铁电体SrBi2Nb2O2电子能带结构的第一性原理研究

采用第一性原理的方法计算了SrBi₂Nb_2O.9（SBN）的顺电相、铁电相的电子结构.顺电相是间接带隙, 铁电相是直接带隙，它们的大小分别为1.57和2.23 eV.顺电相和铁电相的价带顶均主要来自于O₂p态的贡献.而顺电相和铁电相的导带底则分别来自Nb₄d态和Bi₆p态的贡献.计算表明SBN铁电相的低的漏电流与Bi 6p轨道有关.由顺电相到铁电相时，Nb₄d和O_{2 关键词： 顺电相 铁电相 态密度 电子能带结构}     

Effect of Nb doping on morphology,crystal structure,optical band gap energy of TiO2 thin films

Nb–TiO₂ nanofibers and thin films were prepared using a sol–gel derived electrospinning and spin coating, respectively, by varying the Nb/Ti molar ratios from 0 to 0.59 to investigate the effect of Nb doping on morphology, crystal structure, and optical band gap energy of Nb–TiO₂. XRD results indicated that Nb–TiO₂ is composed of anatase and rutile phases as a function of Nb/Ti molar ratio. As the Nb/Ti molar ratio rose, the anatase to rutile phase transformation and the reduction in crystallite size occurred. The band gap energy of Nb–TiO₂ was changed from 3.25 eV to 2.87 eV when the anatase phase was transformed to rutile phase with increasing the Nb doping. Experimental results indicated that the Nb doping was mainly attributed to the morphology, the crystal structure, the optical band gap energy of Nb–TiO₂, and the photocatalytic degradation of methylene blue.     

XPS valence band spectra of Nb3Sn,Nb and Sn

XPS valence band spectra of Nb₃Sn, Nb and Sn have been obtained under high resolution. The 4d band structure in the Nb₃Sn spectrum is similar to that seen in Nb metal.     

1-THz low-noise SIS mixer with a double-dipole antenna

A quasi-optical mixer containing two Nb/Al/AlO_x/Nb superconducting tunnel junctions integrated into a NbTiN/SiO₂/Al microstrip line is studied experimentally in the 800–1000 GHz frequency range. The mixer is developed as an optional front end of the heterodyne receiver operating in frequency band 3 or 4 and incorporated into the HIFI module of the Herschel space-borne telescope. The double-dipole antenna of the mixer is made of NbTiN and Al films; the quarter-wavelength reflector, of a Nb film. The mixer is optimized for the IF band of 4–8 GHz. The double-sideband noise temperature T _RX measured at 935 GHz is 250 K at a mixer temperature of 2 K and an IF of 1.5 GHz. Within 850–1000 GHz, T _RX remains below 350 K. The antenna pattern is symmetrical with a sidelobe level below −16 dB.     

Geometric and electronic structure of epitaxial NbxTi1−xO2 on TiO2(110)

We have investigated the detailed geometric and electronic structure of MBE-grown Nb_xTi_1−xO₂ on TiO₂(110) by means of high-resolution transmission electron microscopy, X-ray photoelectron diffraction, ultraviolet and X-ray photoemission and electron energy loss spectroscopy. We find no measurable change in the NbO bond length relative to that for TiO bonds in TiO₂ in the dilute limit (x = 0.05), and that the epitaxial layers remain strained and coherent with the substrate for x ⩽ ≈ 0.3. However, significant dislocation generation occurs for x > ≈ 0.3. Nb substitution for Ti in the lattice introduces an additional valence electron per atom. The resulting density of states falls in the valence band region, but no new state density occurs in the either the band gap or conduction band. This result is in contrast to what occurs in the very dilute limit (parts per thousand), where Nb electrons occupy a shallow donor level near the conduction band minimum. Based on the electron counting rule, the extra Nb electrons form a non-bonding band which is degenerate with the valence band. The significance of these results for enhanced thermal and photochemistry on Nb_xTi_1−xO₂ surfaces vis a vis TiO₂ is discussed.     

Single-gap s-Wave superconductivity near the charge-density-wave quantum critical point in CuxTiSe2

The in-plane thermal conductivity kappa of the layered superconductor CuxTiSe2 was measured down to temperatures as low as Tc/40, at x=0.06 near where the charge-density-wave order vanishes. The absence of a residual linear term at T-->0 is strong evidence for conventional s-wave superconductivity in this system. This is further supported by the slow magnetic field dependence, also consistent with a single gap, of uniform magnitude across the Fermi surface. Comparison with the closely related material NbSe2, where the superconducting gap is 3 times larger on the Nb 4d band than on the Se 4p band, suggests that in Cu0.06TiSe2 the Se 4p band is below the Fermi level and Cu doping into the Ti 3d band is responsible for the superconductivity.     

Electron transport in Ti1−xNbxO2 solid solutions with x < 4%

By means of conductivity and thermopower measurements in niobium doped rutile TiO₂, we have analysed the electron transport. A previous study made in TiO₂ pointed out the existence of a large polaron transport. Nevertheless, some uncertainties remain as the precise dependence of the mobility with the temperature. The Nb doped TiU₂ studied in the present paper brings some new information: the mobility could follow an optical phonon scattering law. The FHIP model seems to be the more appropriate model to describe the mobility but the lack of theoretical calculation of transport energy does not allow definitive conclusions about the physical processes which occur in the intermediate range of temperature. However, we can say that the ionization activation energy of Nb is found to be close to 14 eV.Moreover, it is found that the localized electrons become mobile by hopping with increasing niobium content. This yields a conductivity composed by electrons which move in the 3d Ti band and localized electrons which hop from site to site. This hopping component and the number of localized electrons increase with niobium content. This leads to the disappearance of the band motion in the conductivity.     

X-ray absorption spectra: K-edges of 3d transition metals,L-edges of 3d and 4d metals,and M-edges of palladium

The X-ray absorption spectra of the 3d and 4d transition metals have been calculated within the single-particle approximation by a new linearized augmented plane wave method. The spectra, calculated with sharp atomic and band-structure single-particle levels, have been convoluted with a Lorentzian broadening function whose width is the sum of that of the core hole and the excited electrons. Plots are shown for (i) the K-edge fine structures up to at least 100 eV above the edge for Ca, Ti, Cr, Co, Cu, and Zn, (ii) the L_{2, 3} white lines for Ca, Ti, Cr, Co, and Cu, (iii) the L₃ white lines for Sr, Zr, Nb, Ru, Rh, and Pd, and (iv) the M_{2, 3} and M_4,5 spectrum of Pd. Systematic features which depend on the crystal structure and the placement of the Fermi level with conduction band are briefly discussed.     

Electronic structure and optical properties of Nb doped Al2O3 on Si by atomic layer deposition

Nb₂O₅ and Nb doped Al₂O₃ have proved to be good candidates as resistive switch materials or optical materials. In this letter, we focus on the complex electronic structure and optical properties of Nb doped Al₂O₃ to give chemical physical images of the films. With the help of SE, XPS and XPS valence band spectra, the detailed electronic structure with atomic bonding structure and optical properties are given. The band gap of a thin oxide film is determined to be 5.05 eV, and the evolution of VBO and CBO of the film on Si are also discussed.     

Electronic structure of AgNbO3 and NaNbO3 studied by X-ray photoelectron spectroscopy

The XPS examinations of the AgNbO₃ and NaNbO₃ single crystals and ceramics allowed estimate their average composition as Ag_1.1Nb_0.9O₃ and Na_1.2Nb_0.9O_2.9. The valence bands of the AgNbO₃ compound, formed mainly of the Nb 4d, Ag 4d and O 2p states, show an energy gap about 3 eV while for the NaNbO₃ compound consist of the O 2p states hybridized with the Nb 3d states and show an energy gap about 4 eV. The chemical shifts of these compounds suggest a mixed ionic and covalent character of the bonds. The broadening of the core level lines of AgNbO₃ suggests a stronger structural disorder in comparison with NaNbO₃ compound.     

Observation of band structures in the odd-odd nucleus126La

The odd-odd nucleus¹²⁶La has been studied in the reactions¹¹¹Cd(¹⁹F,4n) and⁹³Nb(³⁷Cl,p3n). Three band structures have been identified, two of them having both signature members and ΔI=1 character. These are proposed, respectively, to have πh_11/2 ? νh_11/2 and πh_11/2 ? νg_7/2 configurations and tentative 4⁺ and 3^? band heads.     

X-ray spectra and electronic structure of Li x NbO2 superconductor and other niobium oxide compounds

X-ray emission Nb and OK spectra and O1s total photoelectron yield spectra for superconductor Li_0.71NbO₂ and other niobium oxide compounds NbO, BaNb₄O₆, Ba₂Nb₅O₉, BaNb₅O₈. LiNbO₂, NbO₂, LiNbO₃ were investigated. It was found that in stoichiometric LiNbO₂, niobium oxidation degree is 3+. Under the lithium deintercalation down to Li_0.71NbO₂ composition, the niobium ions oxidation degree increases up to 3.3+. As more electrons are supplied by Nb to fill the O2p band, the degree of O2p-Nb4d hybridization is increased. Using the full-potential linear muffin-tin orbital method, the self-consistent band structure calculations of Li _x NbO₂ (x=1.0; 0.7; 0.5) compounds were carried out. Agreement between partial Nb4d and O2p densities of states and corresponding X-ray spectra was observed. For thex=0.7; 0.5 the Fermi level is placed on the slope of sharp density of states peak formed mostly by the Nb4d states. With the decrease ofx thep-d hybridization becomes stronger, although remains well below the values typical for the high-temperature superconductors.     

Electronic structures of MRhO2, MRh2O4, RhMO4 and Rh2MO6 on the basis of X-ray spectroscopy and ESCA data

X-ray O Kα, Rh Mγ and a series of M Lα emission spectra, ESCA spectra of the valence and inner levels, and O K and Rh M_III quantum-yield spectra for X-ray photoemission of the rhodium double oxides MRhO₂ (M = Li, Na, K), MRh₂ O₄ (M = Be, Mg, Ca, Sr, Ba, Co, Ni, Cu, Zn, Cd, Pb), RhMO₄ (M = V, Nb, Ta) and Rh₂MO₆ (M = Mo, W) have been measured and the dependence of electronic structure on the metal M analysed. For all compounds the inner part of the valence band corresponds to O 2p_σ + O 2p_π + Rh 4d states, while the outer part corresponds to Rh 4d. The valence band is separated from the conduction band by a narrow gap of width less than 1 eV. The first empty band, near the bottom of the conduction band, is formed by Rh 4d states, followed by a band due to vacant O 2p states.     

层状钙钛矿结构铁电薄膜SrBi2Ta2O9的掺杂改性研究

研究了Nb掺杂对层状钙钛矿结构铁电薄膜SrBi₂Ta₂O₉(SBT)的改性，分析了其改性机理.利用光声光谱技术对不同含量Nb掺杂SBT薄膜的可见光吸收进行了分析.结果表明掺杂SBT薄膜在580nm处的吸收带随Nb含量的增加发生红移，这暗示掺杂SBT薄膜的能隙与Nb含量有关.对掺杂SBT薄膜的铁电性质研究表明，薄膜的剩余极化值依赖于薄膜中的Nb含量，这与薄膜存在相界有关. 关键词：     

4f–5d hybridization in a high k dielectric

While intra-atomic f–d hybridization is expected, experimental confirmation of f–d hybridization in the photoemission final state leading to 4f band structure has been limited to 5f systems and compound systems with very shallow 4f levels. We demonstrate that core 4f states can contribute to the valence band structure in a wide band gap dielectric, in this case HfO₂ in the photoemission final state. In spite of the complications of sample charging, we find evidence of symmetry in the shallow 4f levels and wave vector dependent band dispersion, the latter consistent with the crystal structure of HfO₂.     

Electronic structure of ZrTiO4 and HfTiO4: Self-consistent cluster calculations and X-ray spectroscopy studies

Total and partial densities of states of the constituent atoms of ZrTiO₄ and HfTiO₄ titanates have been calculated using a self-consistent cluster method as incorporated in the FEFF8 code. The calculations reveal the similarity of the electronic structure of both titanates and indicate that the valence band of the compounds under consideration is dominated by contributions of O 2p states. These states contribute throughout the whole valence-band region; however their maximum contributions occur in the upper portion of the band. Other significant contributors in the valence-band region are Ti 3d and Zr 4d states in ZrTiO₄ and Ti 3d and Hf 5d states in HfTiO₄. All the above d-like states contribute throughout the whole valence-band region of the titanates; however maximum contributions of the Ti 3d states occur in the upper portion, whilst those of the Zr 4d (Hf 5d) states are in the central portions of the valence band. The FEFF8 calculations render that the bottom of the conduction band of ZrTiO₄ and HfTiO₄ is dominated by contributions of Ti 3d^? states, with also smaller contributions of Zr 4d^?/Hf 5d^? and O 2p^? states. To verify the above FEFF8 data, the X-ray emission bands, representing the energy distributions of mainly O 2p, Ti 3d and Zr 4d states, were measured and compared on a common energy scale. These experimental data are found to be in agreement with the theoretical FEFF8 results for the electronic structure of ZrTiO₄ and HfTiO₄ titanates. Additionally, X-ray photoelectron valence-band and core-level spectra were recorded for the constituent atoms of the titanates under study.     

Density of states of Nb and Mo using Slater-Koster interpolation

Slater-Koster parameters are given which have been fit to APW band calculations for Nb and Mo. The density of states at the Fermi surface of the 4d bcc alloys agrees well with a rigid band interpretation of the calculated density of states.     

First-principles energy band calculation for undoped and N-doped InTaO4 with layered wolframite-type structure

The electronic structures of undoped and N-doped InTaO₄ with optimized structures are calculated within the framework of the density functional theory. Calculated lattice constants are in excellent agreement with experimental values, within a difference of 2%. The valence band maximum (VBM) is located near the middle point on the ZD line and the conduction band minimum (CBM) near the middle point on the DX line. This means that InTaO₄ is an indirect-gap material and a minimum theoretical gap between VBM and CBM is ca. 3.7 eV. The valence band in the range from −6.0 to 0 eV mainly consists of O 2p orbitals, where In 4d5s5p and Ta 5d orbitals are slightly hybridized with O 2p orbitals. On the other hand, the conduction band below 5.5 eV is mainly composed of the Ta 5d orbitals and the contributions of In and O orbitals are small. The band gap of N-doped InTaO₄ decreases by 0.3 eV than that of undoped InTaO₄, because new gap states originating from N 2p orbitals appear near the top of the valence band. This result indicates that doping of N atoms into metal oxides is a useful method to develop photocatalysts sensitive to visible light.     

Nb对C-15结构V2(HfZr)系列的超导转变温度的影响

本文中研究了C-15结构V₂Hf_1-xNb_x,V₂Zr_1-xNb_x和V₂Hf_0.5Zr_0.5-xNb_x系列的超导转变温度T_c与Nb合量x的关系,发现V₂Zr,V₂Hf加Nb后与V₂Hf_0.5Zr_0.5加Nb后性能显著不同。测定了V₂Zr,V₂Hf和V₂Hf_0.5Zr_0.5的X射线光电子能谱。结果表明,当Hf原子和Zr原子共存于AB₂化合物的A位上时,发生了一种增强原子间相互作用的新的电荷转移。这个事实支持由角动量分波表象能带论方法分析电声耦合超导原理的结果对四元V₂(HfZrNb)系列的超导行为提出的一种可能解释:4d-5d原子配位可能有助于提高4d导带的杂化程度,从而有利于提高超导T_c。 关键词：     

β-C3N4,β-Si3N4和β-Ge3N4的能带结构

采用基于密度泛函理论的线性丸盒轨道原子球近似(LMTO-ASA)从头计算方法,研究了β-C₃N₄,β-Si₃N₄和β-Ge₃N₄的能带结构,得到了它们的能隙分别为:4.1751,5.1788和4.0279eV。对于β-C₃N₄,由于N的部分2p电子占据了非键轨道,禁带宽度较窄;对于β-Si₃N_{4关键词：}