Voicu Popescu

Magnetic moments, exchange coupling, and crossover temperatures of Co clusters on Pt(111) and Au(111)

The influence of cluster size and of cluster–substrate interaction on the magnetic properties of Co clusters of 1–10 atoms on Pt(111) and Au(111) is studied by fully relativistic ab initio calculations. The focus is on systematic trends of the spin and orbital magnetic moments, the exchange coupling, and the crossover temperature. The spin magnetic moments of Co clusters are larger for the Pt substrate than for the Au substrate, while the reverse is true for the orbital magnetic moments. The local magnetic moments of Co atoms generally increase if the number of Co neighbours decreases. The exchange coupling constants Jij depend on the cluster size and on the location of respective atoms. The crossover temperature increases monotonically with cluster size and is larger for clusters on Pt than for clusters on Au.

Strain-Induced Localized States Within the Matrix Continuum of Self-Assembled Quantum Dots

Quantum dot-based infrared detectors often involve transitions from confined states of the dot to states above the minimum of the conduction band continuum of the matrix. We discuss the existence of two types of resonant states within this continuum in self-assembled dots: (i) virtual bound states, which characterize square wells even without strain and (ii) strain-induced localized states. The latter emerge due to the appearance of “potential wings” near the dot, related to the curvature of the dots. While states (i) do couple to the continuum, states (ii) are sheltered by the wings, giving rise to sharp absorption peaks.

The influence of spin–orbit coupling on the band gap of Heusler alloys

The band gap of half-metallic ferromagnets can be affected by the spin–orbit coupling, which introduces there a small, but non-vanishing, density of states. We study this effect in the case of Heusler alloys. We find that, as a rule, the spin polarization in the middle of the gap decreases for compounds of heavier elements.

Magnetic Dichroism in Electron Spectroscopy

A simple scheme that is based on the Green’s function formalism is introduced, that allows to investigate spin-orbit-induced properties in magnetic solids in a very transparent way. This is demonstrated by investigations on the orbital magnetic moments, on the Fano effect in angle-integrated valence band photoemission and on magnetic circular dichroism in X-ray absorption. Numerical results obtained by additional fully relativistic calculations are presented to support the various conclusions drawn from the analytical considerations. This applies in particular to the interpretation of the magnetic circular dichroism in X-ray absorption (MCXD) on the basis of the so-called sum rules, which relate the dichroic spectra to the spin and orbital magnetic moments of the absorbing atom.

Three-dimensional assemblies of semiconductor quantum dots in a wide-gap matrix providing an intermediate band for absorption

We consider a self-assembled quantum dot (QD) system consisting of the QD itself, the wetting layer and the matrix on a substrate. The electronic structure for various III-V material combinations was determined by atomistic empirical pseudopotential calculations. Taking the widely investigated InAs/GaAs/GaAs(001) system as benchmark, we analyze the changes induced in the energy levels and offsets relevant for a QD-based intermediate band solar cell (IBSC). We explore the effects of (i) the dot material, (ii) the matrix material, and (iii) dot-matrix-substrate combinations that may enable strain balanced structures. Using as unique reference criterion the relative position of the intermediate band inside the band gap of the matrix, we suggest the dot/matrix/substrate combinations InAs/(In,Ga)P/GaAs(001), In(As,Sb)/GaAs/InP(001), and InAs/Ga(As,Sb)/InP(001) as promising candidates for QD-IBSCs.

Fully relativistic theory for the magnetic extended x-ray absorption fine structure

A fully relativistic theoretical description of the magnetic extended x-ray absorption fine structure (MEXAFS) is presented that has been set up within the framework of relativistic multiple scattering theory on the basis of the Dirac equation for spin-polarized, magnetic solids. The multiple scattering processes in the final state are treated using the scattering path expansion technique for finite atomic clusters. Results of the approach for the Fe K and the Gd L edges are presented and are compared with experiment. An analysis of the theoretical spectra clearly shows that the MEXAFS spectra can be interpreted on the basis of the so-called sum rules in a way completely analogous to the case of near-edge x-ray magnetic circular dichroism. This means that the K- and L1-edge spectra directly reflect the orbital polarization of the final states with p character. The L2,3-edge spectra, on the other hand, give, after suitable superposition, access to the spin and orbital polarization of the d states.

Ab initio calculations of magnetoresistance and interlayer exchange constants of the ferromagnet–semiconductor layered systems Fe/Ge

We report on the results of calculations of the electronic, magnetic, and transport properties of Fe/Ge multilayer systems that are meant to modelferromagnetic (FM)/semiconductor (SC)/FM trilayer systems. On the base of the generalized Bloch theorem we have calculated the electronic structure of a set of spin-spiral structures, which allows us to extract averaged interlayer exchange parameters and their dependence on the width of the semiconductor spacer. The dependence of the dc conductivity and magnetoresistance on the width of the semiconductor layer and the relative angle between the moments of adjacent FM layers has also been investigated. The transport properties are studied on the basis of the Drude model.

Magnetic EXAFS at the K- and L2,3-edges of FCC-Ni☆

Abstract Results of a previously developed fully relativistic theory of magnetic EXAFS (MEXAFS), that has been set up in the framework of relativistic multiple scattering theory, are presented. The multiple scattering process in the final states is treated using matrix inversion technique for finite clusters. The K- and L 2,3 -edge spectra of FCC-Ni are calculated on the basis of the Dirac equation for spin-polarized, magnetic solids and compared with experiment. As for the magnetic circular X-ray dichroism in the near-edge region, it is shown that the magnetic EXAFS spectra can be interpreted on the basis of the so-called sum rules.

Native defects in the Co(2)TiZ (Z = Si, Ge, Sn) full Heusler alloys: Formation and influence on the thermoelectric properties

We have performed first-principles investigations on the native defects in the half-metallic, ferromagnetic full Heusler alloys Co(2)TiZ (Z one of the group IV elements Si, Ge, Sn), determining their formation energies and how they influence the transport properties. We find that the Co vacancies (Vc(Co)) and the Ti-Sn as well as the Co-Z or Co-Ti antisites exhibit the smallest formation energies. The most abundant native defects were modeled as dilute alloys, treated with the coherent potential approximation in combination with the multiple-scattering theory Green function approach. The self-consistent potentials determined this way were used to calculate the residual resistivity via the Kubo-Greenwood formula and, based on its energy dependence, the Seebeck coefficient of the systems. The latter is shown to depend significantly on the type of defect, leading to variations that are related to subtle, spin-orbit coupling induced changes in the electronic structure above the half-metallic gap. Two of the systems, VcCo and Co Z, are found to exhibit a negative Seebeck coefficient. This observation, together with their low formation energy, offers an explanation for the experimentally observed negative Seebeck coefficient of the Co2TiZ compounds as being due to unintentionally created native defects.

Interplay of growth mode and thermally induced spin accumulation in epitaxialAl/Co2TiSi/AlandAl/Co2TiGe/Alcontacts

The feasibility of thermally driven spin injectors built from half-metallic Heusler alloys inserted between aluminum leads was investigated by means of {\em ab initio} calculations of the thermodynamic equilibrium and electronic transport. We have focused on two main issues and found that: (i) the interface between Al and the closely lattice-matched Heusler alloys of type Co