquire a rather large dielectron width, ∼130 eV, so that this resonance may become manifest in the e ⁺ e ⁻ experiments. On the contrary, the widths of pure D-wave states are very small, Γ _ee (n ³ D ₁)≤ 2 eV.