Near EF electronic structure of heavily boron-doped superconducting diamond

We have performed soft X-ray angle-resolved photoemission spectroscopy (SXARPES) of a heavily boron-doped superconducting diamond film (T_c=7.2 K) in order to study the electronic structure near the Fermi level (E_F). Careful determination of measured momentum space that across Γ point in the Brillouin zone (BZ) and increase of an energy resolution provide further spectroscopic evidence that E_F is located at the highly dispersive diamond-like bands, indicating that holes at the top of the diamond-like valence band play an essential role for the conducting properties of the heavily boron-doped superconducting diamond for this boron-doping region (effective carrier concentration of 1.6%). The SXARPES intensities at E_F were also mapped out over BZ to obtain experimental Fermi surface sheets and compared with calculations.     

Metal-insulator transition and superconductivity in Hg-doped BaPb0.75Bi0.25O3

The effects of mercury doping on the superconductivity, crystal structure, and electronic structure have been investigated in Hg-doped BaPb_0.75Bi_0.25O₃ (BaPb_0.75−xHg_xBi_0.25O₃, BPHBO) by magnetic measurement, powder X-ray diffraction (XRD), and X-ray photoemission spectroscopy (XPS). At lower doping levels, the system is metallic and superconducting. However, the superconductivity is fully suppressed by Hg doping at x>0.25 and recovered with further increase in Hg content at x>0.3, showing a superconductivity reentrant phenomenon. XPS analysis reveals that BPHBO experiences dual metal-insulator transitions (MITs) at these two superconductivity points, which are accompanied by lattice distortions, suggesting that they may be driven by Peierls transitions. The first MIT may be a Mott-transition, while the second may be due to competition between the band filling effect and modification of the charge-disproportionate state.     

High-resolution photoemission study of FeSr2YCu2O7+δ

We have performed high-resolution photoemission spectroscopy (PES) on FeSr₂YCu₂O_7+δ, of which superconductivity of T_c=49 K was recently reported. We clearly observed opening of a d-wave-like superconducting gap and estimated the maximum gap value (Δ_max) to be 10 meV at 15 K. This gap value gives 2Δ_max/k_BT_c∼5, suggesting a strong-coupling nature of superconductivity in FeSr₂YCu₂O_7+δ. Comparative PES study with superconducting and insulating samples shows that the valence band is rigidly shifted as a function of doping without evolution of additional states within the insulating gap.     

Morphology, structure and photocatalytic performance of ZnIn2S4 synthesized via a solvothermal/hydrothermal route in different solvents

Hexagonal ZnIn₂S₄ photocatalysts with different morphology and crystallinity (micro-structures) were prepared in aqueous-, methanol- and ethylene glycol-mediated conditions via a solvothermal/hydrothermal method. The aqueous- and methanol-mediated ZnIn₂S₄ presented to be Flowering-Cherry-like microsphere, while the ethylene glycol-mediated ZnIn₂S₄ presented to be micro-cluster. In comparison with two other products, aqueous-mediated ZnIn₂S₄ possessed the best crystallinity (micro-structure), which resulted in the highest photocatalytic activity for hydrogen evolution under visible-light irradiation. Additionally, aqueous-mediated ZnIn₂S₄ was found to be more stable than the other two ZnIn₂S₄ photocatalysts while undergoing the photocatalytic process. During the photocatalytic reaction, the average rates for hydrogen production over aqueous-, methanol- and ethylene glycol-mediated ZnIn₂S₄ were determined to be 27.3, 12.4 and 9.1 μmol h^-1, respectively, in the present photocatalytic systems.     

The orbital characters and kz dispersions of bands in iron-pnictide NaFeAs

The electronic structure and orbital characters of iron-pnictide NaFeAs have been studied by polarization dependent angle-resolved photoemission spectroscopy. Some of the bands are mixed with the orbitals of opposite symmetries, which could be interpreted by the hybridization among the bands. According to the photon energy dependent experiment, the k_z dispersions of the bands that cross the Fermi energy are weak in both paramagnetic and spin density wave states. However, a band well below the Fermi level shows a k_z dispersion of 41 meV, which mainly contains the d_z² orbital.     

Ultrahigh-resolution laser photoemission study of URu2Si2 across the hidden-order transition

We have studied the electronic structures of URu₂Si₂ employing ultrahigh-resolution laser angle-resolved photoemission spectroscopy. The change of photoemission spectra is investigated across the hidden-order transition, and the emergence of a narrow band is clearly observed near the Fermi level for both (π,0) and (π,π) directions. In addition, it is shown that tuning of light's polarization allows the signal of a hole-like dispersive feature to enhance. These observations prove that laser angle-resolved photoemission spectroscopy is an effective tool for studying the evolution of electronic structures across the hidden-order transition in URu₂Si₂.     

Optimal levels of oxygen deficiency in the visible light photocatalyst TiO2−x and long-term stability of catalytic performance

The dependence of the visible light-responsive photocatalytic activity of oxygen deficient TiO₂ (TiO_2−x) prepared by Ar/H₂ plasma surface treatment on the degree of oxygen deficiency (x) was assessed to determine the deficiency region associated with highest performance. The highest activity was obtained at x=0.06 (TiO_1.94). The maximum visible light activity for this material, estimated from the formaldehyde (HCHO) removal rate, was three times higher than that exhibited by nitrogen-doped TiO₂ (TiO_2−xN_x). The catalytic ability was found to decrease over the first week after fabrication of the material, after which it became stable, and the performance of TiO_2−x at this point was found to be nearly equal to that of TiO_2−xN_x. The results of ab initio calculations of density of states for TiO_2−x suggest that new oxygen deficiency states emerge at almost the exact center between the valence and conduction bands when x>0.06, which increases the recombination rate between electrons and holes. Therefore the declining performance of TiO_2−x at larger x values is attributed to the emergence of new oxygen deficient states.     

Photoemission studies of Cu intercalated NbSe2

We report growth and angle resolved photoemission studies of Cu intercalated NbSe₂. Cu_xNbSe₂ with x=0, 0.04 and 0.08 were grown by iodine vapor transport method. Intercalation and doping was confirmed by the change in the c-axis lattice constant and the change in the electronic structure, respectively. To investigate the nesting condition, we performed auto-correlation analysis on the data. We find that the magnitude of the nesting vector decreases with doping but the direction remains unchanged. Our observation is consistent with the nesting coming from intra-pocket transition.     

Photoemission study of Ag nanofilm grown on pseudomorphic fcc Fe(1 0 0)

An angle-resolved photoemission study for Ag nanofilm grown on pseudomorphic metastable-fcc-phase Fe(1 0 0) has been done in order to investigate in detail the quantized electronic structures. From the low-energy electron-diffraction and angle-resolved photoemission spectra, it is found that the present Ag nanofilms were grown in the direction of [1 1 1] on pseudomorphic fcc Fe(1 0 0) substrates. The angle-resolved photoemission spectra of Ag nanofilms grown on pseudomorphic fcc Fe(1 0 0) exhibit the features derived from Shockley-type surface state and additional fine-structures derived from the quantized state of Ag sp valence electron. The experimental nanofilm-thickness dependence of binding energies of these quantized states is compared with the theoretical calculation based on the phase accumulation model, taking into account the phase shifts of electron reflection at both interfaces of the Ag nanofilm. From these results, we discuss the quantized electronic structure in Ag nanofilm grown on pseudomorphic fcc Fe(1 0 0).     

Theory of resonant inelastic X-ray scattering spectrum for Ni impurities in cuprates

We perform the numerically exact diagonalization calculation for small Cu-O clusters with a Ni impurity site, representing the Ni-substituted cuprate, to examine the single-particle excitation spectra as well as the resonant inelastic X-ray scattering (RIXS) spectra. We clarify relations between low-energy electronic structures near the Ni site and excitations seen in the RIXS spectra.     

Continuous one-step synthesis of N-doped titania under supercritical and subcritical water conditions for photocatalytic reaction under visible light

N-doped titania was prepared continuously by one-step synthetic method under supercritical and subcritical water conditions using titanium(IV)tetraisopropoxide (TTIP) and nitric acid as a titania precursor and nitrogen source, respectively. The synthesized N-doped titania particles were characterized by XRD, N₂-adsorption, TEM, XPS, UV-vis diffuse reflectance spectroscopy. N-doped titania was successfully synthesized and its crystalline structure was homogenous anatase phase with high surface area. The absorption edge of synthesized N-doped titania shifted into the visible light region compared with commercial titania P25. All synthesized N-doped titania have higher photocatalytic activity than P25 under visible light irradiation. The photocatalytic activity of N-doped titania synthesized under supercritical water condition was the highest for the degradation of methyl orange under visible light due to the larger crystallite size compared with the N-doped titania synthesized under subcritical water condition.     

The pseudogap and superconducting gap in the ab-plane electronic Raman continuum of YBa2Cu4O8

We report plane-polarised Raman spectra from YBa₂Cu₄O₈ single crystals between 300 and 10 K. In the normal state we observe a gap-like depletion of intensity from the electronic continuum extending to around 1200 cm⁻¹ with an onset temperature of around 225 K. We remove the phonons and pseudogap depletion from the spectra using a simple model and recover a characteristic high-T_c superconductor continuum. In the superconducting state, intensity returns to the continuum in the form of a very broad pair-breaking peak.     

Estimation of the cleavage force of crystalline semiconductors with diamond and zinc-blende structure

The cleavage force in several semiconductors with diamond and zinc-blende structure is studied within a simplified approach. It is shown that the cleavage in these systems is characterized by maximum forces from 0.45×10¹⁰ to 1.33×10¹⁰ N/m². For most of the crystals, the cleavage planes are in correspondence with the experimental data.     

Synthesis of homogeneous pentenary chalcopyrite alloys with a classical two-step growth process

Homogeneous single-phase Cu(In_0.75Ga_0.25)(Se_1−yS_y)₂ chalcopyrite alloys were prepared by a novel two-step growth process. CuIn_0.75Ga_0.25 precursors were deposited by DC magnetron sputtering and the subsequent reaction processes in a reactive H₂Se/Ar/H₂S atmosphere was optimized to prevent the formation and separation of stable ternary phases. X-ray diffraction (XRD) analysis of these films revealed characteristic chalcopyrite peaks with a high degree of symmetry, indicative of homogeneous rather than compositionally graded material. The lattice parameters of the single-phase Cu(In_0.75Ga_0.25)(Se_1−yS_y)₂ pentenary alloys decreased linearly with an increase in the S/(S+Se) ratio in accordance with Vegard's law. X-ray photoelectron spectroscopy (XPS) depth profiling confirmed the in-depth compositional uniformity of the pentenary alloys, prepared under optimized selenization/sulfurization conditions.     

Magnetic and electronic properties of the antiferromagnetic YbFe4Al8 compound

The magnetic, electrical and electronic properties of the tetragonal ternary YbFe₄Al₈ compound have been investigated. This compound was supposed to be an antiferromagnetic superconductor due to the negative magnetization signal appearing at a low field of the field cooling mode, however, based on the measurements of the temperature dependence of magnetization and resistivity we do not confirm the presence of superconductivity in this material and we ascribe the negative magnetization to the complicated non-collinear magnetic structure. A switch to the antiferromagnetic order at about 150 K has been visible both on the M(T) and ρ(T) curves. The valence state of the Yb ions has been studied by X-ray photoemission spectroscopy. The valence band spectrum at the Fermi level exhibits the domination of the hybridized Yb(4f) and Fe(3d) states.     

Thermoelectric properties and electronic structure of Te-doped CoSb3 compounds

Co₄Sb_12−xTe_x compounds were prepared by mechanical alloying combined with cold isostatic pressing, and the effects of Te doping on the thermoelectric properties were studied. The electronic structure of Te-doped and undoped CoSb₃ compounds has been calculated using the first-principles plane-wave pseudo-potential based on density functional theory. The experimental and calculated results show that the value of the solution limit x of Te in Co₄Sb_12−xTe_x compounds is between 0.5 and 0.7. The Fermi surface of CoSb₃ is located between the conduction band and the valence band, and its electrical resistivity decreases with increasing temperature. The density of states is mainly composed of Co 3d and Sb 5p electrons for intrinsic CoSb₃.The Fermi surface of Te-doped compounds moves to the conduction band and its electrical resistivity increases with increasing temperature, exhibiting n-type degenerated semiconductor character. Under the conditions of the experiment, the maximum value 2.67 mW/m K² of the power factor for Co₄Sb_11.7Te_0.3 is obtained at 600 K; this is about 14 times higher than that of CoSb₃.     

Synthesis and fluorescence properties of pure and metal-doped spherical ZnS particles from EDTA-metal complexes

Monodispersed spherical ZnS particles as well as doped with Cu, Mn ions were synthesized from metal-chelate solutions of ethylenediamine tetraacetate (EDTA) and thioacetamide (TAA). The characterizations of the ZnS-based particles were investigated via TEM, SEM, XRD, TG/DTA and PL measurements. The sphere size was controlled from 50 nm to 1 μm by adjusting the nucleation temperatures and molar ratio of Zn-EDTA to TAA. The emission intensity continuously increased with the increase of the particle size. When the ZnS microspheres were annealed at 550-800 °C, there were two specific emission bands with the centers at 454 nm and 510 nm, which were associated with the trapped luminescence arising from the surface states and the stoichiometric vacancies, respectively. When Cu²⁺ was introduced into ZnS microspheres, the dominant emission was red-shifted from 454 to 508 nm, fluorescence intensity also sharply increased. However, for the Mn²⁺-doped ZnS, the emission intensity was significantly enhanced without the shift of emission site.     

The effects of O deficiency on the electronic structure of rutile TiO2

Ab initio density functional calculations (plane wave GGA, CASTEP) were performed to determine the effect of O deficiency on the electronic structure of rutile, TiO₂. O deficiency was introduced through either the removal of O or the insertion of interstitial Ti atoms. At physically realistic concentrations of O vacancies in the rutile lattice (i.e. 25% and less) O deficiency results in the population of the bottom of the conduction band, the location of the Ti 3d orbitals in the pure structure, increasingly with increasing vacancy concentration. We propose that this could be confused with the formation and population of gap states especially where O vacancies occur in isolated positions in the lattice. In contrast, Ti interstitials introduce a defect state into the energy gap, without an overall reduction in the size of the energy gap. O vacancies result in a spin polarized solution, whereas Ti interstitials do not.     

TiO2 nanoparticles with high ability for selective adsorption and photodegradation of textile dyes under visible light by feasible preparation

Particular TiO₂ nanoparticles with high selective photocatalytic oxidation of anionic dyes are prepared by a feasible hydrothermal method. Moreover, its photocatalytic selectivity can be easily switched to cationic dyes by a simple post-treatment in ammonia solution, which makes the prepared TiO₂ have bi-directional selectivity in dye photodegradation. Based on the photocatalytic performances and the structure and surface characteristics of the catalyst, the bi-directional selectivity of the catalysts is found to be closely related to the adsorption selectivity. The adsorption selectivity originates from surface charge groups, which are introduced during the preparation and post-treatment progresses. This study provides a facile and economical approach towards selective degradation of dyes with high efficiency by the special TiO₂ nanoparticles synthesized through a simple hydrothermal method, which may be used practically in the future.     

Non-intrinsic superconductivity in InN epilayers: Role of Indium Oxide

In recent years there have been reports of anomalous electrical resistivity and the presence of superconductivity in semiconducting InN layers. By a careful correlation of the temperature dependence of resistivity and magnetic susceptibility with structural information from high-resolution x-ray diffraction measurements, we show that superconductivity is not intrinsic to InN and is seen only in samples that show traces of oxygen impurity. We hence believe that InN is not intrinsically a superconducting semiconductor.