J van der Rest

Off-diagonal disorder and energy of formation in transitional alloys

The authors show that in most cases the energy of formation of disordered transitional alloys is mainly determined by the deformation of the electronic bands upon alloying and that the difference in bandwidth between the metals must be taken into account together with the difference between their energy levels.

Energy of formation, band structure and local environment effects in transitional binary alloys

The electronic contribution to the energy of formation of transitional alloys AcB1-c is determined from their band structure. Preliminary numerical results are presented for nickel and iron paramagnetic alloys and it is shown that the variation of the energy of formation with concentration is not very sensitive to the detailed local environment, but is mainly determined by the average surrounding of A and B atoms.

Local environment and magnetic properties in transitional binary alloys. I. (theory)

A theory is presented for the study of the local environment effects on the magnetic properties in transitional binary alloys; the local environment effects are taken into account for the calculation of the one electron properties by an extension of the cpa and the electron-electron interactions are taken into account in the random phase approximation.

Charge transfer and ordering energy in a model binary alloy

Intra- and inter-atomic Coulomb interactions have been accounted for in the band theory of the order-disorder transition in binary alloys. The variation of the energy levels of the two kinds of atoms on the two sublattices is determined self-consistently using an extension of the coherent potential approximation (CPA). As an application, the authors investigate the role of the charge transfer in the order-disorder transition of CsCl type structure transitional alloys such as VMn, TiFe and ScCo.

Local environment effects on the electronic structure of disordered alloys

An approximate selfconsistent scheme generalizing the single site CPA is derived in order to account for the effect of local environment on electronic and magnetic properties of substitutional alloys.

Local environment and magnetic properties in transitional binary alloys. II. (numerical results)

Numerical results are presented from the study of local environment effects on the magnetic properties of atoms in simple models corresponding to the three typical alloys rh-ni, rh-pd and cu-ni. It is shown that (i) the dependence of the susceptibilities upon concentration and environment, (ii) the importance of the selfconsistent calculation of the charge transfers and the energy levels, (iii) the connection between the relative position of the energy levels and the relative magnitude of the susceptibilities, and (iv) that the susceptibility of the central atom is mainly determined by the electron-electron interactions on the central atom for rh-ni and rh-pd but that in cu-ni atoms further than the first neighbours need to be considered.

Order-disorder transition in a model binary alloy

The authors discuss the connection between the usual Bragg-Williams theory and some recent extension of the coherent potential approximation derived to deal with the order-disorder transition in alloys.

Density of states and superconductivity of vanadium-based alloys

The authors have computed the electron density of states of solid solutions of vanadium-based transition-metal alloys V90X10 using the tight-binding recursion method for degenerate d bands in order to calculate the alloy superconducting transition temperature with the McMillan formula. As observed experimentally, for X on the left-hand side of V in the Periodic Table one obtains an increase of the critical temperature Tc while for X on the right-hand side of V Tc decreases. A detailed comparison with experiment indicates that one cannot neglect the variation of the electron-phonon interactions when the bandwidths of the two constituents are different. Another important conclusion is that for alloys in the split-band limit, such as VAu, VPd and VPt, agreement with the experimental data can be obtained only by assuming that these alloys have a short-range order favouring clusters of pure vanadium.

Theory of binary alloys of different constituent bandwidths

A selfconsistent single site effective Hamiltonian is derived within the multiple scattering theory to describe a disordered tight binding binary alloy for which both constituent bandwidths and site potentials are different. The total density of states and the constituent partial density of states are calculated and discussed. It is shown that the multiple scattering approach provides a suitable formalism for investigating problems such as (i) the extension of the Koster-Slater formula when impurity and host have different bandwidths, (ii) the appearance of a satellite-peak structure in the broader bandwidth component density of states when the difference in bandwidths is great enough, (iii) the generalization of the theory to a two band model relevant for transition and noble metal alloys.

Random transfer integrals in disordered alloys

The Koster-Slater formula expressing the change in the density of states introduced by the presence of localized impurities in metals is generalized to the case of random transfer integrals. It is then shown that the single site approximation first introduced by Shiba (1971) to deal with off diagonal randomness in binary alloys is consistent with this generalization and gives correctly the density of states in the dilute limit. The theory is discussed in the locator formulation and possible extensions to the calculation of transport properties are suggested. Numerical examples are presented to discuss the nature of impurity states when both diagonal and nondiagonal disorder are present.