Electronic structure and optical spectra of oxyanion crystals

Band structure of lithium, sodium, and potassium nitrites, nitrates, chlorates, perchlorates, sulfites, and sulfates together with the state density and the imaginary part of the complex dielectric permittivity is calculated by the DFT-LDA method with the Perdew and Zunger (PZ) correlation potential using the software package CRYSTAL06 on the uniform grid comprising 512 points of the Brillouin zone in the approximation of the direct interband transitions and constant matrix element. Good agreement of experimental and theoretical optical spectra is established. It is demonstrated that the main specific features of the optical spectra at energies of 5–12 eV are determined by the complex anion structure and depend only slightly on the cation structure. The cation excitation energies are 25‒28 eV for sodium salts and 15‒18 eV for potassium ones.     

X-ray spectra and electronic structure of aluminum in AlN and B x Al1 − x N crystals with a wurtzite structure

The electronic band structure and x-ray spectra of aluminum in wurtzite-type crystals of B_xAl_{1 ? x} N compounds are investigated and compared with those of the binary aluminum nitride AlN. The main features and the features revealed for the first time in the L _{II, III} x-ray emission and absorption spectra are interpreted.     

Electronic structure of amorphous Nb1-x Si x films studied by X-ray emission and X-ray photoelectron spectroscopy

We present valence band spectra of the amorphous system Nb_1–x Si _x (0.2x0.8), of bcc-Nb and of a-Si obtained by X-ray photoelectron spectroscopy (XPS, Al K) and X-ray emission spectroscopy (XES, Si K-emission bands). The samples were prepared as thin films by sputtering. The origin of all prominent spectral features was identified and consistently correlated to Si 3s-, Si 3p-and Nb 4d-derived states. The Nb4d-Si3p coupling is stable in binding energy over a wide concentration range. There is strong experimental evidence that the short range order changes considerably within the concentration interval 0.4x0.7, whereas the partial density of states of the Si 3p-electrons is clearly altered in the small concentration range 0.50x0.57.     

Electronic structure of Ag x Au1-x

The fully relativistic version of the Korringa-Kohn-Rostoker-Coherent Potential Approximation (KKR-CPA) method has been used to study the electronic structure of the alloy system Ag _x Au_1-x forx=0.20, 0.40, 0.60 and 0.80. The results of these calculations are discussed in terms of densities of states and Bloch spectral functions and are compared with experimental data.     

Theoretical investigation of the structure of X-ray photoelectron spectra of crystals

In the one-electron approximation general relations are obtained which determine the X-ray photoemission intensity from mono- and poly-crystalline solids. To describe the excited electron states plane waves and orthogonalized plane waves have been used. It is shown that consideration of orthogonalization terms is important when photoionization cross-sections of valence electrons are to be calculated. Results are discussed using the examples of transition metals, noble metals, semiconductors and dielectrics possessing the diamond and zinc-blende structure. The spectra from poly-crystals and single crystals of transition metals can differ markedly, but in the case of dielectrics the difference is small. Calculated X-ray photoelectron spectra of crystals are in good agreement with experimental data. To obtain detailed information of the spectrum structure, especially when transition metals are considered, the many-electron effects must be taken into account.     

Electronic structure of LiMnO : X-ray emission and photoelectron spectra and band structure calculations

The electronic structure of LiMnO₂ and Li₂MnO₃ was studied by means of X-ray photoelectron and soft X-ray emission spectroscopy. For LiMnO₂, LSDA and LSDA+U calculations were carried out. The LSDA+U calculations are in rather good agreement with the measured valence-band structure as well as with the magnetic and electrical properties of LiMnO₂. It is shown that the band gap in LiMnO₂ is determined by the charge-transfer effect. Received 15 March 1999 and Received in final form 14 July 1999     

BxGa1-xAs合金电子结构与光学性质的第一性原理计算

基于密度泛函理论的第一性原理和特殊准随机近似方法,建立64原子的超胞并且对结构进行几何优化.计算和讨论具有闪锌矿结构的三元合金B_xGa_1-xAs的结构参数、电子结构和光学性质.结果表明：B_xGa_1-xAs的晶格常数与使用Vegard定理计算得到的值有微弱的偏离,键长存在明显的弛豫;计算得到的合金带隙弯曲参数变化较小（2.57 eV-5.01 eV）而且对组分变化的依赖很弱;最后分析硼的并入对GaAs光学参数包括介电函数、反射率、折射率、吸收系数和能量耗散函数的影响.     

Electron-energy-loss and X-ray photoelectron spectra. Part 1. Electronic structure of tetraphenylporphyrin and its nickel derivative

Specific information on the energy level diagram and on the energy forbidden gap of tetraphenylporphyrin and its Ni derivative has been obtained by means of electron energy-loss spectroscopy (EELS) and X-ray photoelectron spectroscopy techniques. Comparison of the free ligand and its metal derivative emphasizes the importance of the metal 3d levels in the conduction processes. The interband and plasmon transition present in the EELS spectra of these species are compared with the theoretically calculated energy level diagram and experimental data.     

Electronic structure and resonant X-ray emission spectra of carbon shells of iron nanoparticles

The electronic structure of carbon shells of carbon encapsulated iron nanoparticles carbon encapsulated Fe@C has been studied by X-ray resonant emission and X-ray absorption spectroscopy. The recorded spectra have been compared to the density functional calculations of the electronic structure of graphene. It has been shown that an Fe@C carbon shell can be represented in the form of several graphene layers with Stone-Wales defects. The dispersion of energy bands of Fe@C has been examined using the measured C Kα resonant X-ray emission spectra.     

Electronic structure and X-ray photoelectron spectra of TiC and ZrC: Cluster and band structure approaches

The electronic structure for titanium and zirconium monocarbides have been calculated with the SCF MS X_α-cluster method. The results are compared with the experimental X-ray photoelectron spectra and augmented plane wave (APW) calculations.     

Electronic structure,chemical bonding and X-ray photoelectron spectra of light rare-earth oxides

The electronic structure of the light RE oxides CeO₂, Pr₂O₃ and Nd₂O₃ has been studied by the cluster discrete variational DV-X method and X-ray photoelectron spectroscopy. The interpretation of XPE spectra basing on the MO model is given. The various electronic states contributions to the chemical bonding in the oxides are investigated.     

Electronic structure,chemical bonding and X-ray photoelectron spectra of heavy rare-earth oxides

The electronic structure of the heavy RE sesquioxides from Sm₂O₃ to Lu₂O₃ has been studied by the cluster discrete variational X method and X-ray photoelectron spectroscopy. The interpretation of XPS spectra basing on the MO model is given. An analysis of the dependence of Ln 4f orbitals contribution to chemical bonding on the lanthanide atomic number and bond lengths is made.     

Optical spectra and energy band structure of AgAsS2 crystals

Ground and excited states of three exciton series are observed in the region of fundamental absorption edge of AgAsS₂ crystals. The contours of exciton reflection spectra are calculated and the main parameters of excitons and energy bands are determined in the center of Brillouin zone. The optical reflection spectra are investigated at 30 K in E∥c and E⊥c polarizations in AgAsS₂ crystals in the region of 2-6 eV. The optical functions are calculated from the reflection spectra and a scheme of electronic transitions responsible for peculiarities of reflection spectra deep into the absorption band is proposed.     

Transmission-mode GaN photocathode based on graded Al_xGa_(1-x)N buffer layer

We create a GaN photocathode based on graded Alx Ga1-x N buffer layers to overcome the influence of buffer-emission layer interface on the photoemission of transmission-mode GaN photocathodes.A gateshaped spectral response with a 260-nm starting wavelength and a 375-nm cut-off wavelength is obtained.Average quantum efficiency is 15% and short wavelength responses are almost equivalent to long wavelength ones.The fitted interface recombination velocity is 5×104 cm/s,with negligible magnitude,proving that the design of the graded buffer layers is efficient in obtaining good interface quality between the buffer and the emission layer.     

AlxGa1-xN/GaN调制掺杂异质结构的子带性质研究

通过低温和强磁场下的磁输运测量研究了Al0.22Ga0.78N/GaN调制掺杂异质结构中2DEG的子带占据性质和子带输运性质.在该异质结构的磁阻振荡中观察到了双子带占据现象,并发现2DEG的总浓度随第二子带浓度的变化呈线性关系.得到了该异质结构中第二子带被2DEG占据的阈值电子浓度为7.3×1012cm-2.采用迁移率谱技术得到了不同样品的分别对应于第一和第二子带的输运迁移率.发现当样品产生应变弛豫时第一子带的电子迁移率骤然下降,而且第二子带的电子迁移率远大于第一子带的电子迁移率.用电子波函数分布和应变弛豫时的失配位错散射解释了上述现象.同时进一步说明了界面粗糙散射和合金无序散射是决定AlxGa1-xN/GaN异质结构中2DEG迁移率的主要散射机理.     

Electronic structure and spectra of N-methylfullerenepyrrolidine

A quantum-chemical analysis of the donor-acceptor properties of a C₆₀ + dimethylenemethylamine binary system is performed. It is shown that the term of the intermolecular interaction is two-well, because of which the optimization of the system structure at the initial distances between the molecules of 0.21 nm or smaller leads to the formation of N-methylfullerenepyrrolidine C₆₀. At initial distances exceeding 0.25 nm, a weakly bound charge transfer complex with almost complete retention of the properties of the individual molecules is formed. The low-temperature emission and absorption vibronic spectra of N-methylfullerenepyrrolidine C₆₀ in the crystalline toluene matrix are studied experimentally. It is found that the absorption spectra of the fulleroid and pure fullerene in the region corresponding to the excitation of the lowest electronic states are generally similar. The difference observed between the low-frequency spectra agrees completely with the quantum-chemical analysis performed.     

Electronic structure and spectra of CuO

We report the electronic structure of monoclinic CuO as obtained from first principles calculations utilizing density functional theory plus effective Coulomb interaction (DFT + U) method. In contrast to standard DFT calculations taking into account electronic correlations in DFT + U gave antiferromagnetic insulator with energy gap and magnetic moment values in good agreement with experimental data. The electronic states around the Fermi level are formed by partially filled Cu 3d _x²?y² orbitals with significant admixture of O 2p states. Theoretical spectra are calculated using DFT + U electronic structure method and their comparison with experimental photoemission and optical spectra show very good agreement.     

Electronic structure of bismuth ferrite and hematite single crystals: X-ray photoelectron study and calculation

The X-ray photoelectron spectra of the inner levels and valence bands of the surfaces of BiFeO₃ and Fe₂O₃ single crystals have been measured using X-ray photoelectron spectroscopy on an ESCALAB 250 X-ray photoelectron microprobe with monochromatization of X-ray radiation of the AlK _α line. The ab initio calculations have been performed by the full-potential pseudopotential method in terms of the density functional theory (Quantum-Espresso program package). The band structure and the total and partial densities of states have been calculated for the BiFeO₃ and Fe₂O₃ systems. A comparison of the theoretical and experimental results has demonstrated good agreement between theory and experiment.     

Relationships between X-ray photoelectron spectra,structure and superconductivity in LnBa2Cu3O7-x

X-ray photoelectron spectroscopy has been carried out on HoBa₂Cu₃O_7-x (on abraded, fractured and ion beam reduced surfaces). These measurements show that there are only minor differences with similarly obtained results from YBa₂Cu₃O_7-x. The spectral features of the Ln species (Y and Ho), the O 1s, and Ba 3d envelopes are considered in some detail. The low energy contribution to the O 1s envelope has been assigned in part to the effects of Ba–O bonding, either in the LnBa₂Cu₃O_7-x material, or in BaO in minor phase regions, and in part to the effects of the electronic linkage of the Cu planes in the superconducting compound. The anomalous low binding energy contributions to the Ba 3d envelope are also specific to the electronic structure of these materials. It is likely that Ba (and Sr and Ca for the Bi-Sr-Ca-C-O system) plays an important electronic, as well as structural role, in promoting high temperature superconductivity.