BaTi_2Bi_2O的电子结构与磁性（英文）

采用第一性原理计算,我们对BaTi2Bi2O材料的电子结构和磁性做了详细的研究.其非磁性态的计算结果显示,费米能级处的态密度主要来自d2 z,d x2-y2和dxy三个轨道的电子.费米面主要有三部分组成,并且将其沿着矢量q1=(π/a,0,0)和q2=(0,π/a,0)平移时,第三部分费米面(沿着X-R连线)与第一部分费米面(M-A连线)a嵌套明显,计算得出磁化系数χ0(q)值出现在在X点出现峰值,这一点与峰M点的FeAs基超导体不同.上述磁化率峰值可以诱导产生自旋密度波,这使得BaTi2Bi2O的磁性基态是能量基本简并的bi-collinear antiferromagnetism(AF3)与blocked checkerboard antiferromagnetism(AF4)磁性态.随着空穴掺杂,χ0(q)的峰值降低并且变得有些不对称,而电子掺杂则导致峰值变大.一般认为超导序和磁性序相互竞争,当自旋涨落被完全压制时,超导出现,这正好可以解释为什么超导只能出现在空穴掺杂型化合物而非电子掺杂型里.     

MgNi2Bi4弹性和电子性质的第一性原理研究

Bi基化合物因其奇异的结构和物化性质备受科研人员关注MgNi₂Bi₄是实验上合成的一种新型泡利顺磁性层状金属材料,弱的层间相互作用表明它可能是一种潜在的二维材料本文利用第一性原理计算研究了块体和单层MgNi₂Bi₄材料的弹性和电子性质,结果显示块体MgNi₂Bi₄是一种具有复杂费米面的非磁延展性金属.小的解离能表明在实验上可能可以制备得到单层MgNi₂Bi₄,进一步计算表明单层MgNi₂Bi₄仍为典型的金属材料.我们对MgNi₂Bi₄材料弹性和电子结构的计算和分析有助于未来对该材料的进一步研究.     

Co对单层MoS_2电子结构与磁性的影响

为了研究Co对单层MoS_2电子结构和磁性的影响,本文基于第一性原理,采用数值基组的方法计算了Co吸附式掺杂、Co替代式掺杂单层MoS_2的能带结构、态密度以及分析了其结构的稳定性.结果发现:Co替换式掺杂体系的形成能较低,实验上容易实现;Co在Mo位吸附的稳定性强于在S位吸附;Mo位吸附体系的总磁矩为0.999μB,其磁矩的主要来源于Co原子的吸附所贡献的0.984μB,Co原子的掺杂体系总磁矩为1.029μB,其磁矩的主要由Co原子替代掉一个Mo原子所贡献的磁矩为0.9444μB,相比于吸附体系,Co原子对磁矩的贡献率有所降低;无论是Co吸附在单层MoS_2表面还是Co直接替代掉Mo原子的掺杂体系,Co原子3d轨道的引入是引起单层MoS_2体系磁性的主要原因.     

铁基超导体的角分辨光电子能谱研究进展

最近发现的超导转变温度高达55K铁基高温超导体结束了铜氧化物在超导转变温度高于40K的领域一统天下的局面.与铜氧化物高温超导体一样,超导配对对称性对于理解这一新的体系有着重要的作用.文章利用角分辨光电子能谱实验手段,全面地研究了铁基材料的能带结构和费米面以及它们随载流子掺杂浓度变化的演化过程,发现了铁基超导体中依赖费米面的无节点的超导能隙,指出了费米面间的相互作用对超导配对起着关键作用.     

铁基超导体的角分辨光电子能谱研究进展

最近发现的超导转变温度高达55K铁基高温超导体结束了铜氧化物在超导转变温度高于40K的领域一统天下的局面.与铜氧化物高温超导体一样,超导配对对称性对于理解这一新的体系有着重要的作用.文章利用角分辨光电子能谱实验手段,全面地研究了铁基材料的能带结构和费米面以及它们随载流子掺杂浓度变化的演化过程,发现了铁基超导体中依赖费米面的无节点的超导能隙,指出了费米面间的相互作用对超导配对起着关键作用.     

Cr掺杂ZnO纳米线的电子结构和磁性

采用自旋极化密度泛函理论系统研究了Cr掺杂ZnO纳米线的电学、磁学以及光学属性.计算结果显示,Cr原子沿[0001]方向替代ZnO纳米线中的Zn原子时体系一般呈现铁磁耦合,沿[1010]和[0110]方向替代Zn原子时体系呈现反铁磁耦合,且磁性耦合状态在费米能级附近出现了明显的自旋劈裂现象,发生了强烈的Cr 3d和O 2p杂化效应.自旋态密度计算结果显示,磁矩主要来源于Cr原子未成对3d态电子的贡献,磁矩的大小与Cr原子的电子排布有关.光学性质计算结果显示,Cr掺杂ZnO纳米线在远紫外和近紫外都具有明显的吸收峰,吸收峰发生了明显的红移.这些结果都表明Cr掺杂ZnO纳米线也许是一种很有前途的稀磁半导体材料. 关键词： ZnO 纳米线 第一性原理 磁性     

Ce1-xYxFe2的电子结构和磁性质

关键词：     

碳掺杂BaTiO3的电子结构和磁性研究

基于密度泛函理论,从头计算了C间隙和替位氧掺杂立方结构BaTiO3的电子结构和磁学性质．结果表明C掺杂BaTiO3在自旋极化状态下的总能量比自旋非极化状态下的总能量小,说明C掺杂BaTiO3的基态具有铁磁性．从态密度和自旋电子密度分布可知,C位于BaTiO3间隙的磁性机理和过渡金属掺杂半导体产生磁性的机理类似：C替位掺杂BaTiO3体系磁性源于未配对的C2p电子．     

A-La2O3电子结构和光学性质的第一性原理计算

采用基于密度泛函理论(DFT)框架下广义梯度近似平面波超软赝势法,计算A-La2O3的电子结构和光学性质.结果表明,A-La2O3,属于间接带隙氧化物,禁带宽度为3.72 eV;其价带主要由La的5s,5p和6s态电子以及O的2s和2p态电子构成,导带主要由La的5d态电子构成.经带隙校正后,计算得到A-La2O3在(100)和(001)方向上的光学线性响应函数随光子能量的变化关系,包括复介电函数、复折射率、吸收光谱、反射光谱、损失函数和光电导谱.结果表明,A-La2O3,在(100)和(001)方向上具有光学各向异性,并且具有从近紫外到红外的透明区域,为A-La2O3,的应用提供了理论依据.     

梯形化合物NaV2O4F电子结构的第一性原理研究

采用基于密度泛函理论(DFT)的第一性原理赝势平面波方法, 通过自旋极化的广义梯度近似(GGA)电子结构计算对梯形化合物NaV2O4F进行了研究. 考虑了四种假想的自旋有序态, 计算结果表明该化合物的磁基态具有二维反铁磁(AFM)结构, 即沿梯阶和梯腿方向都表现为AFM作用. 能带结构显示NaV2O4F为绝缘体材料, 带隙约为1.0 eV. 方锥体中的晶体场劈裂使得VO4F方锥体中的 V4+(3d1)离子的未配对电子填充dxy轨道. 电负性极强的F离子使得梯阶上的共价性减弱,并导致梯阶上的交换作用减弱. 采用Noodleman的对称性破缺方法由Ising模型拟合出的自旋交换耦合常数表明NaV2O4F的梯间还存在强度与梯阶的AFM作用相当的铁磁(FM)相互作用, 说明该梯形化合物很可能不是一种自旋梯材料.     

铁基超导体系中的竞争序

文章简要介绍铁基超导体系中存在的一种竞争序,即超导与自旋密度波有序之间的竞争,它是理解铁基超导体物理性质与机理的基础.     

BaTi2As2O电子结构的第一性原理研究

利用基于密度泛函理论的平面波赝势方法,研究BaTi2As2O的能带结构、费米面和态密度.发现:BaTi2As2O是一种非磁性金属,费米能级处的态密度主要来自Ti原子的3d电子,Ti 3d轨道和As 4p轨道有较强的杂化.没有发现其磁性基态,说明Ti原子上没有局域磁矩,与Pickett对Na2Ti2Sb2O的研究结论相吻合.     

Substitution Effects on MgB2 Superconductivity

With the help of supercell method, the first-principle calculations were performed for the study of doping crystal Mg_1-xAl_xB₂ and Mg(B_1-yC_y)₂. Analyzing the variations of the charge distribution and the partial densities of states, we found that the compounds with doping Al to MgB₂ compound and/or replacing boron by carbon exhibit new covalent bond effects and unexpected electronic properties, related to superconductivity. The study of the density of states indicates that superconductivity decreases with the increase of Al fraction and carbon concentration. There exists a transition of superconductor to non-superconductor with the change of Al doping fraction. The substitution of boron by carbon results in the decrease of the transition temperature since the decrease of the electron concentration and the lattice constant. The theoretical predictions agree with experimental observations.     

Unconventional superconductivity pairing induced by antiferromagnetic spin fluctuations in layered organic superconductors

The unconventional character of the superconductivity in organic compounds κ-(ET)₂X is ascribed to an antiferromagnetic spin fluctuation induced pairing. Since the band structure involves two bands (±), we assume that the large amplitude spin fluctuations arise from the band with the best nesting properties (band +), while superconductivity pairing occurs in the other band (-).We show that the nesting properties may mimic either a chemical pressure (deuterization) or a hydrostatic one. Indeed, a change of the nesting ratio t₁/t₂, according to our model, induces a modification of the Fermi surface topology. The spin fluctuations strength in the system is affected and consequently the calculated effective coupling constant of superconductivity for the even-parity singlet pairing channel. Our theory appears to be qualitatively consistent with major experimental reports.     

Superconductivity in octagraphene

This article reviews the basic theoretical aspects of octagraphene, an one-atom-thick allotrope of carbon, with unusual two-dimensional(2 D) Fermi nesting, hoping to contribute to the new family of quantum materials. Octagraphene has an almost strongest sp²hybrid bond similar to graphene, and has the similar electronic band structure as iron-based superconductors, which makes it possible to realize high-temperature superconductivity. We have compared various possible mechanisms of superconductivity, including the unconventional s;superconductivity driven by spin fluctuation and conventional superconductivity based on electron–phonon coupling. Theoretical studies have shown that octagraphene has relatively high structural stability. Although many 2 D carbon materials with C;carbon ring and C;carbon ring structures have been reported, it is still challenging to realize the octagraphene with pure square-octagon structure experimentally.This material holds hope to realize new 2 D high-temperature superconductivity.     

Electronic properties and fermi surface for new Fe-free layered pnictide-oxide superconductor BaTi2Bi2O from first principles

Very recently, as an important step in the development of layered Fe-free pnictide-oxide superconductors, the new phase BaTi₂Bi₂O was discovered which has the highest T _C value (～4.6 K) among all related undoped systems. In this Letter, we report for the first time the electronic bands, Fermi surface topology, total and partial densities of electronic states for BaTi₂Bi₂O obtained by means of the ab initio FLAPW-GGA calculations. The interatomic bonding picture is described as a high-anisotropic mixture of metallic, covalent, and ionic contributions. The structural and electronic factors, which can be responsible for the increased transition temperature for BaTi₂Bi₂O (as compared with related pnictide-oxides BaTi₂As₂O and BaTi₂Sb₂O), are also discussed.     

Structure,Electronic, and Mechanical Properties of Three Fully Hydrogenation h-BN:Theoretical Investigations

The structural, electronic, elastic, mechanical properties and stress-strain relationship of chair, boat, and stirrup conformers of fully hydrogenated h-BN(fh-BN) are investigated in this work using the Perdew-Burke-Ernzerhof(PBE) function in the frame of density functional theory. The achieved results for the lattice parameters and band gaps of three conformers in this research are in good accordance with other theoretical results. The band structures of three conformers show that the chair, boat, and stirrup are direct band gap with a band gaps of(3.12, 4.95, and4.95 e V), respectively. To regulate the band structures of fh-BN, we employ a hybrid functional of Heyd-ScuseriaErnzerhof(HSE06) calculations and the band gaps are 3.84(chair), 6.12(boat), and 6.18 e V(stirrup), respectively.The boat and stirrup fh-BN exhibits varying degrees of mechanical anisotropic properties with respect to the Young's modulus and Poisson's ratio, while the chair fh-BN exhibits the mechanical isotropic properties. Furthermore, tensile strains are applied in the armchair and zigzag directions related to tensile deformation of zigzag and armchair nanotubes,respectively. We find that the ultimate strains in zigzag and armchair deformations in stirrup conformer are 0.34 and0.25, respectively, larger than the strains of zigzag(0.29) and armchair(0.18) deformations in h-BN although h-BN can surstain a surface tension up to the maximum stresses higher than those of three conformers of fh-BN. Furthermore, the band gap energies in three conformers can be modulated effectively with the biaxial tensile strain.     

Electronic and Magnetic Properties of Double Perovskite Ca2CrSbO6

First-principles calculations have been performed for the study of the electronic band structure and ferromagnetic properties of double perovskite Ca2CrSbO6. The density of states, total energy, spin magnetic moment, and charge density were calculated and analyzed in details. It is found that Ca2CrSbO6 has a stable ferromagnetic ground state and the spin magnetic moment per molecule is about 2.99#B. The chromium contributes the most in the total magnetic moments. The results indicate that Ca2CrSbO6 is half-metallic.     

Crossover from a pseudogap state to a superconducting state

This paper deduces that the particular electronic structure of cuprate superconductors confines Cooper pairs to be first formed in the antinodal region which is far from the Fermi surface,and these pairs are incoherent and result in the pseudogap state.With the change of doping or temperature,some pairs are formed in the nodal region which locates the Fermi surface,and these pairs are coherent and lead to superconductivity.Thus the coexistence of the pseudogap and the superconducting gap is explained when the two kinds of gaps are not all on the Fermi surface.It also shows that the symmetry of the pseudogap and the superconducting gap are determined by the electronic structure,and non-s wave symmetry gap favours the high-temperature superconductivity.Why the high-temperature superconductivity occurs in the metal region near the Mott metal-insulator transition is also explained.     

First principles simulations of energy and polarization dependent angle-resolved photoemission spectra of Bi2212

We discuss first-principles simulations of angle-resolved photoemission (ARPES) intensity in Bi2212 where the photoexcitation process is modeled realistically by taking into account the full crystal wavefunctions of the initial and final states in the presence of the surface. Some recent results aimed at understanding the effects of the energy and polarization dependencies of the ARPES matrix element are presented. The nature of the Fermi surface (FS) maps obtained via ARPES by holding the initial state energy fixed at the Fermi energy (E_F) is clarified. The theoretically predicted FS map at 21 eV photon energy displays a remarkable level of agreement with the corresponding ARPES spectrum taken over a large area of the (k_x,k_y) plane. Our analysis shows how the ARPES matrix element can help disentangle closely spaced energy levels and FS sheets and highlight different aspects of the electronic spectrum in complex materials under various experimental conditions.