The electronic structures of epitaxial CrSi2 film prepared on Si(111) substrate

The valence band (VB) electronic structures of CrSi₂ were studied by synchrotron radiation photoemission. Overall features of the VB photoemission spectra measured at room temperature (RT) and 20 K by using synchrotron radiation (photon energy, hν=20–120 eV) were similar. Two characteristic emissions were observed corresponding to the bonding and the nonbonding Cr-d partial density of states (PDOS) in the CrSi₂. The onset of the VB photoemission measured at 20 K was located at about 0.32 eV below Fermi level, due to the energy band gap of CrSi₂ more than 0.32 eV.     

Electron density of states at the edge of the valence band of Cd0.88Fe0.12Se—A photoemission yield study

The electronic structures of Cd_0.88Fe_0.12Se and CdSe have been investigated by photoemission yield spectroscopy (PYS) in the photon energy range from 5 to 12 eV. The () surfaces were obtained by cleavage under ultrahigh vacuum (UHV) conditions. An Fe-related emission appeared at 0.58 eV above the valence band edge. The freshly cleaved surface of Cd_0.88Fe_0.12Se interacted with ambient atmosphere more strongly than CdSe crystal. Leaving the sample in an UHV chamber at room temperature enabled us to identify surface related features and to observe decrease of the ionization energy E_i, energy threshold E_d and the crystal affinity χ due to change of the surface conditions. Effective density of states, derived from the experimental spectra of Cd_0.88Fe_0.12Se exhibits, in contrast with CdSe, a surface-related feature degenerated with the bulk valence band.     

Photoemission and electronic structure of crystalline Co3B and amorphous Co - P - B

We present an XPS and UPS study of crystalline Co, Co₃B and Co₇₈P₁₄B₈ glassy metal. For Co₃B the electronic distribution curves (EDC) of the valence band and the previous specific heat and magnetic results are interpreted in a qualitative model where : i) the s-p cobalt-boron bonding states lie in the low part of the band, the upper levels being mainly cobalt d states and ii) a large density of states of the majority spin band is present at the Fermi level. For amorphous Co₇₈P₁₄B₈ the EDC shows that E_F is located in a high density of states region, it is suggested that the phosphorus p states ae centred at 7.5 eV. The photoemission and the magnetic results can be also interpreted in the framework of the previous model.     

Electronic density of states of high pressure liquid uranium from shock vaporization

Strong shock waves were used to compress and heat initially porous uranium plates to high pressures (≈10⁶atm) and temperatures high enough (some 10⁴K), that one should expect a dominant contribution of the electrons to the thermal state. Information about this contribution, which depends on the electronic density of states distribution D(E), is obtained by measuring the entropy in the shocked state on the basis of the shock vaporization technique. The results indicate that a considerable redistribution of the six valence electrons occurs when uranium is compressed and heated.     

The surface electronic structure of (100) centered tetragonal copper

We present a theoretical investigation of the surface electronic structure for the (100) surface of centered tetragonal copper. The calculated band structure and k_- and layer-resolved densities of states for the surface and bulk are compared with corresponding quantities obtained for the face centered cubic (fcc) structure. We have found a surface state at about 5 eV below the Fermi level which is splitted off the bulk s-d band for both the tetragonal and the fcc structure. Furthermore, we have calculated the normal photoemission spectra within the one-step approach. The direct transition model explains all features of available experimental data. Particularly, we can conclude that the invisibility of the surface state for the fcc structure in most of the photoemission experiments is driven by the final states.     

4p-4d fano-like resonance in rhodium

Photoemission measurements of Rh films in the photon energy range 40–120 eV show that the 4p-4d intrashell interaction has a Fano-like resonance. The 4d photoemission intensity goes through a sharp dip in the vicinity of the 4p⁶4dⁿ+hv → 4p⁵4dⁿ⁺¹ threshold followed by a resonant enhancement before decreasing again. All parts of the 4d band show the same resonant behavior in contrast to previous resonant photoemission results. The resonant behavior of Rh is compared with Co which is the 3d analog of Rh.     

Angle-resolved resonant photoemission of Ni(100) and Ni(110) single crystals

We have studied the valence band photoemission spectra of Ni(100) and Ni(110) single crystals near the excitation threshold for 3p core electrons. The resonant behavior of the 6 eV satellite does not depend on both the surface orientation and the polarization of the electric vector of an incident light for excitation. These results indicate that the 6 eV satellite should be under little influence of spatial symmetry of the valence band. In the angle-resolved photoemission spectra of Ni(100), we have observed another broad feature near the 6 eV satellite. It shows the large energy dispersion and is interpreted as due to the interband transition. In Ni(110), we have observed the weak valence band satellites at binding energies of about 9.3 eV and 13.4 eV. They do not show well-defined resonance around the 3p threshold.     

Synchrotron radiation study of the photoionization cross sections for the whole valence band of 2H-MoS2

The first complete analysis of photoionization cross section in the whole valence band of 2H-MoS₂ is reported. The measurements, taken with synchrotron radiation in the 65–190 eV energy range, allow the identification of the Mo 4d contribution to the electron states in the different regions of the valence density of states. In particular it is found that the deepest valence peak contains a considerable 4d contribution.     

Modulated photoemission spectroscopy: Application to Au

We describe modulated photoemission spectroscopy, in which an internal (sample) parameter such as temperature, or an external (apparatus) parameter such as wavelength is varied. A general formalism is developed for modulated photoemission spectroscopy and then illustrated using temperature modulated photoemission spectra and yields for Au obtained in the ～ 6 to 11.6 eV photon energy range. Modulated s-p band photoemission data are described in terms of photoemission critical points in order to explain the nature of the modulated structures in the s-p band region and relate experiment to energy band thresholds obtained from a recent band calculation for Au. Application of the formalism to modulated d-band emission leads to a method for extracting d-band deformation potentials. For example, we find that the upper d band edge moves upward with respect to E_F at a rate of 2 to 6 × 10^?4 eV/K. Modulation of the quantum yield is described and our measurements are compared with modulated optical data.     

Detection of Fe 3d electronic states in valence band and magnetic properties of Fe-doped ZnO film

Fe-doped ZnO film has been grown by laser molecular beam epitaxy (L-MBE) and structurally characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM), all of which reveal the high quality of the film. No secondary phase was detected. Resonant photoemission spectroscopy (RPES) with photon energies around the Fe 2p-3d absorption edge is performed to detect the electronic structure in the valence band. A strong resonant effect at a photon energy of 710 eV is observed. Fe³⁺ is the only valence state of Fe ions in the film and the Fe 3d electronic states are concentrated at binding energies of about 3.8 eV and 7 eV～ 8 eV. There are no electronic states related to Fe near the Fermi level. Magnetic measurements reveal a typical superparamagnetic property at room temperature. The absence of electronic states related to Fe near the Fermi level and the high quality of the film, with few defects, provide little support to ferromagnetism.     

Yb2.75C60同步辐射光电子能谱

在超高真空系统中制备了Yb275C60薄膜.对样品进行了同步辐射光电子能谱研究.在费米能级以下约5 eV范围内的谱数据由Yb275C60价带（C60分子轨道LUMO，HOMO和HOMO 1导出的能带）和芯态（Yb 4f7/2，4f5/2）能级构成.用紫外能区不同能量的入射光子，用C 2p和Yb 4f光电离截面随光子能量的不同变化规律，通过定量拟合，得到了对谱线有贡献的上述各个组分的峰位、峰宽和强度.结果表明，光子能量高于约300 eV时，芯态4f的贡献使得实验结果远远偏离价带的本征态密度分布.因此，研究Yb275C60价态时，应使用能量低于300 eV的光子.对实验和拟合结果分析，发现了类似纯C60的光电离截面振荡现象.振荡幅度相对于纯C60较小，反映了化合物中C60分子的化学环境对光电离截面振荡现象起着不可忽略的作用. 关键词： Yb275C60 同步辐射光电子能谱 光电离截面振荡     

Study of the chemical bonding of titanium with silicon and oxygen by AES and SEELS

Auger electron Spectroscopy (AES) and slow electron energy loss Spectroscopy (SEELS) have been employed to study the electronic structure of Ti, TiSi₂ and TiO₂. The changes in the Auger and loss spectra when Ti chemically binds with silicon to form TiSi₂ and with oxygen to form TiO₂ have been understood as manifestations of changes in electronic participation. AES spectra show distinct changes in line shapes of transitions involving the Ti valence electrons. The SEELS spectra provide information regarding shallow core levels, valence band and the collective excitation energies of the volume and surface plasmons. By monitoring the changes in the Auger peak at 387 eV and the 3p→ 3d quasiatomic transition (at about 45 eV), the role of d-orbital occupancies are studied in Ti and its compounds. The SEELS studies in the 0-80 eV range have enabled the authors to observe the behaviour of the 3p → 3d quasiatomic transition in Ti, which persists after oxidation but almost disappears during TiSi₂ formation. The values of the plasmon losses are related to the collective behaviour of conduction electrons.     

半氢化石墨烯与单层氮化硼复合体系的电子结构和磁性的调控

采用密度泛函理论第一性原理的PBE-D_2方法,对半氢化石墨烯与单层氮化硼(H-Gra@BN)复合体系的结构稳定性、电子性质和磁性进行了系统的研究.计算了六种可能的堆叠方式,结果表明:H-Gra@BN体系的AB-B构型是最稳定的,为铁磁性半导体,上、下自旋的带隙分别为3.097和1.798 e V;每个物理学原胞具有约1μB的磁矩,该磁矩主要来源于由未氢化的C_2原子的贡献;在z轴方向压应力的作用下,最稳的H-Gra@BN体系的电子性质由磁性半导体转变为半金属,再转变为非磁性金属;预测了一种能方便地通过应力调控电子性质和磁性质的新型材料,有望应用在纳米器件以及智能建筑材料等领域.     

二氧化铀电子结构和弹性性质的第一性原理研究

本文采用第一性原理的方法系统研究了UO₂的晶体结构、电子结构和弹性性质. 在计算中采用广义梯度近似结合Hubbard U项描述电子的局域强关联效应. 首先通过计算能带带隙大小并与理论值比较的方法, 得到了合理的有效库仑相关作用能(U_eff)的取值, 同时通过态密度的计算, 进一步验证了U_eff取值的合理性. 计算得到UO₂中U原子的U_eff值为3.30 eV (U_eff=U-J, U=3.70 eV, J=0.40 eV). 应用此参数计算得到的UO₂晶格常数为5.54 Å, 带隙宽度为2.17 eV. 该结果优于目前现有的研究结果, 同时在同样的U_eff值条件下计算所得到的弹性常数与实验值也符合得较好. 相较于之前的基于实验测量并分析得到的U_eff值, 我们所采用的方法在对UO₂性质描述上更为准确. 不同的有效库仑相关作用能取值下的态密度结果表明, 有效库仑相关作用能的大小可以影响铀原子5f电子轨道的分布.     

高温超导体MgB_2的电子结构研究

用第一性原理能带理论计算了高温超导体MgB-2的电子结构.计算得出的电子能带说明MgB-2是一种宽能带化合物,价带主要由Mg和B原子s和p的杂化形成.费米能级处的态密度N(EF)是0.72(states/eV).根据这些结果,初步推断出MgB-2的超导电性的微观机制不可能是电子-声子耦合的BCS模型,而是有待于探索的新机制.     

Electronic structure of PdO studied by photoemission (XPS,UPS)

In this paper we present the results of photoemission studies (XPS and UPS) performed on a polycrystalline surface of PdO. The electron density of states (EDOS) deduced both from XPS and UPS (He_I and He_II) are very similar. The valence band of PdO, which differs significantly from the Pd one, can be built up by four structures located at 0.5 eV, 2.2eV, 4.5 eV and 6.5 eV below E_F. The various electronic contributions (p or d) in the band are considered and, in order to explain our spectra, we discuss several hypothesis taking into account the possible existence of satellite lines or crystal field effects. Our XPS and UPS spectra show that the energy bands of PdO are narrow (～ 2–3 eV), moreover the energy shift of the core levels (|ΔE^F_B| = 2 eV) is important : these results suggest that the correlations between the d electrons may be important in PdO.     

Theoretical descriptions of surface state photoelectron spectroscopy

Various approximations which are commonly used in describing photoemission from solids are reviewed. To analyze experimental spectra quantitatively recourse must be had to theoretical calculations of surface states. Over a valence band width of 10 eV these calculations can be accurate to less than 0.5 eV for an assumed atomic surface configuration. For many purposes photoemission spectra may yield more information on allowed atomic configurations than LEED (low-energy electron diffraction) for current levels of sophistication.     

Conduction and valence band density of states of SnS2: Theory and experiment

The band structure of SnS₂ has been investigated over a wide energy range by pseudopotential band structure calculations and synchrotron radiation photoemission spectroscopy techniques. A good correspondence has been found between energy positions of the theoretical density of states features and structure in the constant initial state (CIS) and energy distribution curves (EDC's) for the conduction and the valence bands respectively. In the energy region between — 8 eV and 15 eV from the top of the valence band we observe four valence band and six conduction band peaks.     

Effective 4ƒ energy level in virtual excitation and in photoemission processes in Ce-pnictides

Effects of relaxation of occupied band electrons to the ?-hole state through the hybridization between ? and band states are studied based on a detailed model for Ce-monopnictides. The effective 4? level is shifted about 1–2 eV to shallow energy side from the unrenormalized bare level in processes in which the 4? electron is only virtually excited, such as in excitation to the vacant p band states through the p-? mixing. Photoemission spectra show two peaks, one near the Fermi energy and the other about 3 eV below it. The latter is shifted to deep energy side about 0.5–1 eV from the bare level when it lies near the bottom of the valence band. The discrepancy between the 4? level estimated from the low energy phenomena and that from photoemission is resolved.     

Electronic structure of partially hydrogenated Si(100)-( 2 x 1) surfaces prepared by thermal and nonthermal desorption

The electronic structure of partially hydrogenated Si(100)- (2 x 1) surfaces, prepared by controlled thermal annealing and nonthermal photon stimulated desorption of fully hydrogenated Si(100) surfaces, has been investigated by using valence band photoemission. Thermal and nonthermal desorption are found to produce very specific electronic surface structures. This led us to the discovery of two specific surface states having binding energies of 1.0 and 0.7 eV associated with the isolated Si dimers and single Si dangling bonds, respectively.