Energy band structure and X-ray spectra of phenakite Be<Subscript>2</Subscript>SiO<Subscript>4</Subscript>

The electronic structure of crystalline phenakite Be₂SiO₄ is investigated using x-ray emission spectroscopy (XES) (Be K _α XES, Si L _{2, 3} XES, O K _α XES) and x-ray absorption spectroscopy (XAS) (Be 1s XAS, Si 2p XAS, O 1s XAS). The energy band structure is calculated by the ab initio full-potential linearized augmented-plane-wave (FLAPW) method. The total and partial densities of states and the dispersion curves for the Be₂SiO₄ compound are presented. It is shown that the top of the valence band and the bottom of the conduction band of the Be₂SiO₄ compound are predominantly formed by the oxygen 2p states. According to the results obtained, the electron transition with the lowest energy supposedly can occur at the center of the Brillouin zone. The effective masses of electrons (0.5m _e ) and holes (3.0m _e ) for the Be₂SiO₄) compound are estimated.     

Metamagnetic effects in epitaxial BaFe<Subscript>1.8</Subscript>Cr<Subscript>0.2</Subscript>As<Subscript>2</Subscript> thin films

Epitaxial BaFe_1.8Cr_0.2As₂ thin films with the tetragonal c-axis perpendicular to the thin film surface were grown on (LaAlO₃)_0.3(Sr₂AlTaO₆)_0.7 (LSAT) single crystalline substrates using pulsed laser deposition (PLD). Resistive measurements indicate the existence of two transitions at temperatures of about 80 K and 40 K. The transition at 80 K is attributed to the structural transition from the high temperature tetragonal phase to the low temperature orthorhombic phase accompanied with the magnetic transition from a paramagnetic to an antiferromagnetic state as known for doped bulk systems. Below T ≈ 40 K the magnetization curves measured perpendicularly to the orthorhombic c-axis in fields up to 9 Tesla show two inflexion points indicating metamagnetic transitions.     

Properties of an Al<Subscript>2</Subscript>O<Subscript>3</Subscript>/Si interface

The phase chemical composition of an Al₂O₃/Si interface formed upon molecular deposition of a 100-nm-thick Al₂O₃ layer on the Si(100) (c-Si) surface is investigated by depth-resolved ultrasoft x-ray emission spectroscopy. Analysis is performed using Al and Si L_{2, 3} emission bands. It is found that the thickness of the interface separating the c-Si substrate and the Al₂O₃ layer is approximately equal to 60 nm and the interface has a complex structure. The upper layer of the interface contains Al₂O₃ molecules and Al atoms, whose coordination is characteristic of metallic aluminum (most likely, these atoms form sufficiently large-sized Al clusters). The shape of the Si bands indicates that the interface layer (no more than 10-nm thick) adjacent to the substrate involves Si atoms in an unusual chemical state. This state is not typical of amorphous Si, c-Si, SiO₂, or SiO_x (it is assumed that these Si atoms form small-sized Si clusters). It is revealed that SiO₂ is contained in the vicinity of the substrate. The properties of thicker coatings are similar to those of the 100-nm-thick Al₂O₃ layer and differ significantly from the properties of the interfaces of Al₂O₃ thin layers.     

Half-metallicity and magnetism of the quaternary inverse full-Heusler alloy Ti<Subscript>2-x</Subscript>M<Subscript>x</Subscript>CoAl (M = Nb,V) from the first-principles calculations

Using the first-principles calculations based on density functional theory, we investigate the more d-electrons doping effects on the electronic structure and magnetism of the parent inverse Heusler alloy Ti₂CoAl by the substitution of Nb and V atoms for Ti(A) and Ti(B) atoms locating at the two inequivalent sublattices. The Ti₂CoAl is half-metallic with Fermi level near the top of the minority-spin valence band and hence its spin-polarization is easily reduced by the spin-flip excitation. Our total energy calculations show that the V/Nb doping at the Ti(A)/Ti(B) site is energetically favorable compared with the Ti(B)/T(A) site due to the lower total energy. Our band structure calculations indicate that for the V doped compounds, half-metallicity can be well retained regardless of doping sites and percentages except for the case of Ti(A)-site doping with x = 1, while for Nb doped compounds, the half-metallicity persists only in Ti(B)-site doping with different percentages. For the doped compounds with half-metallicity, the Fermi level shifts from the top of minority-spin valence band to the bottom of minority-spin conduction band with increasing content of x, and typically, the doped compounds (V in Ti(A) and Ti(B) sites at x = 0.75 and 0.5, respectively; Nb in Ti(B) site at x = 0.5), whose Fermi levels are adjusted to the expected positions to effectively inhibit the spin-flip excitation are promising candidates for spintronics applications.     

Electronic and magnetic properties of Fe<Subscript>2</Subscript>SiC

The electronic structure and magnetic properties of Fe₂SiC compound have been studiedusing the framework of an all-electron full-potential linearized augmented-plane wave(FP-LAPW) method within the local density (LSDA) and + U corrected(LSDA + U)approximations. An antiferromagnetic spin ordering of Fe atoms is shown to be the groundstate for this compound. From the electronic band structures and density of states (DOS),Fe₂SiC has ametallic character and from the analysis of the site and momentum projected densities, itis deduced that the bonding is achieved through hybridization of Fe-3d with C-2p states andFe-3d withSi-3pstates. It is also pointed out that the Fe-C bonding is more covalent than Fe-Si. In theFM phase, the spin polarized calculations indicate that the total magnetic moment ofFe₂SiC increasesfrom 0.41 to 4.33μ _B when the Hubbard U parameter for iron isconsidered.     

Dielectric response of a (1000 nm)SrTiO<Subscript>3</Subscript> layer epitaxially grown on (001)La<Subscript>0.67</Subscript>Ca<Subscript>0.33</Subscript>MnO<Subscript>3</Subscript> to temperature variation and electric field

The structure and dielectric characteristics of the (1000 nm)SrTiO₃ spacer in a (001)SrRuO₃ ‖ (001)SrTiO₃ ‖ (001)La_0.67Ca_0.33MnO₃ trilayer heterostructure grown on a (001)(LaAlO₃)_0.3+(Sr₂AlTaO₆)_0.7 substrate have been studied. Both oxide electrodes, as well as the strontium titanate layer, were cube-on-cube epitaxially grown. The unit cell parameter in the SrTiO₃ layer measured in the substrate plane (3.908±0.003 Å) practically coincided with that determined along the normal to the substrate surface (3.909±0.003 Å). The temperature dependence of the real part of the permittivity ?′ of the SrTiO₃ layer in the range 70–180 K fits the relation (?′)^?1 ～ ? ₀ ^?1 C ₀ ^?1 (T-T _C ) well, where C₀ and T_C are the Curie constant and the Curie-Weiss temperature, respectively, for bulk strontium titanate crystals and ?₀ is the free-space permittivity. The data obtained on the temperature dependence of the permittivity of SrTiO₃ films enabled us to evaluate the effective depth of electric field penetration into the manganite electrode (L _e ≈ 0.5 nm) and the corresponding capacitance (C _e ≈1×10^?6 F/cm²) of the interface separating the (001)SrTiO₃ layer from the (001)La_0.67Ca_0.33MnO₃ bottom electrode.     

Influence of Si substitution on structural,electronic and magnetic properties of Fe<Subscript>2</Subscript>MnGa Heusler compound

We investigate the electronic and magnetic properties of Fe₂MnGa_1?x Si _x alloy (x = 0, 0.25, 0.5, 0.75, and 1) using first-principles density functional theory within the generalized gradient approximation method. The lattice constant decreases linearly whereas bulk modulus increases with increasing Si content. The total magnetic moment varies linearly with increasing Si content, which follows the Slater-Pauling rule. Electronic band structure calculations indicate that the Fe₂MnGa_1?x Si _x exhibits half-metallic character for all the concentrations studied and the spin polarization and the spin-down band gap both increase with the Si content. Based on the magnetic properties calculations, the Heisenberg exchange coupling parameters give Fe-Mn ferromagnetic coupling and Mn-Mn antiferromagnetic coupling. The T _C first decreases and then increases with Si content, which is in well agreement with the experimental results.     

Band offsets of epitaxial LaAlO3/TiO2 interface determined by X-ray photoelectron spectroscopy

Room-temperature ferromagnetism in doped anatase TiO₂ has previously been observed, ferromagnetic semiconductor heterostructures based on anatase TiO₂ can thus provide a new opportunity to study spin-dependent transport phenomena at room temperature. An accurate determination of barrier heights or band offsets at the TiO₂-based heterojunctions is of great importance for the spintronics application with semiconductors. X-ray photoelectron spectroscopy with high-energy resolution was used to determine the band offsets of epitaxial LaAlO₃/TiO₂ heterojunction on SrTiO₃(001) substrate fabricated by pulsed laser deposition. Results showed an upward band bending of 0.48(0.03) eV when the film thickness of the overlayer LaAlO₃ above 4 unit cells. The valence band offset obtained is about 0.35(0.16) eV. Assuming bulk band gaps for the LaAlO₃ and TiO₂ epitaxial films, the associated conduction band offset is about 2.95(0.16) eV. These results show that LaAlO₃ can be an ideal tunneling barrier for TiO₂-based heterojunctions.     

Electrospun Dy-doped SrFe<Subscript>12</Subscript>O<Subscript>19</Subscript> nanofibers: microstructure and magnetic properties

SrDy _x Fe_12?x O₁₉ (x ≤ 0.08) nanofibers have been synthesized by the electrospinning method followed by calcinations process. The partial substitution of rare earth ions Dy³⁺ (10.5 μ _B of magnetic moments) mainly occupying 12k sublattice sites in the SrFe₁₂O₁₉ crystal structure is investigated and discussed in this work. An enhanced coercivity of 7155 Oe has been obtained when the doped content reached to 0.08 at a relative low calcination temperature of 800 °C. As a result, we believe the synthesized SrDy _x Fe_12?x O₁₉ nanofibers can potentially be useful in high-density recording media as well as permanent magnets.     

Response of the electrical resistivity of La<Subscript>0.67</Subscript>Ba<Subscript>0.33</Subscript>MnO<Subscript>3</Subscript>(20 nm)/LaAlO<Subscript>3</Subscript>(001) films to magnetic field and variation in temperature

The structure, electrical resistivity, and magnetotransport parameters of 20-nm-thick epitaxial La_0.67Ba_0.33MnO₃ films grown by laser ablation on LaAlO₃(001) substrates are studied. The unit cell volume V _eff = 58.80 ų of the as-grown manganite films is found to be less than that for bulk La_0.67Ba_0.33MnO₃ crystals. Maximum values of the negative magnetoresistance MR(μ₀ H = 1 T) = ?0.27 for La_0.67Ba_0.33MnO₃ films are observed at a temperature of about 225 K. For 5 < T < 100 K, the film magnetoresistance depends only weakly on temperature and is on the order of ?0.1. At temperatures below 100 K and for 3 < μ₀ H < 5 T, the electrical resistivity of the as-grown films decreases linearly with increasing magnetic field.     

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.     

Atomic structure of a 1<Emphasis Type="Italic">T</Emphasis>-TiSe<Subscript>2</Subscript> surface layer from photoelectron and Auger electron holography data

A three-dimensional (3D) reconstruction of the atomic structure of the (100) surface of a 1T-TiSe₂ layered dichalcogenide crystal has been performed from X-ray photoelectron and Auger electron diffraction data. The diffraction patterns of the emission of Auger electrons of Se(LMM) selenium and photoelectrons of Ti2p titanium have been considered as holographic diagrams. Being processed with the scattering pattern extraction algorithm using the maximum entropy method (SPEA-MEM), they provide individual 3D images of the nearest environment of selenium and titanium atoms in the TiSe₂ lattice. Using reconstructed 3D images, the positions of 128 atoms in the 2 × 2 × 1.5-nm region of the surface layer of TiSe₂ have been determined. The structure of the surface has a 1T polytype. Interatomic distances in the layer and van der Waals gap are larger than the respective parameters in the bulk of the crystal. It is assumed that titanium layers in two Se-Ti-Se upper surface structural units are displaced along the [001] axis. The structure of the surface layer can be described by a unit cell of the P3 space group with the parameters a = 3.85 Å and c = 14.4 Å.     

The electronic structure and optical and magnetooptical properties of the CaCo<Subscript>2</Subscript> compound synthesized at a high pressure: Experiment and theory

The optical properties (the real ε₁ and imaginary ε₂ permittivity parts, optical conductivity σ, and reflectivity R) of the new ferromagnetic compound CaCo₂ in the Laves cubic phase (C15) synthesized at a pressure of 8.0 GPa were studied over the spectral range ?ω = 0.2–9 eV. The field and spectral (?ω = 0.5–4.2 eV) dependences of the equatorial Kerr effect were determined. The electronic structure and optical characteristics of CaCo₂ were calculated using the electron density functional theory by the linearized augmented-plane-wave method. The main band structure parameters of the compound were determined. The experimental and theoretical σ(ω) and R(ω) dependences were in satisfactory agreement with each other. The formation of the main absorption bands was found to be caused by the (p,d → d,p)-type electronic transitions related to the cobalt and calcium atoms. The exchange splitting of the 3d band of CaCo₂ was estimated, 2Δ_exc ～ (1–1.3) eV.     

First-Principles Study of Pressure-Induced Phase Transition in CuGaO<Subscript>2</Subscript>

We have studied the structural, elastic, electronic properties, and pressure-induced phase transition of CuGaO₂ by using the plane-wave ultrasoft pseudopotential technique based on the first-principles density-functional theory (DFT). The obtained ground state properties of three phases were in agreement with previous works. The calculated enthalpy variations with pressure showed that the structural phase transition (β → 3R/2H) appeared at 65.5 ± 1 GPa. The changes in volume and band gap of β phase showed that there was a break between 30 and 40 GPa. The independent elastic constants of three phases were calculated. The 3R, 2H, and β phases were all mechanical stability and behaved in ductile manner under zero pressure.     

Electronic Structure,Optical Properties,and Pressure Behavior of the CdB<Subscript>4</Subscript>O<Subscript>7</Subscript> and HgB<Subscript>4</Subscript>O<Subscript>7</Subscript> Compounds

Ab initio calculations of the structural, electronic, and optical properties of the CdB₄O₇ and HgB₄O₇ tetraborate compounds in three structural modifications with the Pbca, Cmcm, and Pmn2₁ symmetry have been performed in the framework of the density functional theory using the VASP package. The calculations of the electronic band structure showed that these compounds in all the investigated modifications are dielectrics with a band gap of 2–4 eV. The calculation of the structural properties of the tetraborates under pressure showed that the phase transition between the Pbca and Pmn2₁ structures in cadmium and mercury tetraborates occurs under pressures of 4.8 and 4.7 GPa, respectively.     

Anisotropy of the Complex Permittivity of the Kagome-Staircase Compounds Co<Subscript>3</Subscript>V<Subscript>2</Subscript>O<Subscript>8</Subscript> and Ni<Subscript>3</Subscript>V<Subscript>2</Subscript>O<Subscript>8</Subscript>: Experiment and Ab Initio Calculations

The anisotropy of the components of the complex permittivity of vanadate Co₃V₂O₈ and Co₃V₂O₈ single crystals in the paramagnetic phase are studied by optical ellipsometry in the spectral region 0.5–5.0 eV. Our experimental results support the weak anisotropy of the optical response detected earlier for axes a and c. The optical properties are also investigated along axis b. The properties of both compounds are compared. The optical spectra of both compounds along axis b are shifted toward low energies as compared to axes a and c. The maximum of the main interband absorption band of Co₃V₂O₈ is shifted toward low energies by 0.25–0.3 eV as compared to Co₃V₂O₈. The electronic structure parameters of both compounds are determined. Optical function spectra are analyzed using the results of ab initio band calculations.     

Ordered monoclinic vanadium suboxide V<Subscript>14</Subscript>O<Subscript>6</Subscript>

The monoclinic (space group C2/m) superstructure of the suboxide V₁₄O₆, which is formed as a result of the atomic and vacancy ordering of the tetragonal solid solution of oxygen in vanadium, is investigated using X-ray diffraction and symmetry analysis. The monoclinic suboxide V₁₄O₆ is observed in the vanadium oxide samples VO_0.57, VO_0.81, and VO_0.86 synthesized at 1770 K and the samples VO _y (0.87 ≤ y ≤ 0.98) additionally annealed at 1470 K after the synthesis. It is established that the channel of the disorder-order phase transition associated with the formation of the monoclinic suboxide V₁₄O₆ includes six superstructure vectors belonging to three non-Lifshitz stars of one type {k ₁}. The distribution function of the oxygen atoms in the monoclinic superstructure of the suboxide V₁₄O₆ is calculated. It is demonstrated that the displacements of vanadium atoms distort the body-centered tetragonal metal sublattice, thus preparing the formation of the facecentered cubic sublattice and the transition from the suboxide V₁₄O₆ to the cubic vanadium monoxide with the B1 structure.     

The evolution of electronic configuration and magnetic characterization of Fe<Subscript>9</Subscript>Ni<Subscript>1</Subscript>, Fe<Subscript>8</Subscript>Ni<Subscript>2</Subscript> alloy in theoretical calculation

To analyze the origin of the magnetic enhancement of Fe-Ni alloy, the electronicconfigurations and magnetic properties were investigated using density functional theorybased on the first-principle. The supercell (5 × 1 × 1) of Fe,Fe₉Ni₁ and Fe₈Ni₂ were constructed. Thedefect formation energy, band structure, density of states and electron density differencewere calculated. The results showed that Ni doping changed the electronic configuration ofFe atoms, resulting in the enhancement of spin polarization of Fe and the larger Bohrmagnetic moment in Fe-Ni alloys (Fe₉Ni₁). The results showed thatthe charge transfer and the atomic spacing between Fe atoms and the dopant Ni atoms playedan important role in determination of magnetic moment. The value of Fe supercell(5 × 1 × 1), Fe₉Ni₁ and Fe₈Ni₂ were 23.14,23.34 and 22.61μ _B, respectively.     

The Influence of Copper Impurity on the Electronic Structure and Optical Properties of TmNi<Subscript>5</Subscript> Compound

Investigations of the electronic structure and optical properties of TmNi_5–хСu_x (х = 0, 1, 2, 3) compounds were performed. Self-consistent calculations of the band spectrum have been performed in the local electron-spin-density approximation with a correction on strong electron interactions within the LSDA + U method. Total and partial densities of electron states associated with thulium, nickel, and copper atoms have been calculated. Optical conductivity, the behavior of which is interpreted for each compound taking into account the performed calculations, has been measured by the ellipsometry method in a wide wavelength range of 0.22–16 μm. Concentration dependences of plasma and relaxation frequencies of conduction electrons have been determined.     

Controlled synthesis of photoluminescent Bi<Subscript>4</Subscript>Ti<Subscript>3</Subscript>O<Subscript>12</Subscript> nanoparticles from metal-organic polymeric precursor

Bi₄Ti₃O₁₂ (BIT) nanoparticles with a narrow average particle size distribution in the range of 11–46 nm was synthesized via a metal-organic polymeric precursor process. The crystallite size and lattice parameter of BIT were determined by XRD analysis. At annealing temperatures >550 °C, the orthorhombic BIT compound with lattice parameters a = 5.4489 Å, b = 5.4147 Å, and c = 32.8362 Å was formed while at lower annealing temperatures orthorhombicity was absent. Reaction proceeded via the formation of an intermediate phase at 500 °C with a stoichiometry close to Bi₂Ti₂O₇. The particle size and the agglomerates of the primary particles have been confirmed by FESEM and TEM. The decomposition of the polymeric gel was ascertained in order to evaluate the crystallization process from TG-DSC analysis. Raman spectroscopy was used to investigate the lattice dynamics in BIT nanoparticles. In addition, investigation of the dependence of the visible emission band around the blue–green color emission on annealing temperatures and grain sizes showed that the effect of grain size plays important roles, and that oxygen vacancies may act as the radiative centers responsible for the observed visible emission band.