Frequency dispersion of permeability in ferrite composite materials

The last decade has seen a rapid progress in understanding the electronic structure of disordered alloys and in this article I shall review the progress made by using the Kohn-Korringa-Rostoker (KKR) based approaches. The KKR approach in conjuction with the coherent-potential approximation (CPA) has emerged as a powerful tool to study the electronic structure of disordered alloys. This method called, KKR- CPA, combined with the local density approximation (LDA) provides a first-principles, parameter-free mean-field theory for treating electronic structure of a disordered alloy. Insofar as the average properties are concerned, the KKR-CPA replaces the disordered system by a suitably chosen translationally invariant effective medium, which is determined self-consistently. To illustrate some of its applications I shall present some recent results for Li-Mg and Nb-Mo alloys. Also our recent attempts to go beyond the KKR-CPA and to include short-range order will be briefly discussed.

Electronic Structure of Disordered Alloys

We present a method for calculating the electronic structure of disordered alloys with shortrange order (SRO) which guarantees a positive density of states for all values of the SRO parameter. The method is based on the generalized augmented-space theorem which is valid for alloys with SRO. This theorem is applied to alloys with SRO in the tight-binding linear-muffin-tin-orbital (TB-LMTO) framework. This is done by using the augmented-space formulation of Mookerjee and the cluster-coherent-potential approximation. As an illustration, the method is applied to a single-band model TB-LMTO Hamiltonian. We find that the SRO can induce substantial changes in the density of states

Generalized augmented-space theorem for correlated disorder and the cluster-coherent-potential approximation.

We present here an application of the augmented-space recursion technique for binary disordered alloys The method allows us to incorporate effects like clustering, short-ranged order and off-diagonal disorder arising out of size mismatch and consequent lattice distortions We base our calculations on the TB-LMTO Hamiltonian of Andersen and co-workers We study three alloy systems: AgPd, CuZn and FeTi, and compare our results with earlier work

Electronic structure of random binary alloys

For many years the Korringa-Kohn-Rostoker coherent-potential approximation (KKR-CPA) has been widely used to describe the electronic structure of disordered systems based upon a first-principles description of the crystal potential. However, as a single-site theory the KKR-CPA is unable to account for important environmental effects such as short-range order (SRO) in alloys and spin fluctuations in magnets, among others. Using the recently devised KKR-NLCPA (where NL stands for nonlocal), we show how to remedy this by presenting explicit calculations for the effects of SRO on the electronic structure of the bcc ${\mathrm{Cu}}_{50}{\mathrm{Zn}}_{50}$ solid solution.

Effects of short-range order on the electronic structure of disordered metallic systems

La theorie combine le formalisme de l'espace ameliore et les methodes conventionnels de Korringa-Kohn-Rostoker pour determiner le milieu effectif de facon auto-coherente. De la sorte, on preserve les proprietes de Herglotz de la fonction de Green a configuration moyenne necessaire pour le calcul des diverses proprietes electroniques. La methode introduit des corrections diagonales et hors diagonales dans les matrices de diffusion. Application a un alliage unidimensionnel

Electronic structure of disordered alloys: Korringa-Kohn-Rostoker cluster coherent-potential approximation.

An s phase shift semicircular model of a random substitutional binary alloy has been used as a test case for studying the self-consistent Korringa-Kohn-Rostoker cluster coherent-potential approximation (KKR-CCPA). In addition to being computationally simpler, the model has the advantage that a corresponding model exists in the well known tight-binding framework. A one-to-one correspondence has been shown between the KKR-CCPA equations and the tight-binding CCPA equations. In the tight-binding framework, it has been shown that certain quantities, which were hitherto calculated approximately, can be obtained exactly by the partitioning technique. Results for density of states in the CCPA show rich structure in the impurity band, which arises due to correlated scattering from clusters of atoms.

S. S. Rajput

Papers

Electronic structure of disordered alloys: Korringa-Kohn-Rostoker cluster coherent-potential approximation.

Application of the Korringa-Kohn-Rostoker cluster coherent-potential approximation to an s phase shift semicircular model