Voicu Popescu

University of Duisburg-Essen | uni-due · Theoretical Physics

Spin caloric transport refers to the coupling of heat with spin transport. Its applications primarily concern the generation of spin currents and control of magnetisation by temperature gradients for information technology, known by the synonym spin caloritronics. Within the framework of ab initio theory, new tools are being developed to provide an...

We have performed first-principles investigations on the native defects in the full Heusler alloys Co$_2$Ti$Z$ ($Z$ one of the group IV elements Si, Ge, Sn), determining their formation energies and how they influence the transport properties. We find that Co vacancies (Vc) in all compounds and the Ti$_\text{Sn}$ anti-site exhibit negative formatio...

Smart susceptors are being developed for use as tooling surfaces in molding machines that use apply electro-magnetic induction heating to mold and form plastics or metal powders into structural parts, e.g., on aerospace and automotive manufacturing lines. The optimal magnetic materials for the induction heating process should have large magnetizati...

We present results of first-principles calculations on the transport properties, both under an electric field or a temperature gradient, in the Co/Cu multilayered systems. The various effects brought about by the changes in the morphological parameters, such as the number of repeats and the layer thickness, are discussed in a systematic way. Our ca...

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-matche...

The longitudinal thermopower of a Cu/Co/Cu trilayer system exhibits an oscillatory dependence on the thickness of the Co layer, a behavior related to the formation of quantum well states in the minority spin channel. In addition, it is found to be very sensitive to a switching between an in-plane and out-of-plane magnetization. The resulting magnet...

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...

The supercell approach to defects and alloys has circumvented the limitations of those methods that insist on using artificially high symmetry, yet this step usually comes at the cost of abandoning the language of E versus k⃗ band dispersion. Here we describe a computational method that maps the energy eigenvalues obtained from large supercell calc...

The density of states (DOS) at the Fermi level, (EF), for the 123 superconductor doped with iron was computed. The LCAO model hamiltonian and the Haydock recursive method were used. The iron atoms were located only in one of the copper atom positions Cu1 or Cu2 or in both of them with the same probability. The substitution of Fe in Cu1 positions in...

In the present paper a calculation for the surface states of the 3d transition metals is reported. The scattering method is used in the combined LCAO-OPW representation for the wave functions. The basis set, in the presence of perturbation, is formed by tight-binding orbitals and plane waves. For the unreconstructed (100) surface of Ni, the surface...

DOI:https://doi.org/10.1103/PhysRevB.84.119908

Interface states occur in semiconductor heterojunctions whenever a significant perturbation is present, caused by interface defects, lattice mismatch, discontinuities in the effective mass or sharp variations in the potential across the interface. We discuss the natural interface states appearing in perfectly coherent and isovalent III-V heterojunc...

Random substitutional A(x)B(1-x) alloys lack formal translational symmetry and thus cannot be described by the language of band-structure dispersion E(k(→)). Yet, many alloy experiments are interpreted phenomenologically precisely by constructs derived from wave vector k(→), e.g., effective masses or van Hove singularities. Here we use large superc...

Random substitutional alloys have no long range order (LRO) or translational symmetry so rigorously speaking they have no E(k) band structure or manifestations thereof. Yet, many experiments on alloys are interpreted using the language of band theory, e.g. inferring Van Hove singularities, band dispersion and effective masses. Many standard alloy t...

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 strai...

The concepts of (1) Carrier-Multiplication (CM - ¿two electron-hole pairs from one photon¿) and (2) Intermediate Band Solar Cell (IBSC) based on nanostructures, have created significant interest and excitement, but, at the same time, raised questions pertaining to the approaches used to argue their validity. In part, confusion arose because the i...

Quantum dots (QDs) embedded in a matrix exhibit a coexistence of ``zero-dimensional'' (0D) bound electron and hole states on the dot with ``three-dimensional'' (3D) continuum states of the surrounding matrix. In epitaxial QDs one encounters also ``two-dimensional'' (2D) states of a quantum well-like supporting structure (wetting layer). This coexis...

We investigate the nature of hole and electron states in self-assembled InAs/GaAs and (In,Ga)As quantum dots, using a multi-band atomistic pseudopotential approach. We offer a classification of both hole and electron states based on an analysis of their localization both in the z and xy-directions. We show that the coherent dot-matrix strain presen...

Background A meta-analysis of published data was conducted to investigate the overall risks of hypertension and QTc prolongation in patients with advanced non-small cell lung cancer (NSCLC) who were receiving vandetanib. Methods A computerized search through electronic databases, including PubMed and Embase (until Dec 2014), was performed to obtain...

The intermediate-band solar cell (IBSC) concept has been recently proposed to enhance the current gain from the solar spectrum while maintaining a large open-circuit voltage. Its main idea is to introduce a partially occupied intermediate band (IB) between the valence band (VB) and conduction band (CB) of the semiconductor absorber, thereby increas...

It has been recently proposed that the efficiency of photovoltaic solar cells based on wide-gap III-V absorbing materials can be enhanced if quantum dots are embedded in such matrices, leading to confined electron and hole states that can be excited to the band edges of the wider-gap matrix material, thereby capturing the lower energy (IR) solar ph...

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 dichr...

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...

A fully relativistic theoretical description of magnetic EXAFS (MEXAFS) is presented that has been set up in the framework of relativistic multiple-scattering theory on the basis of the Dirac equation for spin-polarised, magnetic solids. This approach is applied to supply a detailed interpretation of new and very accurate experimental data for the...

The magnetic anisotropy of thin Fe films on (001)-oriented GaAs has been investigated using the fully relativistic TB-KKR method of band structure calculation. In line with experimental findings an in-plane anisotropy has been found with the easy axis pointing along the [110]-direction. The theoretical results for the anisotropy energy ΔE are repro...

Using a fully relativistic implementation of the Landauer-Buttiker formalism, the ballistic conductance and magnetoresistance in the Fe/GaAs/Fe tunnel junction has been calculated. The underlying electronic structure of the system was obtained using a spin-polarized relativistic version of the screened Korringa-Kohn-Rostoker Green-function method....

We present a fully relativistic generalization of the Landauer–Büttiker formalism that has been implemented within the framework of the spin-polarized relativistic screened Korringa–Kohn–Rostoker Green function method. This approach, going beyond the two-current model, supplies a more general description of the electronic transport. It is shown tha...

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.

We report results on calculations of the electronic, magnetic and transport properties of Fe/GaAs and Fe/Ge multilayer systems. On the base of the generalized Bloch theorem we have calculated the electronic structure for a set of spin-spiral structures, which allows us to extract averaged interlayer exchange parameters and their dependence on the w...

The spin-orbit interaction can cause a nonvanishing density of states (DOS) within the minority-spin band gap of half metals around the Fermi level. We examine the magnitude of the effect in Heusler alloys, zinc-blende half metals, and diluted magnetic semiconductors, using first-principles calculations. We find that the ratio of spin-down to spin-...

We report on the results of calculations of the electronic, magnetic, and transport properties of Fe/Ge multilayer systems that are meant to model ferromagnetic (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...

Results of ab initio calculations of the magnetic moments of 3d and 4d transition metal impurities in and on the (001) surface of bcc Fe are presented. These investigations were performed using the spin-polarized relativistic version of the Korringa-Kohn-Rostoker Green’s-function method giving access to the spin as well as the orbital magnetic mome...

The magnetic properties of the disordered alloy systems face-centred-cubic (fcc) CoxRh1-x and fcc FexPd1-x have been investigated by means of the spin-polarized relativistic version of the Korringa-Kohn-Rostoker method of band-structure calculation on the basis of the coherent potential approximation alloy theory. In particular, the spin and spin-o...

The magnetic properties of the disordered alloy systems face-centred-cubic (fcc) Co [iopmath latex="$_x$"] x [/iopmath] Rh [iopmath latex="$_{1-x}$"] 1-x [/iopmath] and fcc Fe [iopmath latex="$_x$"] x [/iopmath] Pd [iopmath latex="$_{1-x}$"] 1-x [/iopmath] have been investigated by means of the spin-polarized relativistic version of the Korringa-Ko...

A theoretical description of spin-resolved CVV (core-valence-valence) Auger electron spectroscopy is presented. The approach is derived on the basis of a fully relativistic description of the electronic structure of magnetic materials. As a consequence, spontaneous exchange splitting as well as spin-orbit coupling-two possible and important sources...

The theoretical description of spin-resolved appearance potential spectroscopy (SR-APS) developed recently to deal with bulk transition-metal systems has been extended to free surfaces of Fen/Cu(001). Results of previous experimental work on fcc-Fe on top of a Cu(001) substrate were interpreted to indicate the coexistence of a few ferromagnetically...

Fully relativistic investigations on the magnetic circular dichroism in emission have been performed for Rh in the three alloy systems Co0.75Rh0.25, Co0.75-xRh0.25Nix, and Co0.75Rh0.25-xPdx. The results obtained using the spin-polarized relativistic Korringa-Kohn-Rostoker method of band-structure calculations together with the coherent-potential ap...

A fully relativistic theory for magnetic extended x-ray absorption fine structure is presented that is based on the Dirac equation for spin-polarized systems and relativistic multiple scattering theory. This scheme not only allows one to treat spin polarization and all relativistic effects on the same level but can also be applied to go beyond the...

A fully relativistic scheme is presented that allows one to describe the magnetic Extended X-ray Absorption Fine Structure (MEXAFS) in disordered systems. This method is based on a combination of the relativistic multiple scattering theory and the Coherent Potential Approximation (CPA) alloy theory. The alloy systems FexCo1-x and FexPt1-x are used...

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 L2,3-edge spectra of FCC-Ni are calcu...

We present spin-resolved appearance potential spectroscopy (APS) calculations at the 2p3/2 edge of Fe in the high-moment ferromagnetic phase of fcc Fe/Cu(001) overlayers, based on a single-particle description of the underlying electronic structure. Within the framework of the real-space Korringa-Kohn-Rostoker method, the spectra were calculated in...

The magnetic and structural properties of bulk hcp Gd and of its (0001) surface have been studied using the linear-muffin-tin-orbital method with the atomic-sphere approximation. This method has been used within the framework of the generalised gradient approximation (GGA) of Perdew, Burke and Ernzerhof. Inclusion of the GGA is found to be necessar...

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 usin...

A theoretical description of spin-resolved appearance potential spectroscopy is presented on the basis of a single-particle description of the underlying electronic structure. The final expression for the signal intensity turns out to be essentially a cross-section-weighted self-convolution of the density of states above the Fermi energy, in close...

The electronic structure of the magnetic compound FeNi3 is investigated by spin-resolved soft-x-ray appearance potential spectroscopy. The spectra which reflect the local densities of empty states on the Fe and Ni sites in this compound are compared with results for the pure elements. A detailed description and interpretation of the experimental sp...

New theoretical developments based on a fully relativistic description of magnetic dichroism (MXD) in X-ray absorption spectroscopy are presented. It is demonstrated that it is possible to achieve within this sophisticated framework an unambiguous identification of the role of exchange splitting and spin-orbit coupling for the MXD by means of model...

Magnetic moments and Curie temperatures of the intermetallic compounds LaCu5–xCox (x = 5) are calculated using a recursion method in the framework of the spin-fluctuation theory of Mohn and Wohlfarth.

The magnetic and orbital moments of Co atoms in YCo5−xNix were computed with the Haydock recursion method in a self-consistent procedure. The decrease in magnetic moment per formula unit with increase in Ni concentration is related to the peculiarities of the density of states for x ≠ 0 and to the decrease in exchange splitting. The transition temp...

The densities of states for LaCo5−xNix substitutional compounds were computed using the Haydock recursion method in a self-consistent procedure. The local magnetic moments were calculated for Co sites for which the Stoner criterion for ferromagnetism was found to be fulfilled. The decrease in magnetic moments with increasing Ni concentration is exp...

Using Haydock's recursion method, the density of states was computed for Y2M17 (M ≡ Fe, Co or Ni) intermetallic compounds, for non-ideal structures. A partial disorder was simulated displacing the Y atoms from the 2b to 2c positions and the M atoms from 4f to 4e positions. The magnetic moments, electron occupation number, electronic specific heat c...

The electronic densities of states in Y2Ni17 are calculated for various occupation numbers of the yttrium and nickel positions using the tight-binding d-band model in conjunction with the recursion method and the virtual crystal approximation. Some yttrium atoms are displaced from the 2b to the 2c positions and some nickel atoms from the 4f and 12j...

Interface states can occur in semiconductor heterojunctions whenever a significant perturbation is present across the interface, for example, interface defects, lattice mismatch, change of sign in the effective mass, or sharp variations in the potential. We discuss here a different type of natural interface states appearing in perfectly coherent an...

The magnetic anisotropy of thin $\chem{Fe}$ films on (001)-oriented GaAs has been investigated using the fully relativistic TB-KKR method of band structure calculation. In line with experimental findings an in-plane anisotropy has been found with the easy axis pointing along the [110]-direction. The theoretical results for the anisotropy energy $\D...