Accurate quasiparticle spectra from self-consistent GW calculations with vertex corrections

Self-consistent GW calculations, maintaining only the quasiparticle part of the Green's function G, are reported for a wide class of materials, including small gap semiconductors and large gap insulators. We show that the inclusion of the attractive electron-hole interaction via an effective nonlocal exchange correlation kernel is required to obtain accurate band gaps in the framework of self-consistent GW calculations. If these are accounted for via vertex corrections in W, the band gaps are found to be within a few percent of the experimental values.     

Five-level k · p model for conduction electrons in GaAs. Description of cyclotron resonance experiments

Five-level k·p model for the conduction electrons in GaAs in the presence of a quantising magnetic field is developed and used to describe spin splittings of the cyclotron resonance and the donor-shifted cyclotron resonance peaks, observed in this material up to fields of 22.5 T. It is shown that the spin splittings are insensitive to polaron effects and that their values can be very well described by the model.Required band parameters correctly account for the rate of electron spin relaxation in GaAs due to inversion asymmetry, as determined by other authors.     

准一维有机铁磁体的磁性机理研究

对准一维有机铁磁体ｐｏｌｙ－ＢＩＰＯ体系的磁性机理进行了研究。详细考虑了该体系中的电子跳跃、电子关联及派尔斯不稳定性，自洽地计算了其能带结构和自旋密度分布，并数值地研究了各因素对系统特性的影响。结果发现该体系基态为稳定的铁磁态． 关键词：     

Intrinsic spin–orbit interaction in carbon nanotubes and curved nanoribbons

We present a theoretical study of spin–orbit interaction effects on single wall carbon nanotubes and curved graphene nanoribbons by means of a realistic multiorbital tight-binding model, which takes into account the full symmetry of the honeycomb lattice. Several effects relevant to spin–orbit interaction, namely, the importance of chirality, curvature, and a family-dependent anisotropic conduction and valence band splitting are identified. We show that chiral nanotubes and nanoribbons exhibit spin-split states. Curvature-induced orbital hybridization is crucial to understand the experimentally observed anisotropic spin–orbit splittings in carbon nanotubes. In fact, spin–orbit interaction is important in curved graphene nanoribbons, since the induced spin-splitting on the edge states gives rise to spin-filtered states.     

基于自洽GW方法的碳化硅准粒子能带结构计算

在多体微扰理论的框架下, 分别采用G₀W₀方法和准粒子自洽GW方法计算3C-SiC和2H-SiC的准粒子能级. 由一个平均Monkhorst-Pack网格点上的准粒子能级和准粒子波函数出发, 结合最局域Wannier函数插值, 得到3C-SiC和2H-SiC的自洽准粒子能带结构. 3C-SiC的价带顶在Γ点, 导带底在X点. DFT-LDA, G₀W₀和准粒子自洽GW给出的3C-SiC间接禁带宽度分别为 1.30 eV, 2.23 eV和2.88 eV. 2H-SiC价带顶在Γ 点, 导带底在K点. 采用DFT-LDA, G₀W₀和准粒子自洽GW方法得到的间接禁带宽度分别为2.12 eV, 3.12 eV和 3.75 eV. 计算基于赝势方法, 对于3C-SiC和2H-SiC的准粒子自洽GW计算给出的禁带宽度均比实验值略大.     

Electronic structure of cubic uranium compounds

Self-consistent cellular multiple scattering techniques and photoemission energy distribution curves obtained at 20<hv<80 eV are used to study the density of states of UN and US. The calculations are based on a model using a finite cluster of atoms in a condensed-matter-like boundary potential. The main results refer to the mixing of thes, p, d, andf-states of uranium into a valence and a conduction band. Thef-states form orbitals with the ligands, within the valence and conduction bands. In the nitride the amount off character in the valence band is only 0.3 electrons and thef electrons are in two resonant levels (of each spin) in the conduction band. Only the first of these levels is occupied for the local, alternate from atom to atom, majority spin. In the sulfide the amount off character in the valence band is 0.59 electrons and the rest of thef-levels are in a resonance state (of majority spin) at the beginning of the conduction band. The conduction band is mainly of localized uranium 6d character. The theoretical results compare favorably with the photoemission data reported here.     

Effect of zero point motion on the hyperfine field at an interstitial positive muon site in ferromagnetic Ni

Using the inhomogeneous electron charge and spin density distribution around the octahedral site in ferromagnetic nickel from a self-consistent band structure scheme, and the abiabatic approximation, we have calculated the muon hyperfine field as a function of the muon displacement. By folding the electron spin density at the vibrating muon site obtained in a self-consistent Kohn-Sham scheme with the finite width of the muon wave function, we find a striking effect on the average muon hyperfine field. The result agrees better with the experiment than earlier calculations based on the jellium model.     

The self-consistent renormalization theory of longitudinal spin fluctuations for weak antiferromagnetic metals with degenerate bands

We develop the self-consistent renormalization theory of spin fluctuations (the SCR theory) for weak antiferromagnetic metals with the single band to the SCR theory with the degenerate bands. The longitudinal spin fluctuations and the longitudinal dynamical susceptibility based on the Hubbard model with the degenerate bands are investigated. As a result, the longitudinal spin fluctuations and the longitudinal dynamical susceptibility are increased with the degeneracy of the band. The Néel temperature is lowered with our increasing the degeneracy of the band.     

Exchange and correlation effects in electronic excitations of Cu(2)O

State-of-the-art theoretical methods fail in describing the optical absorption spectrum, band gap, and optical onset of Cu(2)O. We have extended a recently proposed self-consistent quasiparticle approach, based on the GW approximation, to the calculation of optical spectra, including excitonic effects. The band structure compares favorably with our present angle-resolved photoemission measurements. The excitonic effects based on these realistic band structure and screening provide a reliable optical absorption spectrum, which allows for a revised interpretation of its main structures.     

Electron correlation effects on Compton profiles of copper in the GW approximation

The Compton profiles (CPs) of copper are calculated by the GW approximation with FLAPW basis sets on the LDA. In the quasiparticle band structure in the GW approximation, the width of fully occupied 3d valence band which is overestimated in the LDA, is in good agreement with experimental observation. The dynamical screening effects are important for band width narrowing. The occupation number densities are evaluated from the spectral function calculation within the GW calculations. The CPs obtained using these GW calculations successfully reproduce experimental results.     

Ab initio calculations of electronic structure of anatase TiO2

This paper presents the results of the self-consistent calculations on the electronic structure of anatase phase of TiO2. The calculations were performed using the full potential-linearized augmented plane wave method (FP-LAPW)in the framework of the density functional theory (DFT) with the generalized gradient approximation (GGA). The fullyoptimized structure, obtained by minimizing the total energy and atomic forces, is in good agreement with experiment.We also calculated the band structure and the density of states. In particular, the calculated band structure prefers an indirect transition between valence and conduction bands of anatase TiO2, which may be helpful for clarifying theambiguity in other theoretical works.     

Non-orthogonal tight-binding approach to surface states of covalent semiconductors

A non-orthogonal tight-binding approach to the surface electronic structure of covalent semiconductors is formulated. Using a non-orthogonal basis of bonding, antibonding and dangling-bond orbital surface states, in particular empty surface resonances in the conduction band are computed. Calculations are performed for Si (111) ideal and relaxed surfaces, leading to results in good agreement with self-consistent calculations. The advantage of using a non-orthogonal basis which effectively results in improved orbital localization is demonstrated.     

Magnetism of linear chains

Accurate self-consistent local spin density electronic structure calculations for linear chains of Ni and Fe atoms are presented which realistically treat three-dimensional nature. The one-dimensional character of the bands manifests itself in high density of states arising from van Hove singularities. Both transition metals are “strong” Stoner ferromagnets with large magnetic moments (3.3 and 1.1μ_B for Fe and Ni, respectively) and have large s- and d-exchange splittings. From fits of our results to simple tight-binding models, we find that the d-d effective exchange interaction is similar to that in bulk. However, the use of standard bulk tight-binding parameters is found to be inappropriate since they qualitatively change the results by, for example, misplacing the Fermi level with respect to the band edges. Moreover, in contrast to the bulk, the linear chains also show a rather large s-exchange interaction and hence a large and positive valence contribution to the contract hyperfine field; for Fe this results in a positive contact hyperfine field of the same magnitude as in the bulk. The unique signatures of these linear systems should make their characterization experimentally feasible.     

Band-gap problem in semiconductors revisited: effects of core states and many-body self-consistency

A novel picture of the quasiparticle (QP) gap in prototype semiconductors Si and Ge emerges from an analysis based on all-electron, self-consistent, GW calculations. The deep-core electrons are shown to play a key role via the exchange diagram-if this effect is neglected, Si becomes a semimetal. Contrary to current lore, the Ge 3d semicore states (e.g., their polarization) have no impact on the GW gap. Self-consistency improves the calculated gaps-a first clear-cut success story for the Baym-Kadanoff method in the study of real-materials spectroscopy; it also has a significant impact on the QP lifetimes. Our results embody a new paradigm for ab initio QP theory.     

全相对论多组态原子结构及物理量的精密计算——构建准完备基以及组态相互作用

从第一原理出发计算原子结构有多种理论方法,它们都是基于变分原理的,其关键是构建一组最适合描述真实物理体系的且适用于变分原理的准完备基.本文阐明了如何利用全相对论计算程序GRASPVU,通过单组态Dirac-Fock计算以及多组态Dirac-Fock自洽场计算建立准完备基.然后利用该准完备基进行组态相互作用计算以充分考虑关联作用;在此基础上,进一步考虑电磁相互作用的延迟效应和量子电动力学等修正.该准完备基对原子结构和电磁跃迁等物理量可进行精密的理论计算.最简单的He体系的能级和跃迁速率等物理量的计算值与目前最准确的理论计算值以及精密的实验测量值符合很好,验证了本文提出的方案的适用性.本文计算是全相对论的,可推广到相对论效应很重要的高Z类He体系,以面向重离子储存环相关实验测量.同时该方案也适用于其他任何多电子原子体系;对Mg进行了精密理论计算,阐明了其33D,43D精细结构次序变化的机理.     

D-Type Anti-Ferromagnetic Ground State in Ca_2Mn_2O_5

We study the electronic and magnetic properties of an oxygen-deficient perovskite Ca_2 Mn_2 O_5 based on the first principle calculations. The calculations show that the ground state of Ca_2 Mn_2 O_5 is a D-type anti-ferromagnetic structure with the anti-ferromagnetic spin coupling along the c-direction. The corresponding electronic structure of the D-type state is investigated, and the results display that Ca_2 Mn_2 O_5 is an insulator with an indirect energy gap of ~2.08 eV. By the partial density-of-state analysis, the valence band maximum is mainly contributed to by the O-2 p orbitals and the conduction band minimum is contributed to by the O-2 p and Mn-3 d orbitals. Due to the Coulomb repulsion interaction between electrons, the density of state of Mn-3 d is pulled to-6--4.5 eV.     

Self-consistent calculation of spin transport and magnetization dynamics

A spin-polarized current transfers its spin-angular momentum to a local magnetization, exciting various types of current-induced magnetization dynamics. So far, most studies in this field have focused on the direct effect of spin transport on magnetization dynamics, but ignored the feedback from the magnetization dynamics to the spin transport and back to the magnetization dynamics. Although the feedback is usually weak, there are situations when it can play an important role in the dynamics. In such situations, simultaneous, self-consistent calculations of the magnetization dynamics and the spin transport can accurately describe the feedback. This review describes in detail the feedback mechanisms, and presents recent progress in self-consistent calculations of the coupled dynamics. We pay special attention to three representative examples, where the feedback generates non-local effective interactions for the magnetization after the spin accumulation has been integrated out. Possibly the most dramatic feedback example is the dynamic instability in magnetic nanopillars with a single magnetic layer. This instability does not occur without non-local feedback. We demonstrate that full self-consistent calculations generate simulation results in much better agreement with experiments than previous calculations that addressed the feedback effect approximately. The next example is for more typical spin valve nanopillars. Although the effect of feedback is less dramatic because even without feedback the current can make stationary states unstable and induce magnetization oscillation, the feedback can still have important consequences. For instance, we show that the feedback can reduce the linewidth of oscillations, in agreement with experimental observations. A key aspect of this reduction is the suppression of the excitation of short wavelength spin waves by the non-local feedback. Finally, we consider nonadiabatic electron transport in narrow domain walls. The non-local feedback in these systems leads to a significant renormalization of the effective nonadiabatic spin transfer torque. These examples show that the self-consistent treatment of spin transport and magnetization dynamics is important for understanding the physics of the coupled dynamics and for providing a bridge between the ongoing research fields of current-induced magnetization dynamics and the newly emerging fields of magnetization-dynamics-induced generation of charge and spin currents.     

Role of point defects on conductivity, magnetism and optical properties in In2O3

In order to assess the effects of point defects including transition metal doping on its electronic structure, the self-consistent band structure of the transparent oxide In₂O₃ (in the Ia₃ structure) has been calculated with oxygen vacancies, oxygen and indium interstitial atoms and several transition metal dopants using density functional theory based first principles calculations. An oxygen vacancy alone does not act as a strong native donor but when combined with interstitial indium and (substitutional) transition metal doping, shallow donor levels close to the conduction band are formed. Spin polarized calculations show measurable magnetism in some of the transition metal doped systems while the dielectric functions indicate whether such systems remain transparent among other things.     

Nonequilibrium quantum transport properties of organic molecules on silicon

Electron transport properties of a Si/organic-molecule/Si junction are investigated by large-scale nonequilibrium Green function calculations. The results provide a qualitative picture and quantitative understanding of the importance of self-consistent screening, broadening of quasimolecular orbitals under large bias, and enhancement of transmission, which occurs when the broadened lowest unoccupied molecular orbital aligns with the conduction band edge of the negative lead. The varying coupling can lead to negative differential resistance for a large class of small molecules.