Electronic Structure of Non-oxide Perovskite Superconductor MgCNi3

The electronic structure of new superconducting perovskite MgCNi₃ has been studied using pure Hartree-Fock and density functional theory methods. The main peak of density of states is located below the Fermi level and it is dominated by Ni d. The results of total-energy calculations and electronic density calculations show that MgCNi₃ is not energy favorable in ferromagnetic state. Also we found that the bonding feature between C orbitals and Ni orbitals is covalent, but the bonding between Mg and Ni atoms is ionic or metallic.      

Sn掺杂钙钛矿BaTiO3电子结构的第一原理研究：Ba（SnxTi1-x）O3

采用基于密度泛函理论（DFT）的全势线性缀加平面波（FPLAPW）方法,对Sn掺杂钙钛矿BaTiO3电子结构进行了第一性原理研究,构造了3个不同的超胞,分别为1×2×2、1×1×3和1×1×2,即：分别由4个、3个和2个单胞组成的超胞。研究表明：当X≤0．33时,随着X值的增大,相应的费米面会向Sn掺杂BaTiO3导带的更高的能量处移动,以适应掺杂电子数目的增多,其态密度的演化可以用一个严格的带状模型来描述,所以,掺入BaTiO3体系的每一个电子都对体系的导电过程有贡献,从而使其室温介电常数大幅度提高,介电损失相对减小。复合材料的态密度的重新分布主要是由O—P与所掺入的Sn混合后的杂化而引起的。     

Electronic structures of relaxed BiOX (X = F, Cl, Br, I) photocatalysts

For photocatalysts BiOX (X = F, Cl, Br, I), the atomic sites have been relaxed and the electronic structures have been calculated via the density functional theory (DFT) with or without the adoption of Bi 5d states. BiOF exhibits a direct band gap while the other three species present the indirect feature. The consideration of Bi 5d states results in apparent expansion of the gaps, which are closer to the experimental results. The transition positions are almost independent of the incorporation of Bi 5d states, and the conduction-band bottom flattens with the increase in X atomic number. Both O 2p and X np (n = 2, 3, 4 and 5 for X = F, Cl, Br and I, respectively) states dominate the valence-bands while Bi 6p states contribute most to the conduction-bands. The density peak of the localized X np states in the valence-band shifts towards the valence-band top with the increasing X atomic number, along with certain changes in the valence and conduction bandwidths. Atomic and bond populations, as well as the spatial distribution of orbital density have also been investigated.     

Hole pockets in the doped 2D Hubbard model

The electronic momentum distribution n(k) of the two dimensional Hubbard model is studied for different values of the coupling U/t, electronic density n , and temperature, using quantum Monte Carlo techniques. A detailed analysis of the data on 8 × 8 clusters shows that features consistent with hole pockets at momenta k=(±/2,±/2,) appear as the system is doped away from half-filling. Our results are consistent with recent experimental data for the cuprates discussed by Aebi et al. (Phys. Rev. Lett.72, 2757 (1994)). In the range of couplings studied, the depth of the pockets is maximum at n approximately 0.9, and it increases with decreasing temperature. The apparent absence of hole pockets in previous numerical studies of this model is explained.     

Stable structure of platinum carbides: A first principles investigation on the structure,elastic, electronic and phonon properties

A comprehensive first principles study of structural, elastic, electronic, phonon and thermodynamical properties of novel metal carbide, platinum carbide (PtC) is reported within the density functional theory scheme. The ground state properties such as lattice constant, elastic constants, bulk modulus, shear modulus and finally the enthalpy of PtC in zinc blende (ZB) and rock-salt (RS) structures are determined. The energy band structure and electron density of states for the two phases of PtC are also presented. Of these phases zinc blende phase of PtC is found stable and phase transition from ZB to RS structure occurs at the pressure of about 37.58 GPa. The phonon dispersion curves and phonon DOS are also presented. All positive phonon modes in phonon dispersion curves of ZB-PtC phase indicate a stable phase for this structure. Within the GGA and harmonic approximation, thermodynamical properties are also investigated. All results reveal that the synthesized PtC would favor ZB phase. The compound is stiffer and ductile in nature.     

Geometric and Electronic Structure of Squaric Acid from DFT Calculation

The crystal and electronic structure of antiferroelectric squaric acid is studied using density functional theory method, and the exchange correlation effects are treated by the generalized approximation. In order to understand the ferroelectricity of H2SQ in the molecular plane and the antiferroelectricity in whole crystal, the density of states, charge density distribution and band structure are calculated. The result showed that O2p and C2p play important roles in the interactions between layers. The hybridizations of O2s-Hls and O2p-H1s are responsible for the tendency to ferroelectricity within each layer.     

Interfacial Electronic Structure of Thin Cu Films Grown on Ar^＋—ion Sputter—cleaned α—Al2O3 Substrates

The bonding and electronic structure of Cu/(0001)Al2O3 and Cu/(1120)Al2O3 interfaces has been studied experimentally using spatially-resolved transmission electron energy loss spectroscopy.The specimen were prepared by depositing Cu on single-crystal α-Al2O3 substrates,which have been Ar^ -ion sputter-cleaned prior to the growth of Cu.For both orientations of theα-Al2O3 substrate,atomically abrupt interfaces formed as determined by high-resolution transmission electron microscopy.The investigations of the interfacial Cu-L2,3,Al-L2,3 and O-K energy loss near-edge structures,which are proportional to the site-and angular-momentum-projected unoccupied density of states above the Fermi level,indicate the existence of metallic Cu-Al bonds at the Cu/Al2O3 interface independent of the substrate orientation.     

The Elastic Constants and Anisotropy of Superconducting MgCNi3 and CdCNi3 Under Different Pressure

The second-order elastic constants (SOECs) and third-order elastic constants (TOECs) of MgCNi₃ and CdCNi₃ are presented by using first-principles methods combined with homogeneous deformation theory. The Voigt–Reuss–Hill (VRH) approximation are used to calculate the bulk modulus B, shear modulus G, averaged Young’s modulus E and Poisson’s ratio ν for polycrystals and these effective modulus are consistent with the experiments. The SOECs under different pressure of MgCNi₃ and CdCNi₃ are also obtained based on the TOECs. Furthermore, the Zener anisotropy factor, Chung–Buessem anisotropy index, and the universal anisotropy index are used to describe the anisotropy of MgCNi₃ and CdCNi₃. The anisotropy of Young’s modulus of single-crystal under different pressure is also presented.     

掺杂单层MoS_2电子结构的第一性原理计算

采用第一性原理研究Cu,Ag,Au掺杂单层MoS_2的键长畸变、能带结构和态密度。探讨Cu,Ag,Au掺杂对单层MoS_2电子结构的影响。结果表明:Cu,Ag,Au在S位掺杂的杂质能都低于在Mo位掺杂的杂质能,其在S位掺杂的体系的稳定性强于在Mo位掺杂的体系。在S位掺杂时,杂质与最近邻的Mo,S原子的键长都发生了畸变,畸变率最大的是dAu-Mo,达23.8%。与单层MoS_2的超胞相比,掺杂体系的禁带中出现了4条新能级,导带和价带的能量向低能区移动。杂质原子周围存在着电荷聚集,同时也存在电荷损失。     

SnO2纳米粒子的微波等离子制备及电子结构研究

采用未见报道的ＳｎＯ＿２微波等离子体化学气相沉积法制备出了平均粒径小于１０ｎｍ的ＳｎＯ＿２粒子，ＸＰＳ分析结果表明其内部电子结构发生了变化。     

Electronic structure and transport properties of Co(Si1−xMx) (M = Al, P): First-principles study

In this study, by using the full-potential linear augmented plane wave (FLAPW) method based on the density functional theory (DFT), the lattice parameter of CoSi was calculated theoretically and the calculations of the electronic structures of CoSi and CoSi_1−xM_x (M = Al, P and x = 0.03125, 0.125) were performed. The calculated lattice parameter of binary CoSi is about 0.27% smaller than the experimental value. Calculated electronic structures show that CoSi is a semi-metal and the density of states (DOS) is very small at the Fermi level. M-doping can tune the Fermi level and the hole pockets and the electron ones, which is very valuable to modulate the transport properties. Based on the calculated electronic structures and our experimental results on CoSi [C.C. Li, W.L. Ren, L.T. Zhang, K. Ito, J.S. Wu, J. Appl. Phys. 98 (2005) 063706], the intrinsic relations between electronic structures and transport properties of CoSi and CoSi_1−xAl_x are discussed in detail. The transport properties along main crystallographic directions of binary CoSi and CoSi_1−xAl_x are experimentally examined. The experimental results show that the electrical resistivity of CoSi-based compounds is anisotropic, while the Seebeck coefficient is almost isotropic. The calculated band structures of CoSi_1−xAl_x can theoretically interpret the anisotropy of the electrical transport properties.     

Ni掺杂PtAl2合金电子结构的第一性原理计算

采用第一性原理的密度泛函理论方法从杂质形成能、电荷布居、电子结构等几个方面研究了PtAl2合金的掺杂效应。结果发现,Ni置换Pt元素掺杂形成能的绝对值要低于Ni置换A1的掺杂形成能。进一步从电子结构分析,两者在费米面附近的电子主要是由Pt的4p电子轨道,Ni的3d电子轨道以及部分Al的3p电子轨道上的电子贡献,但前者在费米能附近的反应剧烈程度要高于后者,这说明在PtAl2合金的掺杂中,Ni元素更容易置换Pt原子与Al生成NiAl合金。     

Superconducting state in M3C60: Strong vs. weak coupling

The superconducting state in the doped fullerenes is due to strong coupling (e.g.,2.1 for Rb₃C₆₀) to low-frequency intramolecular modes _L 250 cm^–1 (^21/2). The analysis is based on an equation describingT _c for any strength of the coupling and on recent isotope effect and NMR data.     

Traces of Defects in the Electronic Structure of Porous Ni--Ti Alloys

The electronic structures of Ni—Ti shape-memory alloy samples were investigated by X-ray absorption fine structure(XAFS) spectroscopy both experimentally and theoretically.In the experimental section,the samples were measured at low temperature to determine the persistent traces of both preheating process and atomic concentration effects on the crystal and electronic structure by X-ray absorption near-edge structure(XANES) spectroscopy.As a second step,the extended-X-ray absorption fine structure(EXAFS) calculations,which are based on different choices of one electron potentials according to Ti coordinations by using the real space multiple scattering method FEFF 8.2 code,were performed.The crystallographic and electronic structures of the porous Ni-Ti alloys were tested at various temperatures ranging from 5 to 1323 K.