The influence of short-range order on electronic properties of transition metals: II. Amorphous metals

Local electronic properties have been calculated for the amorphous transition metals Nb, Mo, and Tc using a cluster model which accounts for the topology of the local structure. Trends in the density of states observed in metallic glasses are reproduced, and the behaviour of the averaged electron-phonon coupling strength correlates well with the behaviour of the superconducting transition temperature. The ratios of the critical temperatures in the amorphous and the crystalline state are reproduced correctly. The results show that the change in the critical temperature on going from the crystalline to the amorphous state can be explained by the change in short-range order.     

The influence of short-range order on electronic properties of transition metals: I. Crystalline systems

Densities of states and electronic parameters related to superconductivity have been calculated for the transition metals Nb and Tc, representing the bulk materials by a finite cluster of atoms. The Schrödinger equation is solved for atomic potentials which have been obtained in a self-consistent band structure calculation. The results show that, in these systems, the electronic structure is determind predominantly by short-range order.     

The influence of short-range order on electronic properties of transition metals: I. Crystalline systems

Densities of states and electronic parameters related to superconductivity have been calculated for the transition metals Nb and Tc, representing the bulk materials by a finite cluster of atoms. The Schrödinger equation is solved for atomic potentials which have been obtained in a self-consistent band structure calculation. The results show that, in these systems, the electronic structure is determind predominantly by short-range order.     

Effect of short-range order on electronic and magnetic properties of disordered Co-based alloys

We here study electronic structure and magnetic properties of disordered CoPd and CoPt alloys using augmented space recursion technique coupled with the tight-binding linearized muffin tin orbital (TB-LMTO) method. Effect of short-range ordering present in disordered phase of alloys on electronic and magnetic properties has been discussed. We present results for magnetic moments, Curie temperatures and electronic band energies with varying degrees of short-range order for different concentrations of Co and try to understand and compare the magnetic properties and ordering phenomena in these systems.     

The influence of the short-range ferromagnetic order on the itinerant-electron antiferromagnetism

The short-range spin—spin impurities interaction in the itinerant-electron antiferromagnet is considered in a model with quenched impurities. In the mean field approximation the phase diagram is calculated.     

Amorphous germanium II. Structural properties

The coherent X-ray scattering for momentum transfer, k, between 0·025 and 15·0 Å^?1 has been measured for a series of sputtered amorphous Ge films prepared at various substrate temperatures, T _s, between 0 and 350°C. Differences in the radial distribution function (RDF) of films of different T _s have been determined by an accurate differential scattering technique. The small angle scattering (SAS) of the films is less than 100 electron units for k < 1 Å^?1. From a combination of SAS, RDF and scanning electron microscope studies, it is concluded that an observed increase in film density with increasing T _s occurs through a reduction in the number of voids about 7 Å or less in diameter. No variation of bond length with T _s is found. With increasing T _s, there is an increase in first and second-neighbour coordination and a reduction in bond angle distortion.

The rate of change of coordination, C, with density, ρ₀, is found to be d ln C/d ln ρ₀ = 0·6±0·2. Using a new, general theory of the dependence of the RDF on the dihedral angle distribution, P(θ), it is shown that with increasing T _s there is an increased probability of dihedral angles corresponding to the staggered configuration. For all films, the experimental RDF between r = 4·5 and 6·2 Å agrees with a nearly random P(θ) distribution. Comparison of experimental RDF's of crystalline and amorphous Ge indicates the static distortion of the first-neighbour bond length has a standard deviation of only about 0·04 Å.     

Peculiarity of interrelation between electronic and magnetic properties of HTSC cuprates associated with short-range antiferromagnetic order

We report on the results of measurements of anisotropic resistivity of RBa₂Cu₃O_{6 + x} (R = Tm, Lu) high-temperature superconducting single crystals in a wide range of doping levels, indicating a nontrivial effect of magnetic order on the electronic properties of cuprates. In particular, our results visually demonstrate the crossover from the state with moderate anisotropy of resistivity ρ _c /ρ _ab ～ 30 to a strongly anisotropic state with ρ _c /ρ _ab ～ 7 × 10³ upon cooling as well as upon a decrease in the hole concentration in the CuO₂ planes. It is also shown that anisotropy is sensitive to the magnetic state of CuO₂ planes and attains its maximum value after the establishment of the long-range antiferromagnetic order. The results are discussed in the framework of the theory based on the t-t′-t″-J model of CuO₂ layers taking into account strong electron correlations and short-range magnetic order. In this theory, anomalies of spin correlators and Fermi surface topology for a critical hole concentration of p* ≈ 0.24 are demonstrated. The concentration dependence of the charge carrier energy indicates partial suppression of energy due to the emergence of a pseudogap at p < p*. This theory explains both the experimentally observed sensitivity of anisotropy in conductivity to the establishment of the antiferromagnetic order and the absence of anomalies in the temperature dependence of resistivity ρ _ab (T) in the vicinity of the Néel temperature.     

New theory of binary alloys with short-range order properties

A new method for treating the electronic structure of binary alloys is presented. It is based on the study of a finite size cluster connected at its edges to a Bethe-lattice of the same coordination number. An illustrative example is presented. It includes concentration sequences which are (i) random, (ii) with a tendency to segregation and (iii) with a tendancy to form binary compounds. Energy gaps and localized states appear naturally in the method.     

Features of the electronic spectrum of alloys with electronic topological transition and their influence on physical properties

Occurrence of a discrete spectrum of electrons of small groups as result of the electronic topological transition in Mo-Re alloys is expected on the basis of the experimental results. Such a spectrum arises against the background of the continuous spectrum of electrons of large groups and corresponds to partial localization of electrons.     

The electronic structure of bcc-based random solid solutions of transition metals

The electronic structure of Nb _x Zr_1–x , V _x Nb_1–x and Mo _x V_1–x random solid solutions with the bcc lattice structure is investigated in the frame of the first-principles tight-binding muffin-tin orbital method and the coherent potential approximation. The total and component densities of states as well as the Bloch spectral densities are determined over a broad concentration range.     

Effect of short-range order on the band-edge behavior of the electronic spectrum of disordered alloys

The electronic spectrum of a one-dimentional disordered alloy was calculated both analytically and numerically for energies close to a band edge. The density of states is shown to be strongly dependent on the presence of short-range order in the form of correlations between the occupancies of neighboring sites.     

On the electronic specific heat of transition metals

A simple framework to calculate the electronic specific heat enhancement is presented and applied to the case of the electron-electron interaction in transition metals. A substantial contribution has been found in iron and niobium.     

Absence of first order electronic transitions in liquid metals

High pressure shock wave data on a wide variety of metals indicates that electronic transitions are continuously distributed in the liquid phase and accompanied by melting maxima. A qualitative explanation for this behavior is suggested.     

Effect of short-range atomic order on macroscopic magnetic properties of equiatomic FePd alloy

X-ray diffraction, magnetic measurements, and Mössbauer spectroscopy were employed to comparatively analyse the relation of the basic characteristics of highly anisotropic materials– coercive force H_c and Curie temperature T_C to the peculiarities of short-range atomic order that forms upon annealing of disordered samples of equaiatomic FePd alloys produced by different techniques (casting, melt-spinning, severe deformation). It is shown that for all samples, independently of methods of their preparation, the ordered states with the maximal values of coercivity are inhomogeneous in the composition of short-ordered regions, type of atomic ordering, and degree of tetragonality. The Curie temperature depends on the temperature and duration of annealing for ordering similarly to the conventional course of coercivity, which is peculiar to these alloys. The behaviour of these macroscopic characteristics (H_c and T_C) in the course of annealing is shown to correlate with changes in the local atomic configurations revealed in the Mössbauer spectroscopy experiments.     

Bonding and short-range order effects in the theory of the electronic structure of liquids

A generalized coherent-wave approximation is suggested which takes into account the fact that the average environment of an atom in many liquids is not spherically symmetric but tends to have a point-group symmetry determined by its nearest neighbours. The effect of this generalization upon the theory of Phariseau and Ziman is discussed.