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.     

Nonstoichiometric high-pressure phase of Tm<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Cu<Subscript>3</Subscript>V<Subscript>4</Subscript>O<Subscript>12</Subscript>

Tm _x Cu₃V₄O₁₂, a perovskite-like oxide (space group, Im-3; Z = 2; a = 7.279–7.293 Å) containing vacancies in its cationic sublattice, was obtained barothermally (P = 7.0–9.0 GPa, t = 1000–1100°C) for the first time. The temperature dependences on the electrical resistivity (10–300 K) and the magnetic susceptibility (0–300 K) were investigated. It was shown that the oxide Tm _x Cu₃V₄O₁₂ is characterized by metal-type conductivity and paramagnetic properties.     

Fermion Localization on the <Emphasis Type="Italic">orbifold</Emphasis><Emphasis Type="Italic">S</Emphasis><Subscript>1</Subscript>/<Emphasis Type="Italic">Z</Emphasis><Subscript>2</Subscript>

The hierarchical structure of fermion masses of the Standard Model is explained in split fermion models by localizing the fermions at different points in an extra dimension. We consider split fermion models with two bulk scalars compactified on an orbifold. In the static case we find analytical expression for the localizer. We also address the issue of stability of the localizer. We also find exact solutions for the fermion zero modes. We explore the parameter space of the model. We find ample opportunity for construction of phenomenologically viable theories exist.     

Physical properties,crystal and magnetic structure of layered Fe<Subscript>1.11</Subscript>Te<Subscript>1-</Subscript><Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Se<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> superconductors

The physical and structural properties of Fe_1.11Te and Fe_1.11Te_0.5Se_0.5 have been investigated by means of X-ray and neutron diffraction as well as physical property measurements. For the Fe_1.11Te compound, the structure distortion from a tetragonal to monoclinic phase takes place at 64 K accompanied with the onset of antiferromagnetic order upon cooling. The magnetic structure of the monoclinic phase was confirmed to be of antiferromagnetic configuration with a propagation vector k = (1/2, 0, 1/2) based on Rietveld refinement of neutron powder diffraction data. The structural/magnetic transitions are also clearly visible in magnetic, electronic and thermodynamic measurements. For superconducting Fe_1.11Te_0.5Se_0.5 compound, the superconducting transition with T _c = 13.4 K is observed in the resistivity and ac susceptibility measurements. The upper critical field H _c2 is obtained by measuring the resistivity under different magnetic fields. The Kim’s critical state model is adopted to analyze the temperature dependence of the ac susceptibility and the intergranular critical current density is calculated as a function of both field amplitude and temperature. Neutron diffraction results show that Fe_1.11Te_0.5Se_0.5 crystalizes in tetragonal structure at 300 K as in the parent compound Fe_1.11Te and no structural distortion is detected upon cooling to 2 K. However an anisotropic thermal expansion anomaly is observed around 100 K.     

Formation energy and geometry of vacancies at BN and B<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>C<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript>N<Subscript><Emphasis Type="Italic">z</Emphasis></Subscript> nanocones

We have studied, through ab initio calculations, the stability of 60° and 120° boron nitride nanocones containing mono and multiple boron, nitrogen, and carbon vacancies. The stability of the vacancies as well as the structures reconstruction mechanism have been investigated. Our results indicate that the stability of the cones presenting such vacancies strongly depends on growth conditions. We have also found that multiple vacancies display formation energies that are comparable, and in some cases, even lower to the ones presented by monovacancies. Therefore, our results allow us to conclude that the formation energy does not depend on the vacancy size. Finally, for 120° cones, we can verify that the stability of the boron and nitrogen vacancies depends on the position where the atom has been removed.     

Anomalous temperature dependence of photoluminescence in GeO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> films and GeO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>/SiO<Subscript>2</Subscript> nano-heterostructures

The optical properties of GeO _x film and GeO _x /SiO₂ multilayer heterostructures (with thickness of GeO _x layers down to 1 nm) were studied with the use of Raman scattering and infrared spectroscopy, ellipsometry and photoluminescence spectroscopy including temperature dependence of photoluminescence. The observed photoluminescence is related to defect (dangling bonds) in GeO _x and interface defects for the case of GeO _x /SiO₂ multilayer heterostructures. From analysis of temperature dependence of photoluminescence intensity, it was found that rate of nonradiative transitions in GeO _x film has Berthelot type, but anomalous deviations from Berthelot type temperature dependence were observed in temperature dependences of photoluminescence intensities for GeO _x /SiO₂ multilayer heterostructures.     

Synthesis and properties of high-pressure phase [Er<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Cu<Subscript>3</Subscript>](V<Subscript>4</Subscript>)O<Subscript>12</Subscript>

A new perovskite-like compound Er_0.73Cu₃V₄O₁₂ (space group Im \(\bar 3\), Z = 2, a = 7.266 Å) has been synthesized barothermally (P = 8.0 GPa, t = 1000°C). Its electrical and magnetic properties have been studied. It is found that the temperature dependence of the electrical conductivity (in the range 78–300 K) has of semiconductor type. The behavior of the impedance and admittance has been analyzed at 290 K and frequencies of 200 Hz to 200 kHz under atmospheric pressure and at high (15–42 GPa) pressures.     

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 ).     

Mechanical and Dynamic Stability of Complete and Nonstoichiometric 3<Emphasis Type="Italic">C</Emphasis>-Si<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>C<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript> from Ab Initio Calculations

The energies of formation of vacancies in the carbon and silicon sublattices, the independent elastic constants, the all-round compression, shear and Young’s moduli, and the anisotropy coefficients are determined for the complete and nonstoichiometric cubic phases of 3C-Si_xC_y (x, y = 1.0–0.75) by ab initio methods of the band theory. In the formalism of the density functional perturbation theory (DFPT), the phonon dispersion dependences are obtained for these phases (the comparison with the experiment is given for the complete phase). It is shown that the mechanical characteristics of the phases become strongly anisotropic upon the transition from 3C-SiC_0.875 to 3C-SiC_0.75. It is established from the analysis of the phonon dispersion curves that the 3C-SiC_0.875 and 3C-SiC_0.75 phases, in contrast to the complete 3C-SiC phase, are dynamically unstable at T = 0 K.     

Effect of Oxygen Nonstoichiometry on the Magnetic Phase Transitions in Frustrated YBaCo<Subscript>4</Subscript>O<Subscript>7 + <Emphasis Type="Italic">x</Emphasis></Subscript> (<Emphasis Type="Italic">x</Emphasis> = 0, 0.1, 0.2) Cobaltites

The structural and elastic characteristics of YBaCo₄O_{7 + x} (x = 0, 0.1, 0.2) cobaltites synthesized by various technologies and having various excess oxygen contents x are experimentally studied. The distortion of the crystal structure in stoichiometric samples is found to remove frustrations and to bring about a longrange magnetic order in the cobalt subsystem, which is accompanied by well-pronounced anomalies in the elastic properties in the temperature range of a magnetic phase transition T_N. At a weak deviation from oxygen stoichiometry, the structure distortion disappears, frustrations are retained, and the further development of a long-range magnetic order is hindered. As a result of an absent long-range magnetic order, the anomalies of the elastic characteristics at T_N smooth rapidly and disappear. This finding points to the suppression of structural and magnetic transitions in nonstoichiometric samples and to the conservation of only short-range correlations of order parameter. It is found that nonstoichiometric samples can be separated into two phases depending on the ceramic synthesis conditions.     

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.     

The isospin-symmetry-breaking effects in <Emphasis Type="Italic">K</Emphasis><Subscript><Emphasis Type="Italic">e</Emphasis>4</Subscript> decays

The final-state interaction of pions in K _e4 decay allows to obtain the value of the isospin and angular-momentum-zero ππ scattering length a ₀⁰.We take into account the electromagnetic interaction of pions and isospin-symmetry-breaking effects caused by different masses of neutral and charged pions and estimate the impact of these effects on the procedure of scattering-length extraction from K _e4 decays.     

The electromagnetic effects in <Emphasis Type="Italic">K</Emphasis><Subscript><Emphasis Type="Italic">e</Emphasis>4</Subscript> decay

The final-state interaction of pions in K _e4 decay allows to obtain the value of the isospin and angular-momentum-zero ππ scattering length a ₀ ⁰ .We take into account the electromagnetic interaction of pions and isospin-symmetry-breaking effects caused by different masses of neutral and charged pions and estimate the impact of these effects on the procedure of scattering-length extraction from K _e4 decays.     

Magnetic M<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>O<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript>@N-C as heterogeneous catalysts for the catalytic oxidation of aniline solution with sulfate radicals

Metal nanoparticles have been combined with magnet metal–organic frameworks (MOFs) to afford new materials that demonstrate an efficient catalytic degradation, high stability, and excellent reusability in areas of catalysis because of their exceptionally high surface areas and structural diversity. Magnetic M _x O _y @N-C (M = Fe, Co, Mn) nanocrystals were formed on nitrogen-doped carbon surface by using 8-hydroxyquinoline as a C/N precursor. The Co@N-C, MnO@N-C, and Fe/Fe₂O₃@N-C catalysts were characterized by X-ray diffraction (XRD), Raman, scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), N₂ adsorption/desorption, and X-ray photoelectron spectroscopy (XPS). The catalytic performances of catalysts were thoroughly investigated in the oxidation of aniline solution based on sulfate radicals (SO₄ ^?.) toward Fenton-like reaction. Magnetic M _x O _y @N-C exhibits an unexpectedly high catalytic activity in the degradation of aniline in water. A high magnetic M _x O _y @N-C catalytic activity was observed after the evaluation by aniline degradation in water. Aniline degradation was found to follow the first-order kinetics, and as a result, various metals significantly affected the structures and performances of the catalysts, and their catalytic activity followed the order of Co > Mn > Fe. The nanoparticles displayed good magnetic separation under the magnetic field.

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Chemical state of the surface of mechanically activated Mn<Subscript><Emphasis Type="Italic">m</Emphasis></Subscript>O<Subscript><Emphasis Type="Italic">n</Emphasis></Subscript> oxides

X-ray photoelectron spectroscopy is used to study Mn₃O₄, Mn₂O₃, and MnO₂ manganese oxide surfaces subjected to mechanical activation by means of high intensity grinding. It is found that Mn₂O₃ is the most thermodynamically stable of these oxides; mechanical activation converts the surface layers of Mn₃O₄ and MnO₂ into this intermediate oxide. The chemical stability of activated Mn₂O₃ with respect to actions of the environment was considerably elevated. This result is explained in terms of features of the structural state of the mechanically activated surface.     

Ferromagnetism and ferroelectricity in Eu<Emphasis Type="Italic">X</Emphasis> (<Emphasis Type="Italic">X</Emphasis> = <Emphasis Type="Italic">O</Emphasis>, <Emphasis Type="Italic">S</Emphasis>): pressure effects

Ferromagnetism and ferroelectricity in Eu monochalcogenides have been investigated by ab initio density functional theory in the DFT+U approach. Exchange interaction parameters and Curie temperatures under pressure are studied and discussed using Heisenberg Hamiltonian with first and second-nearest-neighbor interactions. The calculations showed that the hydrostatic pressure perfectly improves the Curie temperature (EuO: T _C = 175 K; EuS: T _C = 33.8 K) and in the other hand it cannot induce the spontaneous polarization (P _s ). The effect of uniaxial and biaxial pressure is also studied. Although the uniaxial strains slightly increases the Curie temperature, it ensures the ferrolectricity in these systems by producing a spontaneous polarization of the order of P _s (EuO) = 57.50 μC/cm² and P _s (EuS) = 42.86 μC/cm² with pressures of 5% and 4%, respectively. The search for new model systems is a necessity to better understand the physics related to multiferroïc materials and to consider possible applications.     

Radiative <Emphasis Type="Italic">K</Emphasis><Subscript><Emphasis Type="Italic">e</Emphasis>3</Subscript> decays revisited

Motivated by recent experimental results and ongoing measurements, we review the chiral perturbation theory prediction for decays. Special emphasis is given to the stability of the inner bremsstrahlung-dominated relative branching ratio versus the K _e3 form factors, and on the separation of the structure-dependent amplitude in differential distributions over the phase space. For the structure-dependent terms, an assessment of the order p ⁶ corrections is given, in particular, a full next-to-leading order calculation of the axial component is performed. The experimental analysis of the photon energy spectrum is discussed, and other potentially useful distributions are introduced.Received: 9 December 2004, Published online: 21 February 2005PACS: 13.20.Eb, 11.30.Rd, 12.39.Fe