J. van der Rest

Density of states, enthalpy of formation and occurrence of the sigma phase in transitional alloys

Abstract We present a simple band model for the computation of the density of states and the enthalpy of formation for transitional alloys in the sigma phase. We apply this model for the computation of phase diagrams of ternary alloys between elements of the first transition row and we show that the good agreement obtained with experiment arises from the particular shape of the enthalpy of formation in the sigma phase, which deviates considerably from the parabolic law obtained in the regular solution approximation.

Influence of long range order on electrical resistivity in alloys

Abstract We have established the CPA expression of electrical conductivity of a one orbital tight binding band model alloy in terms of the long range order parameter. We investigate its variation with temperature from the completely ordered alloy at T = 0 K till the order-disorder transition in terms of band parameters: scattering potential and filling of the band.

Charge states of substitutional S in Si

Abstract We study the electronic structure of the various charge states of a substitutional S impurity in Si in the tight-binding approximation. We take into account the changes in the diagonal elements of the host Hamiltonian on the impurity site and on its nearest neighbors and the changes in the off-diagonal elements between the impurity site and its nearest neighbors. Good agreement is found with experimental results and with more a priori calculations.

Charge states of the sulphur vacancy in ZnS

Abstract We study the various charge states of the undistorted sulphur vacancy in cubic ZnS crystals in the tight-binding approximation. Using Slater transition state theory, we obtain its various excitation energies which are in good agreement with correlated EPR and optical experimental results.

Local environment effect on the superconductivity of transition metal alloys

Abstract We show that for superconductor vanadium based alloys in the split-band limit the calculated critical temperature is in agreement with experimental data if one assumes that these alloys have a short-range order favouring clusters of pure vanadium.

On the Homomorphic Cluster CPA

Among the various extentions of the CPA, the homomorphic cluster CPA (HCPA) was presented recently as the only approximation (i) having the correct analytical properties, (ii) taking multiple scattering into account and (iii) preserving the symmetry of the original lattice. Though very appealing mathematically, we show in this paper that it does not give the right position for the impurity states in the dilute limit with non zero diagonal disorder. Consequently, even if HCPA satisfies points (i) to (iii) and even if it gives the correct dilute limit for purely off-diagonal disorder, it is to be rejected as a method to study systems with non negligible diagonal disorder.

Comparison between the spin polarization energies for VSi− in silicon and VGa0 in gallium phosphide

Abstract We compare the gain in energy upon spin polarization for the undistorted VSi- in silicon and VGa0 in gallium phosphide. We show that this gain in energy is mainly related to the magnitude of the electron-electron interactions on the first neighbors of the vacancy. These interactions being larger for phosphorous atoms than for silicon atoms, the spin polarization energy is larger for VGa0 than for VSi−; this explains why the former is spin polarized while the latter is Jahn-Teller distorted.

On the alloy analogy approximation of the hubbard Hamiltonian

Abstract It is not well established that the ground state of the Hubbard Hamiltonian is not always paramagnetic. It is also well established that the ground state of the alloy analogy approximation of the non-degenerate Hubbard Hamiltonian is always paramagnetic. It has been claimed on the other hand that the ground state of the alloy analogy approximation of the doubly degenerate Hubbard Hamiltonian is not always paramagnetic. We show in this article that this last statement is not fully proved. Contradictory results are in fact obtained, depending upon the physical quantity one computes: total energy calculations have indeed been reported showing a magnetic instability while we prove here that magnetic susceptibility calculations do not. We conclude therefore that the ground state of the alloy analogy approximation of the doubly degenerate Hubbard Hamiltonian is not known at the present time.

Local moment formation in transitional alloys

Abstract Starting from a band picture we compute the magnetic moment of transitional impurities in transitional binary alloys and we study their dependence upon host concentration and upon local environment; we study also the energy difference between the paramagnetic state and the ferromagnetic state of the impurity. Applications to the RhPd(Co) and NbMo(Fe) systems are presented.