Local probing of adsorbate electronic structure using soft X-ray emission; atomic nitrogen on Ni(100) and Cu(100)

The N 2p partial density of states of p4g-N/Ni(100) and c(2 × 2)-N/Cu(100) overlayers have been studied by soft X-ray emission spectroscopy (SXES) and comparisons with corresponding UV photoelectron spectra are made. Broad states (∼10 eV) are observed due to the hybridisation between the N 2p and the substrate 3d and 4sp bands. For both N/Cu and N/Ni the intensity stretches out to the Fermi level, where a prominent peak for N/Cu is observed. These states close to the Fermi level are interpreted as antibonding 2p-3d hybridised states. The higher occupancy of these states in N/Cu is expected due to the higher binding energy of the 3d band in Cu than in Ni. For a complete interpretation of the SXE spectra, especially for N/Cu where many features are observed, further theoretical studies are required.     

掺杂MgCNi3超导电性和磁性的第一性原理研究

从第一性原理出发，计算了MgCNi₃的电子能带结构.MgCNi₃中C 2p与Ni 3d轨道杂化使穿梭费米面上的Ni 3d能带表现出平面性，费米面落在态密度范霍夫奇异(vHs)峰的右坡上.vHs峰上大的电子态密度和铁磁相变点附近的自旋涨落是决定MgCNi₃超导电性的重要因素.研究了三种替代式掺杂对其超导电性和磁性的影响，发现电子掺杂使费米能级下滑到态密度较低的位置，导致体系转变为无超导电性的顺磁相；同构等价电子数的金属间化合物的轨道杂化，引起费米面上态密度的减少，降低了超导电性；而空穴掺杂使费米面向vHs峰值方向移动，虽然费米面上电子态密度增大可能提高超导电性，但增强了的Ni原子磁交换作用产生铁磁序，破坏了超导电性. 关键词： 电子结构 超导电性 磁性 掺杂     

Electronic band structure and inter-atomic bonding in tetragonal BiOCuS as a parent phase for novel layered superconductors

Very recently, the tetragonal BiOCuS was declared as a new superconducting system with a Fe-oxypnictide-related structure. Here, based on first-principle FLAPW-GGA calculations, the structural parameters, electronic band structure, density of states and inter-atomic bonding picture for BiOCuS are investigated. Our results show that, distinct from related metallic-like FeAs systems, the BiOCuS phase behaves as an ionic semiconductor with the calculated indirect band gap at about 0.48 eV. The superconductivity for BiOCuS may be achieved only by doping of this phase. Our data demonstrate that as a result of hole doping, the [CuS] blocks become conducting owing to mixed Cu 3d + S 3p bands located near the Fermi level. For the hole doped BiOCuS the Fermi surface adopts a quasi-two-dimensional-like character, similarly to FeAs SCs.     

Effect of 3d doping on the electronic structure of BaFe2As2

The electronic structure of BaFe(2)As(2) doped with Co, Ni and Cu has been studied by a variety of experimental and theoretical methods, but a clear picture of the dopant 3d states has not yet emerged. Herein we provide experimental evidence of the distribution of Co, Ni and Cu 3d states in the valence band. We conclude that the Co and Ni 3d states provide additional free carriers to the Fermi level, while the Cu 3d states are found at the bottom of the valence band in a localized 3d(10) shell. These findings help shed light on why superconductivity can occur in BaFe(2)As(2) doped with Co and Ni but not Cu.     

Electronic States near the Fermi Level in the Antiferromagnetic and Ferromagnetic Spinels of Zn 1− x Cu x Cr 2 Se 4 ; 0.0 ≤ x ≤ 1.0

The electronic states of normal-type spinels in Zn 1 m x Cu x Cr 2 Se 4 have been studied by X-ray absorption spectroscopy near the K-edge of Zn, Cu, Cr and Se ions. It is found that characteristic white lines occur near the X-ray absorption threshold for Zn and Se ions, but not for Cu and Cr ions. It is also found that the white lines show a slight energy shift, which depends on the Cu concentration. The energy shifts mean that Fermi level gradually approaches the top region of the valence band of Se 2 m , on increasing the Cu ions. The substitution of Zn 2+ with Cu 1+ ions increases the number of positive carriers, holes. The holes occupying the top region of the valence band strongly correlate with Cr 4+ , which forms an empty orbital in the 3d-band of Cr 3+ and exchanges the valence electrons with Cr 3+ . It is interpreted that the ferromagnetism in Zn 1 m x Cu x Cr 2 Se 4 is made by double-exchange interaction between Cr 3+ and Cr 4+ ions and that holes contribute to the electronic conductivity of Zn 1 m x Cu x Cr 2 Se 4 .     

新型超导材料MgCNi3的电子结构与超导电性研究

用MS-Xα方法研究了非氧化物超导材料MgCNi₃的电子结构. 研究结果显示, 态密度分布曲线的主峰靠近Fermi面, 主要来自于Ni的d电子的贡献. 用T(T=Co，Mn，Cu)替代MgCNi₃中的部分Ni形成化合物MgCNi₂T，替代使Ni的价电子数减小, 价态发生变化, Fermi面处态密度N(E_F)减小. 计算结果表明：无论是电子掺杂（Cu）还是空穴掺杂（Co，Mn），MgCNi₃的超导电 关键词： 电子结构 态密度 超导电性     

Fermi level pinning at CdS/Cu(In,Ga)(Se,S)2 interfaces: effect of chalcopyrite alloy composition

We introduce a quantitative model for the band diagram of ZnO/CdS/Cu(In,Ga)(Se,S)₂ heterostructures and for carrier recombination at the CdS/chalcopyrite interface. We derive analytical expressions for the open circuit voltage and the Fermi energy position at the active interface. The open circuit voltage under interface recombination is almost independent from the band gap energy of the chalcopyrite when the valence band edge of the absorber remains at the same energy position. The analytical calculations are in relatively good agreement with numerical simulations. Experimental current–voltage analysis indicates that devices prepared from Cu-poor Cu(In,Ga)(Se,S)₂ chalcopyrites are dominated by recombination in the bulk of the absorber while interface recombination prevails if the absorbers are prepared under Cu-excess. In the latter case, the experimentally determined interface barriers reveal that the interface Fermi energy position shifts upward on the energy scale upon increasing the Ga content into the absorber and remains at a relatively low energy value under S/Se alloying.     

Chern semimetal and the quantized anomalous Hall effect in HgCr2Se4

In 3D momentum space, a topological phase boundary separating the Chern insulating layers from normal insulating layers may exist, where the gap must be closed, resulting in a "Chern semimetal" state with topologically unavoidable band crossings at the Fermi level. This state is a condensed-matter realization of Weyl fermions in (3+1)D, and should exhibit remarkable features, such as magnetic monopoles and Fermi arcs. Here we predict, based on first principles calculations, that such a novel quantum state can be realized in a known ferromagnetic compound HgCr2Se4, with a single pair of Weyl fermions separated in momentum space. The quantum Hall effect without an external magnetic field can be achieved in its quantum-well structure.     

Interplay among superconductivity,charge density waves,and magnetism in EuMo6S8

In a one-dimensional metal, the energy of the electrons can always be lowered by opening an energy gap around the Fermi energy (the Peierls instability): all occupied states are then in the lower-energy band, while the higher-energy band is empty. The opening of such a gap requires a structural distortion, resulting in the formation of a charge density wave. In a three-dimensional system, the gapping takes place in the region where the Fermi surface is nested (i.e., large parallel areas of the Fermi surface are spanned by a certain wave vector), giving rise to partial gapping of the Fermi surface, accompanied by a structural distortion. In this case, a charge density wave can coexist with superconductivity. Both charge-density-wave and superconducting transitions involve the formation of an energy gap at the Fermi energy. A charge-density-wave gap is formed at a region of the Fermi surface where there is a high density of electronic states. In such a material, there is also a strong electronphonon interaction. A region with high density of states and a high electron-phonon interaction is just the portion of the Fermi surface that will enhance the superconducting transition temperature, according to the BCS (Bardeen-Cooper-Schrieffer) theory. When a charge-density-wave gap opens up at the Fermi surface these electronic states are no longer available to form Cooper pairs and to enhance the superconducting transition temperature. The opposite is also true; if a superconducting gap opens, the states involved in forming this gap are no longer available to take part in a charge-density-wave transition. It appears that charge density waves and superconductivity compete for the same portion of the Fermi surface and thus inhibit each other. In this paper, we will review a unique situation with respect to the competition between these two ground states and will also discuss how this competition affects the anomalous behavior of critical field in EuMo6S, at high pressure.     

Distinct fermi surface topology and nodeless superconducting gap in a (Tl0.58Rb0.42)Fe1.72Se2 superconductor

High resolution angle-resolved photoemission measurements have been carried out to study the electronic structure and superconducting gap of the (Tl0.58Rb0.42)Fe1.72Se2 superconductor with a T(c) = 32 K. The Fermi surface topology consists of two electronlike Fermi surface sheets around the Γ point which is distinct from that in all other iron-based superconductors reported so far. The Fermi surface around the M point shows a nearly isotropic superconducting gap of ～12 meV. The large Fermi surface near the Γ point also shows a nearly isotropic superconducting gap of ～15 meV, while no superconducting gap opening is clearly observed for the inner tiny Fermi surface. Our observed new Fermi surface topology and its associated superconducting gap will provide key insights and constraints into the understanding of the superconductivity mechanism in iron-based superconductors.     

点缺陷对单层MoS2电子结构及光学性质的影响研究

采用基于第一性原理的贋势平面波方法,对不同类型点缺陷单层MoS2电子结构、能带结构、态密度和光学性质进行计算。计算结果表明：单层MoS2属于直接带隙半导体,禁带宽度为1.749ev,V-Mo缺陷的存在使得MoS2转化为间接带隙Eg=0.671eV的p型半导体,V-S缺陷MoS2的带隙变窄为Eg=0.974eV,S-Mo缺陷的存在使得MoS2转化为间接带隙Eg=0.482eV; Mo-S缺陷形成Eg=0.969eV直接带隙半导体,费米能级上移靠近价带。 费米能级附近的电子态密度主要由Mo的4d态和s的3p态电子贡献。光学性质计算表明：空位缺陷对MoS2的光学性质影响最为显著,可以增大MoS2的静态介电常数、折射率n0和反射率,降低吸收系数和能量损失。     

Nodeless d-wave superconducting pairing due to residual antiferromagnetism in underdoped Pr2-xCexCuO4-delta

We investigate the doping dependence of the penetration depth versus temperature in electron-doped Pr(2-x)Ce(x)CuO(4-delta) using a model which assumes the uniform coexistence of (mean-field) antiferromagnetism and superconductivity. Despite the presence of a d(x2-y2) pairing gap in the underlying spectrum, we find nodeless behavior of the low-T penetration depth in the underdoped case, in accord with experimental results. As doping increases, a linear-in-T behavior of the penetration depth, characteristic of d-wave pairing, emerges as the lower magnetic band crosses the Fermi level and creates a nodal Fermi surface pocket.     

Penetration depth study of superconducting gap structure of 2H-NbSe2

We report measurements of the temperature dependence of both in-plane and out-of-plane penetration depths (lambda(a) and lambda(c), respectively) in 2H-NbSe2. Measurements were made with a radio-frequency tunnel diode oscillator circuit at temperatures down to 100 mK. Analysis of the anisotropic superfluid density shows that a reduced energy gap is located on one or more of the quasi-two-dimensional Nb Fermi surface sheets rather than on the Se sheet, in contrast with some previous reports. This result suggests that the gap structure is not simply related to the weak electron-phonon coupling on the Se sheet and is therefore important for microscopic models of anisotropic superconductivity in this compound.     

Fermi surface topology and the upper critical field in two-band superconductors: application to MgB2

Recent measurements of the anisotropy of the upper critical field B(c2) on MgB2 single crystals have shown a puzzling strong temperature dependence. Here, we present a calculation of the upper critical field based on a detailed modeling of band structure calculations that takes into account both the unusual Fermi surface topology and the two gap nature of the superconducting order parameter. Our results show that the strong temperature dependence of the B(c2) anisotropy can be understood as an interplay of the dominating gap on the sigma band, which possesses a small c-axis component of the Fermi velocity, with the induced superconductivity on the pi-band possessing a large c-axis component of the Fermi velocity. We provide analytic formulas for the anisotropy ratio at T=0 and T=T(c) and quantitatively predict the distortion of the vortex lattice based on our calculations.     

Density functional theory analysis of electronic structure and optical properties of La-doped Cd_2SnO_4 transparent conducting oxide

The electronic structural, effective masses of carriers, and optical properties of pure and La-doped Cd_2SnO_4 are calculated by using the first-principles method based on the density functional theory. Using the GGA+U method, we show that Cd_2SnO_4 is a direct band-gap semiconductor with a band gap of 2.216 eV, the band gap decreases to 2.02 e V and the Fermi energy level moves to the conduction band after La doping. The density of states of Cd_2SnO_4 shows that the bottom of the conduction band is composed of Cd 5 s, Sn 5 s, and Sn 5 p orbits, the top of the valence band is composed of Cd 4d and O 2p, and the La 5 d orbital is hybridized with the O 2 p orbital, which plays a key role at the conduction band bottom after La doping. The effective masses at the conduction band bottom of pure and La-doped Cd_2SnO_4 are 0.18 m0 and 0.092 m_0, respectively, which indicates that the electrical conductivity of Cd_2SnO_4 after La doping is improved. The calculated optical properties show that the optical transmittance of La-doped Cd_2SnO_4 is 92%, the optical absorption edge is slightly blue shifted, and the optical band gap is increased to 3.263 eV. All the results indicate that the conductivity and optical transmittance of Cd_2SnO_4 can be improved by doping La.     

GaN及其Ga空位的电子结构

用团簇埋入自洽计算法对宽禁带半导体GaN的电子结构进行了自旋极化的、全电子、全势场的从头计算,得到了与实验值符合的GaN晶体禁带宽度以及价带中N2p带、N2s带和Ga3d带之间的相对位置.在此基础上Ga空位计算(无晶格畸变)显示,Ga空位周围的费米面显著高于正常GaN晶格的费米面.因此Ga空位周围N原子的处于费米面上的2p电子很易被激发成正常晶格处的传导电子 关键词： GaN 电子结构 团簇埋入自洽计算     

YNi4Cu: XPS measurements and electronic structure calculation

The valence band and the core levels of the YNi₄Cu compound are studied using the X-ray photoemission spectroscopy. The valence band is compared with the theoretical calculation by the spin-polarized Tight Binding Linear Muffin Tin Orbital method. The dominance of the Ni 3d and Cu 3d states down to 5 eV below the Fermi level is found. The modification of the bands’ widths and positions can be well explained by the d–d repulsion model. The Ni 2p and Y 3d peaks resemble the results for pure metals.     

Spin-triplet superconductivity due to antiferromagnetic spin-fluctuation in Sr2RuO4

A mechanism leading to the spin-triplet superconductivity is proposed based on the antiferromagnetic spin-fluctuation. The effects of anisotropy in spin-fluctuation on the Cooper pairing and on the direction of d vector are examined in the one-band Hubbard model with random-phase approximation. The gap equations for the anisotropic case are derived and applied to Sr2RuO4. It is found that a nesting property of the Fermi surface together with the anisotropy leads to the triplet superconductivity with the d = &zcirc;(sink(x)+/-isink(y)), which is consistent with experiments.     

The electronic and optical properties of Cu doped CdI2

Based on the density functional pseudopotential method, the electronic structures and the optical properties of CdI₂ doped with Cu are investigated in detail. The calculation results indicate that the defect of Cu(Cd) exists steadily with a certain solubility. For the Cu doped CdI₂, the new highly localized impurity bands induced by Cu 3d states lie just across the Fermi energy at the top of the valence band. The doping of Cu induces reduction of band gap of CdI₂; red shifts are revealed in both the imaginary part of dielectric function and the absorption spectra corresponding to the change in band gaps. Moreover, the study of the reflection spectrum and the loss function shows that the doped Cu is responsible for the increased reflection peak intensity and the red shift of the plasma resonant frequency of CdI₂.     

Cr adsorption induced magnetism in Bi2Se3 film by proximity effects

Based on first-principles calculations within density functional theory, we studied the effects of Cr adsorption on the electronic and magnetic properties of Bi₂Se₃ topological insulators employing spin–orbit coupling (SOC) self-consistently. Cr atom induces a spin-polarization with total net magnetic moments of 2.157 μ_B (spin up). There is a p-d hybridization between the Cr 3d states and the nearest neighbor Se 4p states. A peak of density of states appears at Fermi level. The electronic structures change and the energy levels split near the Fermi level. No gap opening has been found at the Dirac point of the surface state from the bottom surface.