Connection between charge transfer and alloying core-level shifts based on density-functional calculations

The measurement of alloying core-level binding energy (CLBE) shifts has been used to give a precise meaning to the fundamental concept of charge transfer. Here, ab-initio density-functional calculations for the intermetallic compound MgAu are used to investigate models which try to make a connection between the core levels shifts and charge transfer. The calculated CLBE shifts agree well with experiment, and permit an unambiguous separation into initial-state and screening contributions. Interestingly, the screening contribution is large and cannot be neglected in any reasonable description. Comparison of the calculated results with the predictions of simple models show that these models are not adequate to describe the realistic situation. On the positive side, the accuracy of the density-functional calculations indicates that the combination of experiments with such calculations is a powerful tool to investigate unknown systems.Comment: RevTeX 10 pages incl 8 figure

Electronic and structural properties of GaN by the full-potential LMTO method : the role of the dd electrons

The structural and electronic properties of cubic GaN are studied within the local density approximation by the full-potential linear muffin-tin orbitals method. The Ga $3d$ electrons are treated as band states, and no shape approximation is made to the potential and charge density. The influence of $d$ electrons on the band structure, charge density, and bonding properties is analyzed. It is found that due to the energy resonance of the Ga 3$d$ states with nitrogen 2$s$ states, the cation $d$ bands are not inert, and features unusual for a III-V compound are found in the lower part of the valence band and in the valence charge density and density of states. To clarify the influence of the Ga $d$ states on the cohesive properties, additional full and frozen--overlapped-core calculations were performed for GaN, cubic ZnS, GaAs, and Si. The results show, in addition to the known importance of non-linear core-valence exchange-correlation corrections, that an explicit description of closed-shell repulsion effects is necessary to obtain accurate results for GaN and similar systems. In summary, GaN appears to be somewhat exceptional among the III-V compounds and reminiscent of II-VI materials, in that its band structure and cohesive properties are sensitive to a proper treatment of the cation $d$ bands, as a result of the presence of the latter in the valence band range.Comment: ( 20 REVTEX-preprint pages (REVTEX macros are included) 8 figures available upon reques

Reconstruction Mechanism of FCC Transition-Metal (001) Surfaces

The reconstruction mechanism of (001) fcc transition metal surfaces is investigated using a full-potential all-electron electronic structure method within density-functional theory. Total-energy supercell calculations confirm the experimental finding that a close-packed quasi-hexagonal overlayer reconstruction is possible for the late 5$d$-metals Ir, Pt, and Au, while it is disfavoured in the isovalent 4$d$ metals (Rh, Pd, Ag). The reconstructive behaviour is driven by the tensile surface stress of the unreconstructed surfaces; the stress is significantly larger in the 5$d$ metals than in 4$d$ ones, and only in the former case it overcomes the substrate resistance to the required geometric rearrangement. It is shown that the surface stress for these systems is due to $d$ charge depletion from the surface layer, and that the cause of the 4th-to-5th row stress difference is the importance of relativistic effects in the 5$d$ series.Comment: RevTeX 3.0, 12 pages, 1 PostScript figure available upon request] 23 May 199

Spreading the European Model by Military Means? The Legitimization of Colonial Wars and Imperialist Interventions in Great Britain and Germany around 1900

Dieser Artikel untersucht, welche Funktionen der Verweis auf Europa in den englischen und deutschen Debatten über Kolonialkriege und imperialistische Interventionen um 900 hatte. Welche Europarepräsentationen dienten den Kriegsbefürwortern als Legitimationsressource, wie instrumentalisierten Kritiker Europabilder, um für ihre politischen Positionen zu werben? So wird etwa gezeigt, wie die Repräsentationen Russlands als mehr oder weniger europäisch dazu dienten, dieses Land als Rivalen oder Kooperationspartner in imperialen Kontexten darzustellen. Im Zentrum des Artikels steht jedoch die Frage, inwieweit Militäreinsätze in der außereuropäischen Welt damit legitimiert wurden, das europäische Modell zu verbreiten (die „Zivilisierungsmission“). Hier ist die zentrale These, dass eine solche Argumentation nur dann eine prominente Rolle spielte, wenn die entsprechenden Militäreinsätze ohnehin populär und erfolgreich waren

Synthese und Charakterisierung von amphiphilen Porphyrin-Perylen Dyaden: Die Grundlagen für optoelektronische Membranen in Wasser

In this dissertation, two unsymmetrically substituted chromophores (porphyrin and perylene) were synthesized and coupled via a linear and rigid phenyl-ethynyl-phenyl linker to a dyad. By introduction of a Newkome dendron with three or nine negative charges, four novel and water-soluble porphyrin–perylene assemblies were produced for the first time. The 18–21 step synthesis procedure was planned, realized and constantly optimized. Using an improved perylene synthesis cascade with novel purification methods, complex perylene imides were obtained in 80–90% yield. A multistep porphyrin synthesis provided unsymmetrical AB2C porphyrins without scrambling in up to 30% yield. The four synthesized dyads were systematically analyzed via NMR, IR, MS, UV-Vis and emission spectroscopy. The absorption and emission behavior in THF and water indicated a photoinduced excited-state interaction between both chromophores and the formation of a porphyrin(+)–linker–perylene(–) charge-separated state. Energy and electron transfer between porphyrin and perylene were calculated, simulated and measured by spectroscopic experiments. The aggregation behavior could be adjusted by addition of base or by amine/nitrogen compounds (such as pyridine) which were found to coordinate axially to the zinc porphyrin in the dyad.Im Rahmen dieser Dissertation wurden zwei unsymmetrisch substituierte Chromophore (Porphyrin und Perylen) synthetisiert und mittels einer linearen und starren Phenyl-ethinyl-phenyl-Brücke zu einer Dyade verknüpft. Durch Anfügen eines Newkome-Dendrons mit drei bzw. neun negativen Ladungen wurden erstmalig vier neuartige und wasserlösliche Porphyrin–Perylen-Systeme hergestellt. Der 18–21-stufige Syntheseprozess wurde geplant, durchgeführt und durchgehend optimiert. Durch eine verbesserte Perylen-Synthese-Kaskade mit neuentwickelten Aufreinigungs-methoden wurden komplexe Perylenimide in 80–90% Ausbeute erhalten. Eine mehrstufige Porphyrinsynthese ermöglichte es, unsymmetrische AB2C-Porphyrine ohne Scrambling in bis zu 30% Ausbeute zu erhalten. Die vier synthetisierten Dyaden wurden intensiv mittels NMR, IR, MS, UV-Vis und Emissionsspektroskopie untersucht. Das Absorptions- und Emissionsverhalten in THF und Wasser wies auf eine photoinduzierte Interaktion zwischen beiden Chromophoren und auf die Entstehung eines ladungsgetrennten Porphyrin(+)–Linker–Perylene(–) Zustandes hin. Der Energie- und Elektronentransfer zwischen Porphyrin und Perylen wurde berechnet, simuliert und in spektroskopischen Experimenten nachgewiesen. Das Aggregationsverhalten konnte reguliert werden durch Zugabe von Base oder durch Amin- bzw. Stickstoffverbindungen (z.B. Pyridin), welche axial an das Zink-Porphyrin der Dyade koordinierten

Extracting convergent surface energies from slab calculations

The formation energy of a solid surface can be extracted from slab calculations if the bulk energy per atom is known. It has been pointed out previously that the resulting surface energy will diverge with slab thickness if the bulk energy is in error, in the context of calculations which used different methods to study the bulk and slab systems. We show here that this result is equally relevant for state-of-the-art computational methods which carefully treat bulk and slab systems in the same way. Here we compare different approaches, and present a solution to the problem that eliminates the divergence and leads to rapidly convergent and accurate surface energies.Comment: 3 revtex pages, 1 figure, in print on J. Phys. Cond. Mat

GW correlation effects on plutonium quasiparticle energies: changes in crystal-field splitting

We present results for the electronic structure of plutonium by using a recently developed quasiparticle self-consistent $GW$ method (\qsgw). We consider a paramagnetic solution without spin-orbit interaction as a function of volume for the face-centered cubic (fcc) unit cell. We span unit-cell volumes ranging from 10% greater than the equilibrium volume of the $\delta$ phase to 90 % of the equivalent for the $\alpha$ phase of Pu. The self-consistent $GW$ quasiparticle energies are compared to those obtained within the Local Density Approximation (LDA). The goal of the calculations is to understand systematic trends in the effects of electronic correlations on the quasiparticle energy bands of Pu as a function of the localization of the $f$ orbitals. We show that correlation effects narrow the $f$ bands in two significantly different ways. Besides the expected narrowing of individual $f$ bands (flatter dispersion), we find that an even more significant effect on the $f$ bands is a decrease in the crystal-field splitting of the different bands.Comment: 9 pages, 7 figures, 3 table

Many-body effects in iron pnictides and chalcogenides -- non-local vs dynamic origin of effective masses

We apply the quasi-particle self-consistent GW (QSGW) approximation to some of the iron pnictide and chalcogenide superconductors. We compute Fermi surfaces and density of states, and find excellent agreement with experiment, substantially improving over standard band-structure methods. Analyzing the QSGW self-energy we discuss non-local and dynamic contributions to effective masses. We present evidence that the two contributions are mostly separable, since the quasi-particle weight is found to be essentially independent of momentum. The main effect of non locality is captured by the static but non-local QSGW effective potential. Moreover, these non-local self-energy corrections, absent in e.g. dynamical mean field theory (DMFT), can be relatively large. We show, on the other hand, that QSGW only partially accounts for dynamic renormalizations at low energies. These findings suggest that QSGW combined with DMFT will capture most of the many-body physics in the iron pnictides and chalcogenides.Comment: 4+ pages, 3 figure

Ab-initio procedure for effective models of correlated materials with entangled band structure

We present a first-principles method for deriving effective low-energy models of electrons in solids having entangled band structure. The procedure starts with dividing the Hilbert space into two subspaces, the low-energy part ("$d$ space'') and the rest of the space ("$r$ space''). The low-energy model is constructed for the $d$ space by eliminating the degrees of freedom of the $r$ space. The thus derived model contains the strength of electron correlation expressed by a partially screened Coulomb interaction, calculated in the constrained random-phase-approximation (cRPA) where screening channels within the $d$ space, $P_d$, are subtracted. One conceptual problem of this established downfolding method is that for entangled bands it is not clear how to cut out the $d$ space and how to distinguish $P_d$ from the total polarization. Here, we propose a simple procedure to overcome this difficulty. In our scheme, the $d$ subspace is cut out from the Hilbert space of the Kohn Sham eigenfunctions with the help of a procedure to construct a localized Wannier basis. The $r$ subspace is constructed as the complementary space orthogonal to the $d$ subspace. After this disentanglement, $P_d$ becomes well defined. Using the disentangled bands, the effective parameters are uniquely determined in the cRPA. The method is successfully applied to 3$d$ transition metals.Comment: 14 pages, 4 figure