R M Singru

Positron annihilation in defect KCl crystals

Positron lifetime spectra in pure KCl crystals as well as in crystals with cation vacancies, anion vacancies and color centers clearly show three components. The results show that defects play an important role in the annihilation.

A study of the Fermi surfaces of lithium and disordered lithium-magnesium alloys: theory and experiment

The authors present a theoretical and experimental study of the Fermi surfaces (FSs) of Li and the disordered alloys Li1-xMgx (0.6>or=x>0.0) in the BCC phase. Theoretical calculations employ the first-principles fully charge-self-consistent Korringa-Kohn-Rostoker coherent-potential-approximation scheme within the local-density approximation. The experiments involve the two-dimensional angular correlation of positron annihilation radiation measurements on four Li1-xMgx single-crystal specimens with x=0.0, 0.28, 0.40 and 0.60. Good overall agreement is found between experiment and theory with regard to the size and shape of the FS as x is increased from 0.0 to 0.6, although some discrepancies are noted. The question of the critical Mg concentration x=xc at which the FS first makes contact with the Brillouin zone boundary in Li1-xMgx alloys is considered in some detail.

Electronic structure of disordered Nb - Mo alloys studied using the charge-self-consistent Korringa - Kohn - Rostoker coherent potential approximation

We present an all-electron fully charge-self-consistent Korringa - Kohn - Rostoker coherent potential approximation (KKR-CPA) study of the electronic structures of disordered bcc alloys over the entire composition range. Specific computations are reported for x = 0.0, 0.25, 0.50, 0.75, and 1.0. Extensive comparisons with the predictions of the Nb-based rigid-band model (RBM) and other theoretical results, as well as with the relevant experimental results insofar as they are available are made. The particular issues that we focus on concern the evolution of the Fermi surface (FS), and the changes in the density of states and the superconducting transition temperature of Nb with increasing Mo content. The N- and H-centred FS sheets of Nb are found to shrink essentially rigidly, but the -centred sheets evolve in a highly non-rigid-band manner. The x = 0.25 Mo alloy displays an especially large disorder-induced smearing of the -centred FS sheets. Direct experimental information concerning the FS is available only for the N-centred sheet via positron annihilation and in this regard our results are in accord with the measurements. Concerning superconductivity, we have computed the Hopfield parameters and for the Nb and Mo sites, and used the results to obtain for via the McMillan formula. We find that this simple scheme describes the observed composition dependence of in reasonably well.

The electronic structure, and magnetic and structural properties of Fe - Cu and Fe - Ag alloys

We present the electronic structure, magnetic properties, and structural properties of metastable and alloys over the whole composition range. Although in the equilibrium state (Fe, Cu) and (Fe, Ag) do not form solid solutions, recent experiments show that by using special techniques, continuous and homogeneous solid solutions of (Fe, Cu) and (Fe, Ag) can be formed over the whole concentration range. In the present work, we have used the tight-binding linear-muffin-tin-orbital method with the coherent-potential approximation to calculate the electronic structure and magnetic moment of these alloys. Our calculation of the density of states shows two very clear peaks due to Fe and Cu (Ag), in agreement with the experimental x-ray photoemission spectroscopy results. Also our calculated values of the magnetic moments for different compositions of the alloys are found to be in good agreement with the experimental results. Our calculation shows that an Fe atom in an Fe - Cu alloy has a lower magnetic moment than an Fe atom in an Fe - Ag alloy for the same composition. We have also calculated the heat of formation of these alloys, which we always find to be positive, implying the non-existence of stable phases of these alloys under equilibrium conditions.

Positron lifetimes in some ionic compounds

Abstract Positron lifetime measurements in some ionic compounds like sulphates and hydroxides are reported. The results show that positrons annihilate in these compounds with two lifetimes: τ 1 ≈ 0.3 nsec and τ 2 ≈ 1.5 nsec. The data seem to support the possibility of positronium formation in these compounds.

Electron momentum distribution in palladium

Abstract Results of a band structure calculation of the electron momentum distribution (EMD) for positron annihilation and Compton scattering in Pd are reported. Significant anisotropy in the EMD for 〈100〉, 〈110〉 and 〈111〉 direction is indicated.

X-ray form factors and Compton profiles for some solids derived using an atom-in-jellium-vacancy model

X-ray form factors f(k), Compton profiles J(q) and momentum expectation values (pn) are calculated for Mg,Al,Si,Ge and Ag using the local-density approximation with the spherical atom-in-jellium-vacancy model. These theoretical results show satisfactory agreement with experiment, thus indicating the success of the present method for these solids in the position (r) as well as the momentum (p) space.

Transformation from metallic electron charge density to electron momentum density

There is no direct and exact relation between the electron charge density, rho (r), and electron momentum density, gamma (p). Two approximate methods for transforming from rho (r) to gamma (p) developed in the literature are briefly outlined, and these two methods are applied to metallic Al, V, and Cu. The results are compared with each other and with experiment.

Calculation of electronic properties of some solids using the non-local density approximation

The fully self-consistent implementation of the non-local density functional as suggested by Langreth and Mehl (see Phys. Rev. B, vol.28, p.1809, 1983) has been used in conjunction with the atom-in-jellium model to calculate some electronic properties of metallic Li Be, Mg and Al and solid Si. It is found that the non-local density approximation of Langreth and Mehl, when compared with the local density approximation, shows an improvement in the orbital eigenvalues and total energies.

Effect of different density functionals on the core-level binding energies in some metals

Core level binding energies in metallic Mg, Al, Cd, In, Sn, Sb and Te have been calculated for various popular exchange correlation potentials. Calculations have been performed within the atom-in-jellium-vacancy mode using Slaters transition state theory. Theoretical results have been compared with the experiments.