E. S. Giuliano

Influence of the Coulomb correlation in partially filledd-bands on the electronic spectra of disordered alloys

SummaryThe XVV Auger spectra of Cu−Ni, Zr−Nb, Nb−Mo and Ta−W alloys at different concentrations have been computed by using CPA densities of states and a theory which takes into account the effect of the Coulomb correlation between the two Auger holes in partially filledd-bands. The renormalized one-particle spectra and the Auger spectra are presented as a function of the Coulomb repulsion.RiassuntoUsando densità degli stati elettronici, calcolate mediante CPA, e una teoria che tiene conto degli effetti della correlazione coulombiana in banded non completamente piene, sono stati calcolati gli spettri Auger XVV e lo spettro a una particella rinormalizzato per le leghe disordinate Cu−Ni, Zr−Nb, Nb−Mo e Ta−W, in funzione della repulsione coulombiana.

Electronic properties of nonhomovalent alloys: A study of the b.c.c. Li−Mg solid solution

SummaryThe influence of the alloying on the electronic properties of the b.c.c. Li−Mg solid solution has been investigated within the KKR-CPA framework. Such an influence has been studied by computing the band structure, Fermi surface, soft X-ray spectra, residual resistivity and Auger spectra for chosen values of the composition. The theoretical results we obtained are in fair agreement with the surface existing experimental data and support the view that also in «simple metals» alloys care has to be taken in studying such systems in terms of low-order perturbation approaches.RiassuntoSono state studiate varie proprietà elettroniche delle leghe di Li−Mg nell’intervallo di concentrazione in cui questi sistemi cristallizzano nel reticolo b.c.c. In particolare sono state calcolate la struttura a bande, le superfici di Fermi, la resistività residua, gli spettri X ed Auger e si sono confrontati i risultati con i dati sperimentali esistenti. Il buon accordo fra risultati teorici e dati sperimentali indica che, anche per leghe di metalli semplici, gli approcci perturbativi a basso ordine possono essere insufficienti a descrivere il comportamento di tali sistemi.РезюмеВ рамках KKR-CPA исследуется влияние процесса сплавления на электронные свойства объемноцетририованного кубического твердого раствора Li−Mg. Проводятся вычисления зонной структуры, поверхности Ферми, спектров мягкого рентгеновского излучения, остаточного сопротивления и Ож-ye-спектров для выбранных величин состава. Полученные теоретические величины хорошо согласуются с существуюими экспериментальными данными.

Quasi-particle lifetimes effects on deviations from Vegard’s rule in Ag-Pd disordered alloys

SummaryThe deviations from linearity in the concentration dependence of lattice spacings in AgcPd1-c have been recently related to the topology changes shown by the Fermi Surface, or Electronic Topological Transitions (ETTs), of this alloy. Using an extension of the ETTs theory to finite-temperature and impurity scattering case, and in terms of a simple Rigid Band Model, we study such deviations and interpret them as the results of the filling of the hole pocket centered at theX point of the Brillouin Zone, remarking the role of the finiteness of the quasi-particle lifetime of the relevant states. We also show howab initio calculations based on the Coherent Potential Approximation underestimate these lifetimes.

Pseudostructural disorder in metallic solid solutions: A study of Ag−Cd and Ag−Mg random alloys

SummaryWe have investigated some Ag−Cd and Ag−Mg alloys within the KKR-CPA framework in order to give insights on their electronic properties. Notwithstanding Cd and Mg have different electronic configurations it appears that their effect on the pure Ag is quite the same. In fact measurements on their optical properties and Fermi surface indicate that most of the spectra and Fermi surface parameters are comparable. In the meantime it also appears that they cannot be classified as rigid band alloys. From the results we obtain it is possible to give sensible explanations to this peculiar behaviour in terms of the angularmomentum channels of the coherent scattering amplitude obtained within the KKR-CPA theory, the main conclusion being thats- andp-like states remain essentially free-electron-like, whiled-like states are very much perturbed by the alloying.

Relativistic korringa-kohn-rostoker CPA theory: Extension tol=3 scattering channel

SummaryWe have extended the well-known relativistic KKR-CPA theory up tol≤3 angular momentum, in order to make this framework suitable to deal with rare-earth alloys. However, such an extension has also relevance in judging the convergence of the KKR-CPA calculations even for those atomic species with empty valencef-states. To do that we worked out the transformation from the usual real spherical harmonics to the cubic double-point group representatin. We have tested the present theory and corresponding codes by computing the electronic structure of Cu0.75−Au0.25 and by comparing our results with earlierl≤2 R-KKR-CPA calculations. In order to reduce the large amount of computing time required, we have also used a new technique for the Brillouin zone integration.

Cluster calculations of the electronic density of states in a Cu−Zr metallic glass

SummaryThe electronic density of states in glassy Cu−Zr has been computed by averaging the density of states of a number of clusters of atoms. The atomic positions in the clusters were chosen in such a way to reproduce the pair distribution function of the system. For each cluster the density of states was computed within the multiple-scattering theory by using Lloyds formula. The results obtained are in fair agreement with the experimental dta. The computed density of states shows a clear split-band regime with a distance between the two main peaks comparable with the experimental results.RiassuntoLa densità di stati elettronici nel vetro Cu−Zr è stata calcolata mediando le densità di stati di piú cluster di atomi. Le posizioni degli atomi nei cluster sono state scelte in modo da riprodurre la funzione di distribuzione di coppia del sistema. La densità di stati è stata calcolata per ciascun cluster con la formula di Lloyd nella teoria dello scattering multiplo. I risultati ottenuti sono in buon accordo con i dati sperimentali e mostrano un chiaro regime di split-band, con una distanza fra i due picchi principali confrontabile con i risultati sperimentali.РезюмеВычисляется плотность электронных состояний в стекле Cu−Zr с помощью усреднения плотности состояний ряда кластеров атомов. Положения атомов в кластерах выбираются таким образом, чтобы воспроизвести парную функцию распределения этой системы. Для каждого кластера плотность состояний вычисляется с помощью формулы Ллойда теории многократного рассеяния. Полученные результаты довольно хорошо согласуются с экспериментальными данными. Вычисленная плотность состояний обнаруживает явный режим расщепления зоны, причем расстояние между двумя главными пиками сравнимо с экспериментальными результатами.

Electronic Topological Transitions and Phase Stability in Ag-Pd and Cu-Pt Alloys

Metallic random alloys can display Electronic Topological Transitions, i.e. changes of the Fermi Surface connectivity, on varying the atomic concentrations. We present a study of Ag-Pd and Cu-Pt Fermi Surfaces, based on the LDA-KKR-CPA theory, and argue that the different phase equilibria of such isoelectronic systems are related to the spectacular differences in their Fermi Surfaces.

The Influence of Quasi-Particle Lifetimes and Electronic Topological Transitions on the Deviations from Vegard’s Rule in AG-PD Alloys

In this paper, we discuss about the variations of the equilibrium lattice parameter versus the alloy concentration in AgPd fcc solid solutions. We interpret the experimentally observed deviations from Vegard’s rule in terms of the Electronic Topological Transition (ETT) related to the d band filling which occurs about the equiatomic concentration. Through simple Rigid Band Model (RBM) calculations, we show how the finiteness of quasi-particle lifetimes affects these deviations, namely reducing their amplitude. RBM calculations are also used to compare versus the experiment the predictions of ab initio Local Density Approximation, Korringa-Kohn-Rostoker Coherent Potential Approximation (LDA-KKR-CPA) calculations. On this basis we shall discuss the apparent underestimation of the relevant quasi-particle lifetime in the CPA theory.