Electronic structure of nin-isocoric transition metal alloys: CuxRh1-x

The electronic structure of the copper—rhodium alloys is determined in the frame of recently developed LCAO-based theory which allows to treat also non-isocoric alloys with strong off-diagonal disorder exactly within the single-site approximation. The total and component densities of states are calculated and compared with available experimental data.     

Spin-polarized tunneling through a thin-film

The effect of spin-disorder scattering on perpendicular transport in a magnetic monolayer is considered within the single-site Coherent Potential Approximation (CPA). The exchange interaction between a conduction electron and localized moment of the magnetic ion is treated with the use of the s–f model. Electron-spin polarization is evaluated in the tunnel current which comes from the different densities of spin-up, spin-down conduction electrons at the Fermi level in a ferromagnetic semiconductor (EuS). Calculated results are compared with some tunneling experiments.     

Real space renormalization of a mixed valence hamiltonian on a bethe lattice

A mixed valence system may be described by two hybridized bands: a narrow one with strong Coulomb interaction and a conduction band. We study this model by means of a decimation method originally developed for a linear chain and recently extended to a Bethe lattice. Different values of the hybridization are compared. The densities of states so obtained exhibit a more detailed structure than equivalent CPA calculations.     

Analytic evaluation of resonating valence bond states

Expectation values of the one-particle density matrix, the Cooper-pair amplitude and the Hubbard interaction are calculated analytically within locally correlated superconducting wave functions. For weak correlations or small densities, a rapidly converging series-expansion of exact expectation values is derived, while for strong correlations a sum-rule conserving single-site approximation is developed. Explicit results for the kinetic energy of holes within resonating valence bond states are compared with corresponding Monte Carlo data. The characteristic difference between wave functions with fixed and variable particle number, respectively, is pointed out.     

Electron correlations and many-body techniques in magnetism

Recent progress in the theory of magnetism and electron correlations is reviewed to clarify the theories developed in the last decade and their mutual relations. A historical development of the theory of magnetism is outlined, and the dynamical coherent potential approximation (CPA) which completely takes account of the dynamical spin and charge fluctuations within the single-site approximation is introduced. Both the dynamical effects on various magnetic properties and the many-body band structure are shown to be explained on the same footing. It is shown that the dynamical CPA is equivalent to the other single-site theories of strongly correlated electrons: the many-body CPA, the dynamical mean-field theory (DMFT), and the projection operator method CPA (PM-CPA). These theories are elucidated with use of a common concept of effective medium or coherent potential. The effects of orbital degeneracy and the realistic calculation scheme are discussed with an emphasis on Hund’s rule coupling. Non-local theories of magnetism and electron correlations which go beyond the single-site approximation are presented. They include the molecular dynamics approach to the magnetic short range order, the dynamical cluster methods as a direct extension of the DMFT, and the self-consistent projection operator approach as an extension of the PM-CPA with use of the incremental cluster expansion. The current problems of their approaches and their future perspective are discussed.     

无序合金中相干势近似的另一种推导——紧束缚模型

本文用Matsubara和Toyozawa的locator展开方法重新推导单粒子和双粒子格林函数的相干势近似(CPA)方程,文中不采用对累积量做自洽处理的方法来考虑原子之间的体积排斥效应,而采用Yonezawa早先提出的一种图形法,本文的重点是提出一个可以用来生成单格点图形自洽方程的系统方法,并用在处理双粒子平均格林函数上,这种处理方法可以推广到处理包括液态金属或非晶态金属等具有短程序系统电导率的CPA方程。 关键词：     

The single-site approximation for disordered canonical d-band metal alloys

The LCAO theory of substitutionally disordered transition metal alloys, which allows to treat the different bandshapes and naturally leads to a matrix generalization of the concept of the multiplicative off-diagonal randomness, is developed within the single-site approximation. The novel expression for the full averaged resolvent is obtained, and applied to the evaluation of the density of states and the spectral density for the silver-palladium alloys. The results quantitatively agree with the recent KKR-CPA data.     

Magnetic properties and a combined recursive-CPA calculation of pseudobinary intermetallics: Application to Hf(FeAl)2

The pseudobinary compounds Hf(Fe_1−xAl_x)₂, 0.15×0.30, show a pure C14 phase. Since the structure is stabilized in the presence of impurities, a combined recursive method — extended CPA is applied. The local densities of states for C14 are obtained within the recursive method and then the CPA describes the disorder. The changes with x of the local densities, and Fe magnetic moment are estimated.     

Intrinsic normal-ordered vertex operators from the multiloop N-tachyon amplitude

We construct vertex operators for arbitrary mass level states of the closed bosonic string. Starting from a generalization of the Koba-Nielsen amplitude which is suitable for an arbitrary genus Riemann surface, we read the vertex operators from the residues of the poles for the intermediate states. Since the original expression is metric independent and normal ordered without the need of inventing any regularization scheme, our vertex operators also possess these properties. We discuss their general features.     

Electronic density of states of binary alloys within thev-cluster-approximation

The properties of the 1-cluster-approximation (1 CA) as an extension of the CPA which preserves translational invariance are investigated. The electronic density of states of one-dimensional disorderedAB-alloys is calculated analytically in the limit of low concentrations within the 1 CA; for arbitrary concentrations the electronic densities of states of one-dimensional and of three-dimensional simple cubicAB-alloys are evaluated numerically. In the impurity band region the 1 CA shows more structure than the CPA even in the low-concentration-limit in agreement with the exact calculations for finite crystals. The 1CA-self-energy, however, is not analytic; in particular it has a branchpoint of order 2 off the real energy axis.Work performed within the research program of the Sonderforschungsbereich 125 Aachen-Jülich-Köln     

Spin fluctuation theory of ferromagnetic metals

Ferromagnetic metals at finite temperatures are discussed using the functional integral method along the lines of the unifield spin fluctuation theory of Moriya and Takahashi. We develop a simple approach which makes use of only the density of states but still takes account of the nonlocal nature of the spin fluctuations with the aid of a single-site coherent potential approximation. The effect of the charge density fluctuation is also taken into account within the saddle point approximation. The results of numerical calculations for various model densities of states including those for bcc and fcc d-metals are presented. Particularly, the Curie temperatures and the Curie-Weiss magnetic susceptibilities for Fe, Co and Ni seem to be fairly well reproduced.     

界面扩散对超晶格电子结构的影响

本文用CPA方法研究了界面扩散对超晶格电子结构的影响。我们看到,无论是扩散区还是无扩散区,所对应的电子局部态密度都要受到这种原子互扩散过程的影响。 关键词：     

无序对金属超晶格电子结构的影响——CPA方法

本文用一种简单的哈密顿模型,在紧束缚近似下,只计及对角矩阵元的无规分布,用CPA方法自洽地计算了无序所引起的金属超晶格电子结构的变化,给出了某些参数下局部态密度和自能曲线。 关键词：     

Can the Pruned-Enriched Method be Used for the Simulation of Fluids?

The flat histogram version of pruned and enriched Rosenbluth method (FLATPERM) is an effective Monte Carlo method for calculating densities of states of polymers on a lattice. In this paper we generalize this method to calculate the densities of states of off-lattice systems. To demonstrate the feasibility of the approach, we perform sample calculations for the Lennard-Jones fluids. The densities of states of Lennard-Jones fluids simulated by Pruned-enriched method, i.e., the generalization of FLATPERM, agree with the densities simulated by Wang-Landau method in the range of high potential energy. However the direct extension of FLATPERM fails at low energy and a useful extension still needs to be found.     

Magnetic properties of ZrZn2 calculated by the functional integral method

The functional integral method based upon the Stratonovitch-Hubbard transformation is used to calculate the temperature dependence of magnetization, amplitude of the local moment, paramagnetic susceptibility and specific heat of the weak itinerant ferromagnet ZrZn₂. Calculations are performed within the static approximation, using the alloy-analogy approximation and single-site CPA.     

Rigorous mean-field model for coherent-potential approximation: Anderson model with free random variables

A model of a randomly disordered system with site-diagonal random energy fluctuations is introduced. It is an extension of Wegner'sn-orbital model to arbitrary eigenvalue distribution in the electronic level space. The new feature is that the random energy values are not assumed to be independent at different sites, but free. Freeness of random variables is an analog of the concept of independence for noncommuting random operators. A possible realization is the ensemble of randomly rotated matrices at different lattice sites. The one- and two-particle Green functions of the proposed Hamiltonian are calculated exactly. The eigenstates are extended and the conductivity is novanishing everywhere inside the band. The long-range behavior and the zero-frequency limit of the two-particle Green function are universal with respect to the eigenvalue distribution in the electronic level space. The solutions solve the CPA equation for the one- and two-particle Green function of the corresponding Anderson model. Thus our (multisite) model is a rigorous mean-field model for the (single-site) CPA. We show how the Lloyd model is included in our model and treat various kinds of noises.     

ELECTRON STATES IN SIMPLE METALLIC COMPOSITIONALLY MODULATED STRUCTURE

We present a generalization of the coherent potential approximation (CPA) in order to make it applicable to simple metallic compositionally umdulated structure. The medium Green's function and the coherent potential in the CPA are calculated by using the infinite order perturbation theory and the CPA self-consistent equation.     

Jensen-Tsallis divergence and atomic dissimilarity for ionized systems in conjugated spaces

A generalization of the original Jensen-Shannon divergence (JSD) is presented in this work, which gives rise to a non-extensive one-parameter divergence providing a powerful dissimilarity measure between electronic distributions. The analysis performed in this study employs the JTD measure to compare one-particle densities of neutral and ionized atomic systems, that generalizes and improves some previous results based on other measures of divergence. Such an improvement mainly arises from the capability of JTD to modify, by means of its order parameter, the relative contribution of specific relevant regions of the atomic densities under comparison, in both position and momentum spaces. Relevant information of the ionization processes attending to structural pattern and periodicity is found, as well as the strong correlation between extremal values of the neutral-cation JTD and those of the atomic ionization potential. Similar conclusions are obtained from processes involving an anion. The analysis includes a study of the dependence of JTD on its order for fixed atomic couples.     

Lack of Hohenberg-Kohn theorem for excited states

For a given excited state there exist densities that arise from more than one external potential. This is due to a qualitatively different energy-density relationship from that of the ground state and is related to positive eigenvalues in the nonlocal susceptibility for excited states. Resulting problems with the generalization of the density functional methodology to excited states are discussed.     

Congruences and States on Pseudoeffect Algebras

We study congruences on pseudoeffect algebras, which were recently introduced as a non-commutative generalization of effect algebras. We introduce ideals for these algebras and give a sufficient condition for an ideal to determine a congruence. Furthermore, states on pseudoeffect algebras are considered. It is shown that any interval pseudoeffect algebra maps homomorphically into an effect algebra whose states are in a one-to-one correspondence to the states of the original algebra.