Gd2Co2Al电子结构和磁性的第一性原理研究

从第一性原理出发，在局域自旋密度近似(LSDA)和LSDA+U(在位库仑能)近似下，采用FPLAPW密度泛函能带计算方法研究了Gd₂Co₂Al的电子结构和磁性. 从平均场近似出发，估算了体系的居里温度，并分析了导致体系居里温度偏低的原因.研究结果显示Gd₂Co₂Al为金属导体，其强的铁磁性的提供者主要是Gd，且Co的局域铁磁性是不稳定的. 基于LSDA近似的计算表明Gd₂Co₂关键词： 稀土过渡族金属间化合物 密度泛函理论 电子结构 磁性性质     

U impurities in Au

The properties of U impurities in Au have been studied by magnetic susceptibility, electrical resistivity, and electron spectroscopic measurements. The results indicate that U forms a well-defined local magnetic moment of 3.6 _B , giving rise to a Kondo resistance anomaly at low temperature. This is in agreement with spectroscopic data, which show localized impurity states well removed from the Fermi level. Comparison to calculated atomic multiplets shows the need for inclusion of crystal or ligand field effects for the multiplets. The intra-5f Coulomb correlation energy is found to be 2.6 eV.     

Renormalization of optical excitations in molecules near a metal surface

The lowest electronic excitations of benzene and a set of donor-acceptor molecular complexes are calculated for the gas phase and on the Al(111) surface using the many-body Bethe-Salpeter equation. The energy of the charge-transfer excitations obtained for the gas phase complexes are found to be around 10% lower than the experimental values. When the molecules are placed outside the surface, the enhanced screening from the metal reduces the exciton binding energies by several eVs and the transition energies by up to 1 eV depending on the size of the transition-generated dipole. As a striking consequence we find that close to the metal surface the optical gap of benzene can exceed its quasiparticle gap. A classical image charge model for the screened Coulomb interaction can account for all these effects which, on the other hand, are completely missed by standard time-dependent density functional theory.     

5d过渡金属氧化物中的奇异量子物性研究

相比于3d和4d过渡金属元素, 5d过渡金属元素既具有很强的自旋轨道耦合相互作用, 同时它们的电子关联作用也不可忽略. 因而5d过渡金属氧化物体系具有许多奇异的量子特性. 这篇综述主要介绍我们在5d过渡金属氧化物中的一些理论进展. 首先介绍烧绿石结构铱氧化物(A₂Ir₂O₇, A=Y或稀土元素)中的Weyl拓扑半金属性. 我们确定出A₂Ir₂O₇这一类具有阻挫结构材料的磁基态, 并预言其是Weyl半金属; 其Weyl 点受到拓扑保护而稳定, 而且它的表面态在费米能级形成特别的费米弧. 其次预言尖晶石结构锇氧化物(AOs₂O₄, A=Ca, Sr)是具有奇异磁电响应的Axion绝缘体; 然后分析了电子关联、自旋轨道耦合对钙钛矿结构的锇氧化物(NaOsO₃)的影响, 并成功定出它的基态磁构型, 最终确定其为Slater绝缘体. 最后介绍了LiOsO₃中铁电金属性的成因.     

Effect of the electronic structure of a substrate on the photoinduced behavior of adsorbed NO and CO molecules

Photoprocesses in systems produced by adsorption of NO and CO molecules on the Pt(111) and Ni(111) surfaces, as well as on the (111) surface of Pt-Ge alloy, is studied by the IR absorption spectroscopy, resonant multiphoton ionization, and UV photoelectron spectroscopy methods. The energy of photons varies between 2.3 and 6.4 eV. The character of the processes depends on the type of the metallic substrate. On the Pt(111) surface, NO molecules dissociate or are desorbed, depending on the degree of coverage. On the Ni(111) surface, the molecules only dissociate. Conversely, NO molecules adsorbed on the (111) surface of the Pt-Ge alloy are only desorbed from the surface. In the CO/Pt(111) and CO/Pt(111)-Ge systems, CO molecules adsorbed on on-top adsorption sites are desorbed under the action of the photons, while those occupying bridging adsorption sites change their properties insignificantly. A model of photoinduced processes is suggested. According to this model, the lifetime of a state excited by charge transfer between the valence band of the metal and the 2π-antibonding molecular orbital plays a decisive part in the occurrence of one or the other of these processes.     

SrCoO3电子结构和磁学性质的第一性原理研究

通过基于密度泛函理论的广义梯度近似GGA+U方法对铁磁相SrCoO₃的电子结构和磁学性质进行了系统研究.结果表明:随着U值的增大,对于Co离子,主自旋方向的t_2g和e_g态向低能级移动,而次自旋方向的t_2g和e_g态向高能级移动;O2p电子态的分布基本不随U变化.能带结构表明,U大约在7-8eV之间时,SrCoO₃由金属性转变为半金属性.U值小于7eV时,Co离子的磁矩随着U值的增大几乎成线性增大,而当U大于7eV后基本保持不变.结合实验结果,本文认为U取8eV时得到的计算结果更为合理,Co离子的磁矩为3.19μв,且SrCoO₃表现出半金属特性.     

Ni4NdB电子结构和磁性能第一性原理研究

采用第一性原理,在局域自旋密度近似LSDA及LSDA+U近似,对Ni₄NdB化合物进行结构优化,计算体系晶格常数,电子结构和磁性能.结果表明,Ni₄NdB为带隙很小的金属导体,存在Nd-Ni铁磁耦合,体系总磁矩由Nd原子局域磁矩提供.体系原子成键较为复杂,Nd原子与近邻Ni原子成金属键,Nd原子与近邻B原子成较强离子键,Ni原子与近邻Ni原子间存在间接交换相互作用.在U作用下,体系磁矩与Nd原子磁矩变化一致,Ni原子磁矩在2.75 eV呈现磁有序-磁有序崩溃转变 关键词： 密度泛函理论 电子结构 磁性能 稀土过渡金属间化合物     

Triangular Mott-Hubbard insulator phases of Sn/Si(111) and Sn/Ge(111) surfaces

The ground state of Sn/Si(111) and Sn/Ge(111) surface alpha phases is reexamined theoretically, based on ab initio calculations where correlations are approximately included through the orbital dependence of the Coulomb interaction (in the local density+Hubbard U approximation). The effect of correlations is to destabilize the vertical buckling in Sn/Ge(111) and to make the surface magnetic, with a metal-insulator transition for both systems. This signals the onset of a stable narrow gap Mott-Hubbard insulating state, in agreement with very recent experiments. Antiferromagnetic exchange is proposed to be responsible for the observed Gamma-point photoemission intensity, as well as for the partial metallization observed above 60 K in Sn/Si(111). Extrinsic metallization of Sn/Si(111) by, e.g., alkali doping, could lead to a novel 2D triangular superconducting state of this and similar surfaces.     

The semiconductor-to-ferromagnetic-metal transition in FeSb2

We propose FeSb₂ to be a nearly ferromagnetic small gap semiconductor, hence a direct analog of FeSi. We find that despite different compositions and crystal structures, in the local density approximation with on-site Coulomb repulsion correction (LDA+U) method magnetic and semiconducting solutions for U=2.6 eV are energetically degenerate similar to the case of FeSi. For both FeSb₂ and FeSi (FeSi_1-xGe_x alloys) the underlying transition mechanism allows one to switch from a small gap semiconductor to a ferromagnetic metal with magnetic moment ≈1 μ_B per Fe ion with external magnetic field.     

利用扫描隧道显微镜表征和操纵单分子

扫描隧道显微镜（STM）提供给我们一种表征单分子的局域物理和化学特性的特殊方法，甚至还能帮助我们操纵单分子以构造分子尺度的新型器件。本文中我们采用了两种新型STM技术分别来表征封装在富勒烯笼里面的金属原子和构造一种具有较强Kondo效应的分子器件。空间dI/dV映像谱被用来探索单个Dy@C82分子中能量分辨的金属-笼杂化态，揭示了有关Dy原子在碳笼中的空间位置和Dy-碳笼之间相互作用的重要信息。我们也通过控制STM针尖诱导的高电压脉冲来诱导CoPc分子的边缘脱氢化，从而改变了这个分子在Au(111)表面的吸附构型，导致吸附在Au表面的完整CoPc分子所不具备的Kondo效应产生。     

Adsorption and valence electronic states of nitric oxide on metal surfaces

Among fundamental diatomic molecules, the adsorption of carbon monoxide (CO) and nitric oxide (NO) on metal surfaces has been a subject of intensive research in the surface science community, partly owing to its relevance to heterogeneous catalysis used for environmental control. Compared to the rather well-defined adsorption mechanism of CO, that of NO is less understood because the adsorption results in much more complex reactions. The complexity is ascribed to the open-shell structure of valence electrons, making the molecule readily interact with the metal surface itself as well as with co-adsorbed molecules. Furthermore, the interaction crucially depends on the local structure of the surface. Therefore, to elucidate the interaction at the molecular scale, it is essential to study the valence state as well as the bonding geometry for individual NO molecules placed in a well-defined environment on the surface. Scanning tunneling microscopy (STM) is suitable for this purpose. In this review, we summarize the knowledge about the interaction of NO with metal surfaces, mainly focused on the valence electronic states, followed by recent studies using STM and atomic force microscopy (AFM) at the level of individual molecules.     

Ni5Nd2B4的电子结构和磁性能研究

采用LSDA（Local spin-density approximation）近似及LSDA+U（在位库伦势）近似模拟金属间化合物Ni5Nd2B4的磁性能对于R-M-B合金特性的研究具有重要意义。研究结果显示,LSDA近似下,Ni5Nd2B4具备金属导体性质,晶体结构中最紧邻Ni、B原子间杂化成键,最紧邻Ni-Ni共价成键,Nd、B原子形成成键分子轨道作用,Ni原子间存在自旋消弱现象;LSDA+U近似下,Nd原子磁矩提供体系磁性来源,由于自旋排斥作用Ni原子电子与Nd原子电子自旋方向相反,体系在U值约为6.35eV的作用下能较理想的处理体系电子作用。     

Pd与CeO2(111)面的相互作用的第一性原理研究

采用基于广义梯度近似的投影缀加平面波(projector augmented wave) 赝势和具有三维周期性边界条件的超晶胞模型，用第一性原理计算方法，计算并分析了Pd在CeO₂(111)面上不同覆盖度时的吸附能，价键结构和局域电子结构. 考虑了单层Pd和1/4单层Pd两种覆盖度吸附的情况. 结果表明：1)在单层吸附时，Pd的最佳吸附位置是O的顶位偏向Ce的桥位；在1/4单层吸附时，Pd最易在O的桥位偏向次层O的顶位吸附.2) 单层覆盖度吸附时，吸附原子Pd之间的作用较强；1/4单 关键词： 三元催化剂 Pd 2')" href="#">CeO₂ 吸附 密度泛函理论     

Interplay between chiral magnetic and Kondo effects in Weyl metal phase

We investigate the Kondo effect in a Weyl metal state, which occurs from a spin-orbit coupled Dirac metal phase under magnetic fields. We start from an effective field theory in terms of low-energy fermions on a pair of chiral Fermi surfaces, which takes into account both the Berry curvature and chiral anomaly. Resorting to the U(1) slave-boson mean-field theory, we find that the effective Kondo temperature increases monotonically as a function of the external magnetic field due to enhancement of the density of states. The enhancement is originated from the chiral magnetic effect which is novel feature of Weyl metals. This leads to the prediction of the magnetic-field dependence in the logarithmic temperature dependence of the longitudinal magnetoconductivity.     

Kondo state of Co impurities at noble metal surfaces

We use scanning tunneling microscopy and spectroscopy to study the properties of magnetic Co adatoms on noble metal surfaces at 6 K. Due to spin-flip scattering of the substrate electrons at the impurity the many-body Kondo state forms. This state is characterized by an energy, the Kondo temperature T_K. We measure T_K of adatom systems and a resonant scattering phase shift locally and are thus able to discuss the coupling of the Co adatom to the metal electronic system. From the resonant scattering phase shift of the surface-state electrons scattering off a Co adatom on Ag(111), we find that the coupling to the surface state is rather weak. On the other hand, increasing the number of nearest neighbor substrate atoms increases the coupling of a Co adatom to the host metal and increases T_K. This shows the dominant character of the coupling of the Co atom to the bulk states of the substrate crystal. PACS 72.10.Fk; 68.37,Ef; 72.15.Qm     

Kondo lattice state within the slave boson approach: spin-split masses and effective interaction among heavy quasiparticles

We provide a brief overview of our Hamiltonian approach to the the Kondo-lattice state at both finite Coulomb interaction U and nonzero applied magnetic field. In the mean-field approximation we introduce the spindependent effective masses, as well as calculate the effective Kondo (hybridization) temperature as a function of U. The effective interaction coming from the slave-boson exchange is shown to contain both the local (intraband) pairing and the Kondo (interband) interaction.     

Density functional investigation of Kondo behavior,electronic structure and magnetic properties of CeRuPO-nano-layer

In this study, Kondo behavior, electronic structure and magnetic properties of CeRuPO-nano-layer are investigated using the first principles calculations. The calculations are performed by employing the full potential linearized augmented plane wave (FP-LAPW) method based on the density functional theory (DFT). These properties are calculated in the presence of spin-orbit interaction. The exchange-correlation interaction is calculated within generalized gradient approximation (GGA). In addition, the GGA+U approach (where U is the Hubbard correlation term) is also employed to improve treatment of the f-electrons. The calculated partial electron density of states demonstrates that the hybridization between Ce-4f and Ru-5d orbitals and consequently Kondo effect changes at the surface of the CeRuPO-nano-layer compared to the bulk. The results show that due to the weaker Kondo effect at the surface of CeRuPO-nano-layer, the magnetic moment of Ce atoms enhances and the effective mass of the conduction electron reduces.     

Ar adsorptions on Al (111) and Ir (111) surfaces: a first-principles study

We investigate the adsorptions of Ar on Al (111) and Ir (111) surfaces at the four high symmetry sites, i.e., top, bridge, fcc- and hcp-hollow sites at the coverage of 0.25 monolayer (ML) using the density functional theory within the generalized gradient approximation of Perdew, Burke and Ernzerhof functions. The geometric structures, the binding energies, the electronic properties of argon atoms adsorbed on Al (111) and Ir (111) surfaces, the difference in electron density between on the Al (111) surface and on the Ir (111) surface and the total density of states are calculated. Our studies indicate that the most stable adsorption site of Ar on the Al (111) surface is found to be the fcc-hollow site for the (2 × 2) structure. The corresponding binding energy of an argon atom at this site is 0.538 eV/Ar atom at a coverage of 0.25 ML. For the Ar adsorption on Ir (111) surface at the same coverage, the most favourable site is the hcp-hollow site, with a corresponding binding energy of 0.493 eV. The total density of states (TDOS) is analysed for Ar adsorption on Al (111) surface and it is concluded that the adsorption behaviour is dominated by the interaction between 3s, 3p orbits of Ar atom and the 3p orbit of the base Al metal and the formation of sp hybrid orbital. For Ar adsorption on Ir (111) surface, the conclusion is that the main interaction in the process of Ar adsorption on Ir (111) surface comes from the 3s and 3p orbits of argon atom and 5d orbit of Ir atom.     

钴原子及其团簇在Rh(111)和Pd(111)表面的扫描隧道显微学研究

利用扫描隧道显微镜和扫描隧道谱(STM/STS)及单原子操纵,系统研究了单个钴原子(Co) 及其团簇在Rh (111)和Pd (111)两种表面的吸附和自旋电子输运性质. 发现单个Co原子在Rh (111)上有两种不同的稳定吸附位,分别对应于hcp和fcc空位, 他们的高度明显不同,在针尖的操纵下单个Co原子可以在两种吸附位之间相互转化. 在这两种吸附位的单个Co原子的STS谱的费米面附近都存在很显著的峰形结构, 经分析认为Rh (111)表面单个Co原子处于混价区,因此这一峰结构是d轨道共振 和近藤共振共同作用的结果.对于Rh (111)表面上的Co原子二聚体和三聚体, 其费米面附近没有观测到显著的峰,这可能是由于原子间磁交换相互作用 和原子间轨道杂化引起的体系态密度改变所共同导致.与Rh (111)表面不同, 在Pd (111)表面吸附的单个Co原子则表现出均一的高度.并且对于Pd (111)表面所有 单个Co原子及其二聚体和三聚体,在其STS谱的费米面附近均未探测到显著的电子结构, 表明Co原子吸附于Pd (111)表面具有与Rh (111)表面上不同的原子-衬底相互作用与自旋电子输运性质.