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On the basis of detailed first-principles calculations and semiclassical Boltzmann transport, the anisotropic thennoelectric transport properties of Bi2Te3 and Sb2Te3 under strain were investigated. It was found that due to compensation effects of the strain-dependent thermopower and electrical conductivity, the related power factor will decrease under applied in-plane strain for Bi2Te3, while being stable for Sb2Te3. A clear preference for thermoelectric transport under hole doping, as well as for the in-plane transport direction was found for both tellurides. In contrast to the electrical conductivity anisotropy, the anisotropy of the thermopower was almost robust under applied strain. The assumption of an anisotropic relaxation time for Bi2Te3 suggests that already in the single crystalline system strong anisotropic scattering effects should play a role. [ABSTRACT FROM AUTHOR]

On the basis of detailed ab initio studies, the influence of strain on the anisotropy of the transport distribution of the thermoelectrics Bi2Te3 and Sb2Te3 was investigated. Both tellurides were studied in their own as well as in their copartner's lattice structure to gain insight into the electrical transport in epitaxial heterostructures composed of both materials. It is shown that the anisotropy of the transport distribution overestimates the experimental findings for Bi2Te3, implying anisotropic scattering effects. An increase of the in-plane lattice constant leads to an enhancement of the transport anisotropy for p doping, whereas the opposite occurs for it doping. The recent findings and special features of the transport distribution are discussed in detail in relation to the topology of the band structures. [ABSTRACT FROM AUTHOR]

By means of ab initio calculations in conjunction with the random-phase approximation (RPA) within the full-potential linearized augmented plane wave method, we study the screening of the Coulomb interaction in Nbx Co (1 ≤ x ≤ 9) clusters. In addition, these results are compared with pure bcc Nb bulk. We find that for all clusters the on-site Coulomb interaction in RPA is strongly screened, whereas the intersite nonlocal Coulomb interaction is weakly screened and for some clusters it is unscreened or even antiscreened. This is in strong contrast with pure Nb bulk, where the intersite Coulomb interaction is almost completely screened. Furthermore, constrained RPA calculations reveal that the contribution of the Co 3d → 3d channel to the total screening of the Co 3d electrons is small. Moreover, we find that both the on-site and intersite Coulomb interaction parameters decrease in a reasonable approximation linearly with the cluster size and for clusters having more than 20 Nb atoms a transition from 0D to 3D screening is expected to take place. [ABSTRACT FROM AUTHOR]

We report first-principles study of the magnetic structure of Mn3Si. The Mn atoms form two sublattices. One of them is (MnI) characterized by large atomic magnetic moments and the other one (MnII) possesses induced moments. It is shown that in agreement with experiment the magnetic ground state is a spin spiral. We found that the spiral magnetic structure is formed as a result of the competition between direct antiferromagnetic interaction of the MnI magnetic moments and indirect ferromagnetic interaction of the same moments through the magnetic moment of the MnII sublattice. We demonstrate that this competition is strongly volume dependent and leads to volume dependence of the wave vector of the spin spiral. These properties are related to the volume dependence of the induced moments of MnII atoms. [ABSTRACT FROM AUTHOR]

The aim of this article is to present ab initio calculations of interlayer exchange coupling for Fe/Au (001) multilayers with the same spacer thickness as in recently performed experiments. The structure is characterized by a bcc–fcc transition. The calculations are based on density functional theory using a scalar relativistic screened Korringa–Kohn–Rostoker method. The results show that in case of ideally smooth interfaces the differences of the theoretically and experimentally obtained coupling strengths by one order of magnitude remain. It will be demonstrated that the usual explanation by means of a mesoscopic roughness fails. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We studied the structural, electronic, and magnetic properties of a cubic perovskite BaFeO3-δ (0 ≤ δ ≤ 0.5) within the density-functional theory using a generalized gradient approximation (GGA) and a GGA + U method. According to our calculations, BaFeO3 in its stoichiometric cubic structure should be half-metallic and strongly ferromagnetic, with an extremely high Curie temperature (TC) of 700-900 K. However, such an estimate of TC disagrees with all available experiments, which report that TC of the BaFeO3 and undoped BaFeO3-δ films varies between 111 and 235 K, or, alternatively, that no ferromagnetic order was detected there. Fitting the calculated x-ray magnetic circular dichroism spectra to the experimental features seen for BaFeO3, we concluded that good agreement can be obtained when oxygen vacancies are included in our model. Thus, the relatively low TC measured in BaFeO3 can be explained by oxygen vacancies intrinsically present in the material. Since iron species near the O vacancy change their oxidation state from 4+ to 3+, the interaction between Fe4+ and Fe3+, which is antiferromagnetic, weakens the effective magnetic interaction in the system, which is predominantly ferromagnetic. With increasing S in BaFeO3-δ, its TC decreases down to the critical value when the magnetic order becomes antiferromagnetic. Our calculations of the electronic structure of BaFeO3-δ illustrate how the ferromagnetism originates and also how one can keep this cubic perovskite robustly ferromagnetic far above room temperature. [ABSTRACT FROM AUTHOR]

Using surface x-ray diffraction in combination with ab initio calculations we have studied the atomic structure of ultra-thin ZnO films deposited on Fe(l 10). In contrast to expectation that ZnO adopts the "graphitic" hexagonal Boron-nitride structure to the Wurtzite (WZ) structure is observed. Its formation is related to oxygen impurities in Fe(110) hollow sites inducing an anisotropic charge redistribution within the film which is characterized by a metallic surface. Our results provide a deeper understanding of depolarization mechanisms in ultrathin polar films at the atomic scale. [ABSTRACT FROM AUTHOR]

Using a first-principles Green's function approach we study magnetic properties of the magnetic binary tetradymite chalcogenides Bi2Se3, Bi2Te3, and Sb2Te3. The magnetic coupling between transition-metal impurities is long range, extends beyond a quintuple layer, and decreases with increasing number of d electrons per M atom. We find two main mechanisms for the magnetic interaction in these materials: the indirect exchange interaction mediated by free carriers and the indirect interaction between magnetic moments via chalcogen atoms. The calculated Curie temperatures of these systems are in good agreement with available experimental data. Our results provide deep insight into exchange interactions in magnetic binary tetradymite chalcogenides and open a way to design new materials for promising applications. [ABSTRACT FROM AUTHOR]

Magnetoelectric coupling at the interface between PbTi(Zr)O3 (001) (PZT) and a Co film as well as its tuning by Zr substitutions is studied here from first principles. Chemically disordered 25% Zr-doped compositions were modeled within the supercell approach. Our calculations suggest that ferroelectric ionic displacements in PZT can be locally enhanced, especially when the interfacial Ti cations are replaced with Zr. Regarding Co films their structure tends to become hexagonal for thicker films, while rather significant changes in Co magnetization are anticipated in response to the electric polarization reversal in PZT. [ABSTRACT FROM AUTHOR]

We present ab initio electronic structure calculations based on density functional theory for the thermoelectric properties of Bi2Te3 films. Conductivity and thermopower are computed in the diffusive limit of transport based on the Boltzmann equation. Bulk and surface contributions to the transport coefficients are separated by a special projection technique. As a result we show clear signatures of the topological surface state in the thermoelectric properties. [ABSTRACT FROM AUTHOR]

Using surface x-ray diffraction in combination with ab initio calculations, we demonstrate that the atomic structure of ultrathin BaTiO3 (BTO) films grown on Me(001) surfaces (Me=Fe, Pd, Pt) depends on subtle modifications of the interface chemical composition. A complete reversal of the surface termination from a BaO- [BTO on Fe(001)] to a TiO2-terminated film [BTO on Pt(001)] is observed which goes in parallel with the adsorption of submonolayer amounts of oxygen at metal hollow sites of the interface. Our results may suggest a new route to an overall control of both the surface and the interface geometry in BaTiO3/metal contacts. [ABSTRACT FROM AUTHOR]

We provide, by a detailed first-principles investigation, evidence for weak electronic correlations in SrRu03. The magnetism in SrRu03, in terms of the equilibrium magnetization and critical temperature, is well described by the generalized gradient approximation. Including Hubbard-type correlations results in worse agreement with experiment. [ABSTRACT FROM AUTHOR]

In nonmagnetic crystals with inversion symmetry the electronic bands are twofold degenerate. As a consequence, any orthonormalized linear combination of the two corresponding eigenfunctions can represent the electron wave function. A priori it is not obvious which superposition, gauge, should be chosen to calculate a quantity which is not gauge invariant within a certain approximation. Here we consider gauge options appropriate under particular physical conditions. [ABSTRACT FROM AUTHOR]

First-principles and model calculations show that the Dirac surface state of the topological insulator Bi2Te3 survives upon moderate Mn doping of the surface layers but can lose its topological character as a function of magnetization direction. The dispersion depends considerably on the direction of the Mn magnetization: for perpendicular magnetization, a gap of 16 meV opens up at the Dirac point; for in-plane magnetization, a tiny gap can be opened or closed in dependence on the magnetization azimuth. The ground state is ferromagnetic, with a critical temperature of 12 K. The results provide a path towards a magnetic control of the topological character of the Dirac surface state and its consequences to spin-dependent transport properties. [ABSTRACT FROM AUTHOR]

We have studied the thermoelectric properties of porous silicon, a nanostructured, yet single-crystalline form of silicon. Using electrochemical etching, liquid-phase doping, and high-temperature passivation, we show that porous Si can be fabricated such that it has thermoelectric properties superior to bulk Si, for both n- and p-type doping. Hall measurements reveal that the charge carrier mobility is reduced compared to the bulk material which presently limits the increase in thermoelectric efficiency. [ABSTRACT FROM AUTHOR]

Using surface x-ray diffraction and ab initio calculations we present a model of the BaBaTiO3(001)-(2 x 1) surface structure, which has not been considered so far. While the crystal is terminated by two TiO2 layers similarly to SrTiO3(001)-(2 x 1), we find that one out of two surface layer Ti-atoms resides in a tetragonal pyramidal oxygen environment. This peculiar geometry leads to a metallic and magnetic surface involving local magnetic moments up to 2&mgr;B in magnitude located at surface Ti and O atoms. Our results are important for the understanding of the intrinsic surface metallicity of insulating oxides in general. [ABSTRACT FROM AUTHOR]

Topological insulators are characterized by the presence of spin-momentum-locked surface states with Dirac points that span the fundamental bulk band gap. We show by first-principles calculations that the surface state of the insulator Bi2Te3 survives upon moderate Mn doping of the surface layers. The spin texture of both undoped and Mn-doped Bi2Te3 is much more complicated than commonly believed, showing layer-dependent spin reversal and spin vortices. [ABSTRACT FROM AUTHOR]

Magnetoelectric coupling allows for manipulating the magnetization by an external electric field or the electrical polarization by an external magnetic field. Here, we propose a mechanism to electrically induce 180° magnetization switching combining two effects: the magnetoelectric coupling at a multiferroic interface and magnetic interlayer exchange coupling. By means of first-principles methods, we investigate a ferroelectric layer in contact with a Fe/Au/Fe trilayer. The calculations show mat the interface magnetism is strongly coupled to the ferroelectric layer. Furthermore, under certain conditions a reversal of polarization causes a sign reversal of the interlayer exchange coupling which is results in a 180° switching of the free layer magnetization. We argue that this magnetoelectric coupling mechanism is very robust and can find applications in magnetic data storage. [ABSTRACT FROM AUTHOR]

First-principles calculations of x-ray absorption spectra (XAS) and the related x-ray magnetic circular dichroism (XMCD) are presented for ultrathin Fe layers on a BaTiO3 (BTO) single-crystal surface within the framework of density functional theory. We have investigated the dependence of XAS and XMCD of Fe L2,3 edges as a function of Fe layer thickness (1-3 ML) and on polarization direction of BTO. The calculations give a detailed insight concerning the relation between structural, electronic, and magnetic properties at the multiferroic interface Fe/BTO. In dependence on the Fe layer thickness we find characteristic features in the related XMCD, which depend strongly on the interface structure. The critical temperature of the ultrathin Fe layer is calculated to be between 170 to 230 K. [ABSTRACT FROM AUTHOR]

The magnetic properties of ATc03 (A = Ca, Sr, Ba) perovskites were studied from first principles using multiple-scattering theory. For each system, the most preferable magnetic order is antiferromagnetic where the magnetic moments of the nearest Tc atoms are aligned antiparallel to each other. In the case of CaTc03 and SrTc03, their recently observed antiferromagnetism is supported by our calculation of the Tc-Tc exchange coupling parameters. The Néel temperatures TN estimated for CaTc03 and SrTc03 are in a good agreement with experiment. For BaTc03, which is not synthesized so far, we predict a fundamental band gap of 0.3 eV and TN below 900 K. In general, TN increases with volume expansion, whereas the presence of vacancies on both the oxygen and cation sites of ATc03 should significantly suppress the critical temperature as well as the magnetic moment of Tc. Electronic correlations play a minor role in the formation of the magnetic state, although they may improve the calculated value of TN. [ABSTRACT FROM AUTHOR]

Photoemission electron microscopy performed on a well-prepared surface of BaTi03 reveals the persistence of surface domains at temperatures well above the bulk Curie temperature. Their patterns follow the ferroelectric domain structure observed at 300 K. The contrast between formerly outward polarized domains and in-plane polarized domains is preserved across the transition, while the contrast of inward polarized domains changes sign. The work functions of different possible structures are compared by first-principles calculations. The domain contrast in photoemission above the bulk Curie temperature is associated with a remaining tetragonal distortion of the topmost unit cells which is stabilized by an ionic surface relaxation. [ABSTRACT FROM AUTHOR]

We present a first-principles study of the electronic structure and exchange interactions in Si/Mn digital magnetic alloy (DMA) consisting of Mn monolayers embedded in a Si matrix stacking along [001] direction. The main focus of our study is on the dependence of magnetic properties on the morphology of the Mn monolayer. Three different structural models for the Mn monolayer are considered: manganese in substitutional (i), interstitial (ii), and both interstitial and substitutional (iii) positions. The atomic positions in Si/Mn DMA are determined by means of the VASP code and then serve as input for multiple-scattering calculations of the electronic and magnetic structure. The magnetic force theorem is used to evaluate the exchange coupling parameters. A Heisenberg model based on those parameters is used to estimate magnon frequencies and magnetic phase-transition temperatures for different anisotropies. The magnetic properties of Si/Mn DMA are found to be strongly dependent on the underlying crystalline structure. [ABSTRACT FROM AUTHOR]

The electronic structure of (Zn,V)O dilute magnetic semiconductors (DMSs) was investigated theoretically from first principles, using the fully relativistic Dirac linear muffin-tin orbital band structure method and the Korringa-Kohn-Rostoker Green's function approach within the local spin-density approximation. X-ray magnetic circular dichroism (XMCD) spectra at the V L2,3 edges were calculated. Comparing the experimental results and the theoretical simulations we discuss possible crystal and magnetic structures of (Zn,V)O and reach the best agreement with the experimental XMCD spectra for the antiferromagnetically ordered DMSs in the presence of an oxygen vacancy. The corresponding structure was used to determine the electronic and magnetic properties of (Zn,V)O. [ABSTRACT FROM AUTHOR]

Abstract: The evolution of electronic structure of the organic semiconductor iron-phthalocyanine with potassium doping has been studied by means of photoemission spectroscopy, near-edge X-ray absorption fine structure and density functional theory (DFT) calculations. The DFT study and detailed analysis of the core-level spectra permit us to suggest possible lattice sites for the potassium ions. The data disclosed filling of the lowest unoccupied molecular orbital upon doping and associated changes of the core level absorption spectra. None of the films prepared in our studies showed a finite electronic density of states at the Fermi level. [Copyright &y& Elsevier]

Abstract: We present a first-principles study of the electronic and magnetic properties of cubic hafnium dioxide stabilized by Mn. We find this material to be ferromagnetic and half-metallic, with the Mn-impurity electronic states lying in the band gap of hafnia for a wide range of manganese concentration. Our ab initio calculations, within the local spin-density approximation, demonstrate that Mn-doped hafnia may be ferromagnetic at 700K while its high-T C ferromagnetism is robust to the oxygen vacancy defects and to how the Mn impurities are distributed over the cation sublattice. [Copyright &y& Elsevier]

The electronic structure and morphology of organic semiconducting cobalt-phtalocyanine (CoPc) films in situ prepared on the Au(001)-5×20 superstructure have been studied by a combination of experimental and theoretical work. The CoPc molecular film was characterized by photoemission spectroscopy (PES, valence band and core-level). The experimental results were simulated and have been explained in the framework of density functional theory (DFT) calculations. The C 1 s and N 1 s core level spectra were analyzed by taking into account the fact that both types of atoms have different nonequivalent positions in the molecule. And finally, the experimentally obtained electronic valence band structure of CoPc is in very good agreement with ab initio density of state results, allowing a detailed site-specific insight into the system. [ABSTRACT FROM AUTHOR]

Abstract: The filled and unoccupied electronic states of the organic semiconductor copper-phthalocyanine (CuPc) have been determined by a combination of direct and resonant photoemission, near-edge X-ray absorption and first principles calculations. The experimentally obtained electronic states of CuPc are in very good agreement with results of ab initio density of states, allowing to derive detailed site specific information. [Copyright &y& Elsevier]

We study structure and electronic states of Cu nanocontacts. The Korringa–Kohn–Rostoker (KKR) Green''s function method for low-dimensional systems and the density functional theory in the local density approximation are used in our calculations. Atomic relaxations of nanocontacts and electrodes are performed using ab initio based many body potentials constructed by means of the KKR Green''s function method and the tight-binding approach. We demonstrate that nanocontacts exhibit stress oscillations during stretching caused by changes in their structure. Performing ab initio calculations of electronic states in nanocontacts in a fully relaxed geometry, we reveal a strong enhancement in the density of electronic states at the Fermi level before breaking. [Copyright &y& Elsevier]

Hydrogenation provides a novel way to tune the electronic properties of graphene. Recent scanning tunneling microscopy experiments have demonstrated that local graphene magnetism can be selectively switched on and off by hydrogen (H) dimers. Employing first-principles calculations in conjunction with the constrained random-phase approximation we determine the strength of the effective Coulomb interaction U in hydrogenated graphene. We find that the calculated U parameters are smaller than the ones in graphene and depend on the H concentration. Moreover, the U parameters are very sensitive to the position of H atoms adsorbed on the graphene lattice. We discuss the instability of the paramagnetic state of the hydrogenated graphene towards the ferromagnetic one on the basis of calculated U parameters within the Stoner model. Spin-polarized calculations reveal that the itinerant ferromagnetism in hydrogenated graphene can be well accounted for by the Stoner model. [ABSTRACT FROM AUTHOR]