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ABSTRACT: Angular momentum seems to not be conserved in chemical reactions where one of the reactants is magnetic; consequently, such reactions show a high activation barrier. An example is the production of hydrogen by electrolysis of water: practically all losses occur in the production of (magnetic) oxygen. Anodes with a low overvoltage (a measure of the losses) are based on the ruthenium dioxide (110) surface. First-principles electronic structure calculations show that this surface itself carries magnetic moments. This magnetic surface enables the production of oxygen in the ground state while conserving angular momentum.
The Journal of Physical Chemistry C 01/2013; 117(12):6353–6357. · 4.80 Impact Factor
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ABSTRACT: The lithium amide (LiNH(2)) + lithium hydride (LiH) system is one of the most attractive light-weight materials options for hydrogen storage. Its dehydrogenation involves mass transport in the bulk (amide) crystal through lattice defects. We present a first-principles study of native point defects and dopants in LiNH(2) using density functional theory. We find that both Li-related defects (the positive interstitial Li(i)(+) and the negative vacancy V(Li)(-)) and H-related defects (H(i)(+) and V(H)(-)) are charged. Li-related defects are most abundant. Having diffusion barriers of 0.3-0.5 eV, they diffuse rapidly at moderate temperatures. V(H)(-) corresponds to the [NH](2-) ion. It is the dominant species available for proton transport with a diffusion barrier of ∼0.7 eV. The equilibrium concentration of H(i)(+), which corresponds to the NH(3) molecule, is negligible in bulk LiNH(2). Dopants such as Ti and Sc do not affect the concentration of intrinsic defects, whereas Mg and Ca can alter it by a moderate amount. Ti and Mg are easily incorporated into the LiNH(2) lattice, which may affect the crystal morphology on the nano-scale.
Physical Chemistry Chemical Physics 02/2011; 13(13):6043-52. · 3.57 Impact Factor
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ABSTRACT: The optical and electronic properties of Mg-Ti hydrides are studied using first-principles density functional theory. Dielectric functions are calculated for MgxTi(1-x)H2 with compositions x = 0.5, 0.75, and 0.875. The structure is that of fluorite TiH2 where both Mg and Ti atoms reside at the Ti positions of the lattice. In order to assess the effect of randomness in the Mg and Ti occupations we consider both highly ordered structures, modeled with simple unit cells of minimal size, and models of random alloys. These are simulated by super cells containing up to 64 formula units (Z = 64). All compositions and structural models turn out metallic, hence the dielectric functions contain interband and intraband free electron contributions. The former are calculated in the independent particle random phase approximation. The latter are modeled based upon the intraband plasma frequencies, which are also calculated from first-principles. Only for the models of the random alloys we obtain a black state, i.e. low reflection and transmission in the energy range from 1 to 6 eV.
Physical Review B 05/2008; 79:125117. · 3.69 Impact Factor
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ABSTRACT: The relationship between the crystal structures, band structures, and electronic properties of acene-TCNQ complexes has been investigated. We focus on the newly synthesized crystals of the charge-transfer salt tetracene-TCNQ and similar to it perylene-TCNQ, potentially interesting for realization of ambipolar transport. The band structures were calculated from first principles using density-functional theory (DFT). Despite the similarity in the crystal structures of the acene-TCNQ complexes studied here, the band structures are very different. Hole and electron transport properties are predicted to be equally good in perylene-TCNQ, in contrast to the tetracene-TCNQ, which has good transport properties for electrons only. The estimated degree of charge transfer for tetracene-TCNQ is 0.13e and for perylene-TCNQ 0.46e.
The Journal of Physical Chemistry A 04/2008; 112(11):2497-502. · 2.95 Impact Factor
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ABSTRACT: We provide compelling evidence to establish that, contrary to one's elementary guess, the tunneling spin polarization (TSP) of amorphous CoFeB is larger than that of fcc CoFeB. First-principles atomic and electronic structure calculations reveal striking agreement between the measured TSP and the predicted s-electron spin polarization. Given the disordered structure of the ternary alloy, not only do these results strongly endorse our communal understanding of tunneling through AlO(x), but they also portray the key concepts that demand primary consideration in such complex systems.
Physical Review Letters 03/2008; 100(5):057205. · 7.37 Impact Factor
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ABSTRACT: The wave character of an F-type color center in sodium chloride has been investigated using ab initio band structure calculations in the super-cell approach. Although the F-center, consisting of a zero-charge nucleus and an electron, can be regarded as the most delocalized 'atom', calculations show a localized character of its 1s state. The influence of the F-center extends over 12 Å. The wave character of the defect band originates mainly from the chlorine 4s states rather than the sodium 3s states, in line with the available experimental results.
Journal of Physics Condensed Matter 01/2008; 20(7):075219. · 2.55 Impact Factor
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ABSTRACT: A review of new developments in theoretical and experimental electronic structure investigations of half-metallic ferromagnets (HMF) is presented. Being semiconductors for one spin projection and metals for another ones, these substances are promising magnetic materials for applications in spintronics (i.e., spin-dependent electronics). Classification of HMF by the peculiarities of their electronic structure and chemical bonding is discussed. Effects of electron-magnon interaction in HMF and their manifestations in magnetic, spectral, thermodynamic, and transport properties are considered. Especial attention is paid to appearance of non-quasiparticle states in the energy gap, which provide an instructive example of essentially many-body features in the electronic structure. State-of-art electronic calculations for correlated $d$-systems is discussed, and results for specific HMF (Heusler alloys, zinc-blende structure compounds, CrO$_{2},$ Fe$_{3}$O$_{4}$) are reviewed.
12/2007;
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ABSTRACT: Several aspects of half-metallic magnetism at finite temperature are discussed. Since NiMnSb is the simplest half-metal and the longest known it will be used as an example. Also it is a half-metal with remarkable little on-site Coulomb repulsion. Consequently it is a half-metal that is not notably corrupted by non-quasiparticle states. There exists an anomaly at 90 K, described before, that will be shown to be unrelated to the position of the Fermi level in the bandgap. Several substitutions are investigated that could shed some light on the origin of the transition. The calculated phonon spectrum is compared with experimental neutron scattering data. Finally, the spin-polarization of interfaces of NiMnSb with the transition metal based non-magnetic semiconductors NiTiSn and CoTiSb is investigated and the electronic structure of an infinite two-dimensional array of NiMnSb quantum dots embedded in NiScSb is reported.
Journal of Physics Condensed Matter 08/2007; 19(31):315212. · 2.55 Impact Factor
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ABSTRACT: Recently, half-metallic magnetism was predicted for rubidium sesquioxide at relatively high temperatures. Here, we present density-functional calculations on related potassium and caesium oxides as well as hypothetical continuous series. Finally, we consider ammonium sesquioxide and estimate a Curie temperature of 250 K. All these half-metals posses band gaps for the majority spin direction.
Journal of Physics Condensed Matter 04/2007; 19(16):165203. · 2.55 Impact Factor
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ABSTRACT: We have grown the charge-transfer salt of the electron acceptor 7,7,8,8-tetracyanoquinodimethane (TCNQ) and the electron donor tetracene using physical vapor transport. The crystal structure was solved by single-crystal X-ray diffraction and the symmetry was found to be triclinic (space group P1̄). The 1:1 complex grows by alternating face to face stacking of tetracene and TCNQ molecules. Calculations on the electronic band structure show that the tetracene−TCNQ complex is an indirect band gap semiconductor and provide insight in the conductive properties of tetracene−TCNQ.
02/2007;
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ABSTRACT: Lanthanum hexaboride is one of the cathode materials most used in high-power electronics technology, but the many experimental results do not provide a consistent picture of the surface properties. Therefore, we report the first ab initio calculations of the work functions and surface energies of the (001), (011), (111), (112), and (012) surfaces by considering the different surface terminations and structural relaxation. Either the (111)B- or the (001)La-terminated surface is the most stable, depending on La chemical potential. The work function of the latter is the lowest (2.07 eV) of the surfaces considered. Both the work function and surface energy decrease further when surface La is replaced by Ba and become, respectively, 1.43 and 7.7 eV/nm(2) at the chemical potentials of elemental lanthanum and barium bulk. These results compare favorably with previous work on the intermetallics BaAl(4), CaAl(4), and BaAuIn(3). Their most stable surfaces possess the lowest work function. Now, we study a compound with a decidedly different crystal type and with its constituting elements from column 3 of the periodic table, of which one is nonmetallic.
The Journal of Physical Chemistry B 10/2006; 110(37):18459-65. · 3.70 Impact Factor
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P J W Weijs,
G Wiech,
W Zahorowski,
W Speier,
J B Goedkoop,
M Czyzyk,
J F van Acker,
E van Leuken, R A de Groot,
G van der Laan,
D D Sarma,
L Kumar,
K H J Buschow,
J C Fuggle
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ABSTRACT: The valence bands and conduction bands of about 30 transition metal silicides (of which we concentrate on 4 here) have been investigated by measurements of Si X-ray emission bandsspectra, X-ray absorption spectra near the Si K (1s) edge, photoemission spectra, and Bremsstrahlung Isochromat spectra. The densities of states have also been calculated for the materials in their real crystal structures. The influence of the core hole on some spectra has been investigated using supercell calculations, a (Greens function) generalized Clogston-Wolff model, and Auger spectroscopy. A selection of results is presented to illustrate the utility of site and selective methods in investigations of the electronic structure of silicides and the nature of the "quasi-gap" of the partial density of Si p states in the region of the transition metal d bands.
Physica Scripta 09/2006; 41(4):629. · 1.20 Impact Factor
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ABSTRACT: In this article, based on electronic structure calculations, the conditions are discussed under which a genuine half-metallic interface between a heusler C1b half-metal and a semiconductor can exist. An explanation is given why for the III–V semiconductors the double anion terminated (111) interface is the only possible interface. For semiconductors, based on transition metals, a much wider variety of interfaces are found to be possible.
Journal of Physics D Applied Physics 02/2006; 39(5):793. · 2.54 Impact Factor
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ABSTRACT: The current status of half-metallicity is briefly reviewed. Three origins of half-metallicity are distinguished: In heusler
C1b compounds covalent interactions similar as in III-V semiconductors are responsible. The effect is intimately related with
the crystal structure, while the electron-electron interactions are weak. A second category is found in the limit of strong
magnetism. Chromium-dioxide is an archetype here. In the third category, strong electron-electron interactions can be the
origin of half-metallicity like in magnetite and the double perovskites. Experiments probing half-metallicity are reviewed
with a distinction between the behaviour of genuine bulk, surfaces and interfaces. The effects of temperature and disorder
on half-metallicity are discussed. In general increased temperature will reduce spin-polarisation as magnons are excited.
Calculations show that suitably chosen impurities counteract this effect, and may actually improve the spin-polarization at
finite temperature. Finally, spin injection from nano-scale structured half-metallic contacts is discussed.
11/2005: pages 199-216;
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ABSTRACT: The relationship between the work function (Phi) and the surface stability of compounds is, to our knowledge, unknown, but very important for applications such as organic light-emitting diodes. This relation is studied using first-principles calculations on various surfaces of BaAl4. The most stable surface [Ba terminated (001)] has the lowest Phi (1.95 eV), which is lower than that of any elemental metal including Ba. Adding barium to this surface neither increases its stability nor lowers its work function. BaAl4 is also strongly bound. These results run counter to the common perception that stability and a low Phi are incompatible. Furthermore, a large anisotropy and a stable low-work-function surface are predicted for intermetallic compounds with polar surfaces.
06/2005;
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ABSTRACT: Correlation effects on the electronic structure of half-metallic CrAs in zinc-blende structure are studied for different substrate lattice constants. Depending on the substrate the spectral weight of the non-quasiparticle states might be tuned from a well developed value in the case of InAs substrate to an almost negligible contribution for the GaAs one. A piezoelectric material that would allow the change in the substrate lattice parameters opens the possibility for practical investigations of the switchable (tunable) non-quasiparticle states. Since the latter are important for the tunneling magnetoresistance and related phenomena it creates new opportunities in spintronics. Comment: 12 pages, 3 figures, 2 tables. accepted PRB 71, 1 (2005)
01/2005;
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ABSTRACT: The influence of intrinsic defects in half-metals is calculated in the case of NiMnSb. Of the 14 cases of intrinsic defects, five affect the half-metallic properties. They are energetically very unlikely to occur. Circumstances are discussed under which defects may even have a beneficial effect on the spin polarization of the conduction electrons. Non-intrinsic defects, like deliberate doping by rare-earth atoms, as well as the effect of nano-structured contacts may influence the magnon spectrum, improving the behaviour at finite temperature.
Journal of Physics Condensed Matter 11/2004; 16(48):S5517. · 2.55 Impact Factor
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ABSTRACT: Spintronic materials, especially half-metallic ferromagnets exhibit
unusual sensitivity to different perturbations, in particular to
impurities close to the Fermi level. We carried out finite temerature
calculations for model systems that combine correlation and disorder
effects in the framework of the CPA+DMFT approach. Results of the
realistic electronic structure calculations in the clean limit of NiMnSb
within the spin-spiral and LDA+DMFT methods were compared. The nature of
non-quasiparticle states in the minority gap above the Fermi level is
discussed. The ab-initio supercell technique was used to investigate the
interaction of non-magnetic impurities with the non-quasiparticle
states.
02/2004; -1:26010.
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ABSTRACT: Polymer diodes require cathodes that do not corrode the polymer but do have low work function to minimize the electron injection barrier. First-principles calculations demonstrate that the work function of the (1000) surface of the compound Ca2N is half an eV lower than that of the elemental metal Ca (2.35 vs. 2.87 eV). Moreover its reactivity is expected to be smaller. This makes Ca2N an interesting candidate to replace calcium as cathode material for polymer light emitting diode devices. Comment: 3 pages, 4 figures, accepted by J. Appl. Phys
11/2003;
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ABSTRACT: Nonquasiparticle states above the Fermi energy are studied by first-principle dynamical mean field calculations for a prototype half-metallic ferromagnet, NiMnSb. We present a quantitative evaluation of the spectral weight of this characteristic feature and discuss the possible experimental investigation (BIS, NMR, STM and Andreev reflection) to clarify the existence of these states. Comment: 15 pages, 4 figures, acepted in PRB
10/2003;
