Dynamic Electronic Structure in Transition Metal Oxides

The isomer shift of La1-xCaxFeO3 decreases gradually from the value for trivalent iron to that for tetravalent iron, as the Ca content x increases from 0 to 1. This indicates that iron in La1.x-CaxFeO3 has an intermediate valence state. The intermediate valence state of iron increases gradually from trivalence to tetravalence. This can be interpreted as being due to electron delocalization among iron ions on identical octahedral sites. The Mossbauer spectra at various temperatures of La0.5Ca0.5FeO3 and their theoretical treatment show that electron delocalization slows down with decreasing temperature.     

Dynamic Electronic Structure in Transition Metal Oxides

The isomer shift of La1-xCaxFeO3 decreases gradually from the value for trivalent iron to that for tetravalent iron, as the Ca content x increases from 0 to 1. This indicates that iron in La1.x-CaxFeO3 has an intermediate valence state. The intermediate valence state of iron increases gradually from trivalence to tetravalence. This can be interpreted as being due to electron delocalization among iron ions on identical octahedral sites. The Mossbauer spectra at various temperatures of La0.5Ca0.5FeO3 and their theoretical treatment show that electron delocalization slows down with decreasing temperature.     

Theoretical Model for Superconductivity in Magnetic Borocarbides

The interaction between magnetism and superconductivity is investigated within the theory of superconductivity for electrons in magnetic Bloch-states. We show that the presence of an incommensurate helical magnetic structure suppresses the superconducting order parameter only slightly, via the reduction of the effective attraction. Consequences for the upper-critical-field curve of HoNi₂B₂C and related compounds are discussed.     

Superconductivity (and Magnetism) in the Nickel Borocarbides

An overview of superconductivity in the rare-earth nickel borocarbides (RNi₂B₂C, where R represents a rare earth element or Y) is presented, with an emphasis on the interplay between magnetism and superconductivity in general and the behavior of the nonmagnetic superconductors YNi₂B₂C and LuNi₂B₂C, in particular. Some open questions are identified.     

Absence of a Competition Between Magnetism and Superconductivity in Layered Nickel Borocarbides: Common Correlations of Tc with Crystal Chemical Parameters for Magnetic and Nonmagnetic Compounds

In the present work, an analysis of the crystal chemical parameters of nickel borocarbides RNi₂B₂C (R = rare earth) is given. The reasons for the dependence of superconducting transition temperature (T _c) on crystal chemical parameters by two separate curves for magnetic and nonmagnetic R are considered. For all R, a common pattern of dependence of T _c on crystal chemical parameters similar to that existing in layered quasi two-dimensional systems (HTSC cuprates and diborides) is established. The absence of the influence on the T _c of borocarbides of magnetic properties R is also established. On the basis of the correlations found, the radii of a number of rare earths are more precisely defined, and T _c of compounds at various substitutions R are calculated.     

Electronic Structure of Doped Insulators and High Temperature Superconductivity

The Fermi liquid theory of the normal state and the BCS-Eliashberg theory of the superconducting state were designed for good metals not for doped antiferromagnetic insulators, such as the high temperature superconductors. Consequently, it is necessary to understand the electronic structure of the doped insulator and to develop a new mechanism and many-body theory of superconductivity for these materials. It will be argued that, since the motion of a single hole in an antiferromagnet is frustrated, the driving force for the physics of a finite concentration of doped holes is the need to reduce their zero-point kinetic energy. This proceeds in three steps that are reflected in a sequence of crossovers and phase transitions. First of all, the system forms a charge-inhomogeneous state – an electronic liquid crystal phase, involving an array of metallic stripes, which lowers the kinetic energy along a stripe. In the direction perpendicular to the stripes, the kinetic energy is lowered by pair hopping, which proceeds in two steps. Local pair hopping induces spin pairing and then, at a lower temperature, pair hopping from stripe to stripe produces superconducting phase coherence. Some of the experimental support for the various aspects of this model will be described.     

Local Electronic Structure, O–T Phase Transition and Superconductivity in the Ba-site Nd-substituted YBCO System

A series of YBa_2-xNd_xCu₃O_y (x = 0–0.4) samples have been systematically studied by means of X-ray diffraction, transport property measurements and positron annihilation technology. The positron lifetime parameters show strong Nd substitution dependence. There is an obvious change of positron lifetime parameters around the O–T phase transition. The local electron density n_e and vacancy concentration C_v as a function of x were calculated from the positron lifetime results. The correlations between local electronic structure, O–T phase transition and superconductivity are discussed. The results confirmed that n_e mainly has an effect on high-T_c superconductivity by affecting the charge transfer between CuO₂ planes and Cu–O chains region or Ba–O layer. The vacancy properties in the orthorhombic phase and tetragonal phase are two intrinsic different types. Positron lifetime is very sensitive to the O–T phase transition in the YBCO systems that can be used as a useful technique to determine the O–T phase transition in these systems.     

Absence of a Competition Between Magnetism and Superconductivity in Layered Nickel Borocarbides: Common Correlations of Tc with Crystal Chemical Parameters for Magnetic and Nonmagnetic Compounds

In the present work, an analysis of the crystal chemical parameters of nickel borocarbides RNi₂B₂C (R = rare earth) is given. The reasons for the dependence of superconducting transition temperature (T _c) on crystal chemical parameters by two separate curves for magnetic and nonmagnetic R are considered. For all R, a common pattern of dependence of T _c on crystal chemical parameters similar to that existing in layered quasi two-dimensional systems (HTSC cuprates and diborides) is established. The absence of the influence on the T _c of borocarbides of magnetic properties R is also established. On the basis of the correlations found, the radii of a number of rare earths are more precisely defined, and T _c of compounds at various substitutions R are calculated.     

Ab Initio Study of Electronic and Magnetic Properties of Germanene with Different Nonmagnetic Metal Adatoms

Electronic and magnetic properties of two-dimensional (2D) germanene (Ge) with five different adatoms have been analyzed and discussed by the DFT method. Different nonmagnetic metals adsorbed at different sites, on one hand, a magnetic moment is induced by Mg adatom in the germanene. On the other hand, the adsorptions of Al, Ga, Li, and Na show no magnetism. We further study the effect of strain on the magnetism in Mg-adsorbed germanene; we apply an isotropic tensile and compressive strain on the system. On the basis of our calculations, a tunable magnetism shows as the strain increases. The analysis of the PDOS shows that the s–p hybridization between Mg and Ge atoms results in such magnetic behavior. These magneticadsorbed germanene materials might show potential applications in the nanoscale devices.     

金属Ni的电子结构

用纯金属单原子(OA)理论、全势线性Muffin-Tin轨道(FP-LMTO)及第一原理离散变分方法(DVM)对Ni的电子结构与磁性进行了系统的研究,得到了金属Ni的轨道占据数、平衡晶格常数、结合能、原子磁矩、体弹性模量、态密度、能带结构等信息,并对结果进行了分析.     

Synthesis and Electronic Properties of Transition Metal Containing Polymers

1. IntroductionSince the discovery of the first organic light emitting polymer in 1991, research in this area has been mainly focused on conjugated organic polymers. The use of polymer metal complexes for electroluminescence applications has received relatively few attention.     

Synthesis and Electronic Properties of Transition Metal Containing Polymers

1. Introduction Since the discovery of the first organic light emitting polymer in 1991, research in this area has been mainly focused on conjugated organic polymers. The use of polymer metal complexes for electroluminescence applications has received relatively few attention.     

金属Pt的电子结构和物理性质

依据纯金属单原子理论(OA)确定了面心立方结构(fcc)金属Pt的电子结构为[Xe](5dn)6.48(5dc)2.02-(6sc)1.48(6sf)0.02,由这种电子结构计算的晶格常数、结合能等与实验值很符合,并与第一原理方法计算的结果较符合,在此基础上初步解释了金属Pt的电子结构与其催化性能的关系,通过计算得到了fcc金属Pt的势能曲线、热膨胀系数、晶格常数、比热、焓和Gibbs能随温度变化的曲线.     

Superconductivity and Structure

The degree of the isolation of the CuO₂ planes (e.g. distance or bond valence to the apical coordination) has been shown by quantitative algorithms to be the major factor in determining aspects of the doping curves. They include the magnitude of the optimal number of doped holes (h_op) and the corresponding T _cop. It is shown that the roots of these phenomenological laws lie in a related structural dependence of super-exchange. The latter is expressed in the pseudo-gap or Neel temperature of the undoped parent compound. A fruitful language can be developed which deals with a buildup of complex quantum chemical features by bringing two holes into vicinity of a super-exchange O, forming a “local” Cu₂O₇ pair. Structural considerations also dictate that stress is relieved by alternate orthogonal pair orientation. This leads to plaid patterns with primary and secondary channels of charge. The presence of these two types of charge channels is involved in the mechanism of superconducting charge transport. Similar structuring of doped charge into plaid patterns of “local” pairs has been proposed for “all” high T _c superconductivity. STM now gives pictorial representation of the remnants of such an electronic crystal structure. The response of these bond-ordering motifs to structural details is further discussed. These ideas supply organization to the manifold experimental situation and provide opportunities for a unifying theory for high T _c superconductivity in terms of real space structuring of “local” pairs, largely on crystal-chemical principles.     

纳米颗粒的超导转变温度

首次给出一组适合声子特点的共轭复量子数算符,由此建立声子的量子算符代数理论,根据该理论得到声子的量子化能量、纳米颗粒的晶格点阵热容和纳米颗粒的超导转变温度的具体表示。结果表明,声子的量子化能量、纳米颗粒的晶格点阵热容和纳米颗粒的超导转变温度均与声子的量子尺寸、状态量子数及时间量子数有关。     

Influence of a Finite Tc on Enhanced Superconductivity Near the Metal/Insulator Transition

We adapted the BCS equation for T _c so that it applies to disordered conductors near the metal/insulator transition (MIT). The resulting expression for T _c as a function of the conductivity accounted quite well for several disparate classes of superconducting materials near the MIT [disordered metals, oxide conductors (including high-T _c superconductors), semiconducting materials, organic conductors, and C₆₀]. This function, however, was applicable only to situations for which T _c was zero in the pure limit—which was the case for the data chosen. This paper extends the model to the nonzero case and compares the predictions to the appropriate data.     

Superconductivity in Transition Metal Doped MoB<Subscript>4</Subscript>

Polycrystalline ingots of MoB₄ have been found to be superconducting near 6 to 8 K when doped with Ti or Nb. Magnetic susceptibility measurements performed on these alloys indicate that a substantial fraction of the sample volume is superconducting below the transition temperature, while X-ray diffraction scans provide insight into the crystallographic effects of the dopants on the layered MoB₄ structure, revealing both a systematic increase of the lattice constant in the direction of the boron planes and a systematic decrease in the inter-plane spacing as the dopant amount is increased. This structural expansion and contraction corresponds to an increase in T _c until the dopant solubility limit is reached. These findings present a stark contrast to the effects of lattice parameter modification on superconductivity in the structurally related AlB₂-type alloys.