Search results for "Approx"
showing 10 items of 922 documents
New Materials with High Spin Polarization Investigated by X-Ray Magnetic Circular Dichroism
2013
We investigate element-specific spin and orbital magnetic moments of polycrystalline bulk Heusler alloys that are predicted to be half-metallic with composition Co2YZ (Y = Ti, Cr, Mn, Fe and Z = Al, Ga, Si, Ge, Sn, Sb) using magnetic circular dichroism in X-ray absorption spectroscopy (XAS/XMCD). In addition to stoichiometric compounds we also investigate composition series with partly replaced elements on the Y-site (Co2Fe x Cr1−x Si, Co2Mn x Ti1−x Si and Co2Mn x Ti1−x Ge) and on the Z-site (Co2MnGa1−x Ge x ) promising a tailoring of the Fermi level with respect to the minority band gap. We compare experimental results with theoretical predictions elucidating the influence of local disorde…
Realistic investigations of correlated electron systems with LDA + DMFT
2006
Conventional band structure calculations in the local density approximation (LDA) [1–3] are highly successful for many materials, but miss important aspects of the physics and energetics of strongly correlated electron systems, such as transition metal oxides and f-electron systems displaying, e.g., Mott insulating and heavy quasiparticle behavior. In this respect, the LDA + DMFT approach which merges LDA with a modern many-body approach, the dynamical mean-field theory (DMFT), has proved to be a breakthrough for the realistic modeling of correlated materials. Depending on the strength of the electronic correlation, a LDA + DMFT calculation yields the weakly correlated LDA results, a strong…
The limits of the rotating wave approximation in electromagnetic field propagation in a cavity
2005
We consider three two-level atoms inside a one-dimensional cavity, interacting with the electromagnetic field in the rotating wave approximation (RWA), commonly used in the atom-radiation interaction. One of the three atoms is initially excited, and the other two are in their ground state. We numerically calculate the propagation of the field spontaneously emitted by the excited atom and scattered by the second atom, as well as the excitation probability of the second and third atom. The results obtained are analyzed from the point of view of relativistic causality in the atom-field interaction. We show that, when the RWA is used, relativistic causality is obtained only if the integrations …
Superfluidity of fermionic pairs in a harmonic trap. Comparative studies: Local Density Approximation and Bogoliubov-de Gennes solutions
2020
Abstract Experiments with ultracold gases on the lattice give the opportunity to realize superfluid fermionic mixtures in a trapping potential. The external trap modifies the chemical potential locally. Moreover, this trap also introduces non-homogeneity in the superconducting order parameter. There are, among other approaches, two methods which can be used to describe the system of two-component mixtures loaded into an optical lattice: the Local Density Approximation (LDA) and the self-consistent Bogoliubov–de Gennes equations. Here, we compare results obtained within these two methods. We conclude that the results can be distinguishable only in the case of a small value of the pairing int…
Phase transitions in polymer blends and block copolymer melts: Some recent developments
2005
The classical concepts about unmixing of polymer blends (Flory-Huggins theory) and about mesophase ordering in block copolymers (Leibler's theory) are briefly reviewed and their validity is discussed in the light of recent experiments, computer simulations and other theoretical concepts. It is emphasized that close to the critical point of unmixing non-classical critical exponents of the Ising universality class are observed, in contrast to the classical mean-field exponents implied by the Flory-Huggins theory. The temperature range of this non-mean-field behavior can be understood by Ginzburg criteria. The latter are also useful to discuss the conditions under which the linearized (Cahn-li…
Calculations of the atomic and electronic structure for SrTiO3 perovskite thin films
2001
The results of calculations of SrTiO3 (100) surface relaxation and rumpling with two different terminations (SrO and TiO2) are presented and discussed. We have used the ab initio Hartree–Fock (HF) method with electron correlation corrections and the density functional theory (DFT) with different exchange–correlation functionals, including hybrid exchange techniques. All methods agree well on surface energies and on atomic displacements, as well as on the considerable increase of covalency effects near the surface. More detailed experiments on surface rumpling and relaxation are necessary for further testing of theoretical predictions.
Two-LO-Phonon Resonant Raman Scattering in II-VI Semiconductors
1996
Recently, absolute values of socond-order Raman scattering efficiency have been measured around the E 0 and E 0 + Δ 0 critical points of several II-VI semiconductor compounds. The measurements were perfomed in the z(x,x)z backscattering configuration on (001) (ZnSe and ZnTe) and (110) (CdTe) surfaces. They show strong incoming and outgoing resonances around the baud gap and larger scattering efficiencies as compaered to III-V compounds. A theoretical model which includes excitons as intermediate states in the Raman process is shown to give a very good quantitative agreement between theory and experiment. Only a small discrepancy exists, while III-V compounds the discrepancies were close to …
Partial self-consistency and analyticity in many-body perturbation theory: Particle number conservation and a generalized sum rule
2016
We consider a general class of approximations which guarantees the conservation of particle number in many-body perturbation theory. To do this we extend the concept of $\Phi$-derivability for the self-energy $\Sigma$ to a larger class of diagrammatic terms in which only some of the Green's function lines contain the fully dressed Green's function $G$. We call the corresponding approximations for $\Sigma$ partially $\Phi$-derivable. A special subclass of such approximations, which are gauge-invariant, is obtained by dressing loops in the diagrammatic expansion of $\Phi$ consistently with $G$. These approximations are number conserving but do not have to fulfill other conservation laws, such…
Characterization of the LGBTI-speaking community discourse. An approach to Hispanic queer linguistics.
2020
Toda subcultura forma una comunidad lingüística particular en la que, aunque se comparta la lengua con el resto de la sociedad, varía el habla en ciertos aspectos. Es el caso de la comunidad de habla LGTBI, que se hace servir de un léxico peculiar, así como una fraseología característica, unas estructuras gramaticales especiales y, en definitiva, unos recursos pragmáticos muy llamativos que tienen como objetivo mitigar ciertos enunciados, intensificar otros en determinados contextos y construir actos de habla afiliativos que expresen solidaridad social. Por otro lado, cumple una función muy importante en la construcción identitaria de sus usuarios. En este trabajo se propone un análisis pra…
Exception-Tolerant Hierarchical Knowledge Bases for Forward Model Learning
2021
This article provides an overview of the recently proposed forward model approximation framework for learning games of the general video game artificial intelligence (GVGAI) framework. In contrast to other general game-playing algorithms, the proposed agent model does not need a full description of the game but can learn the game's rules by observing game state transitions. Based on hierarchical knowledge bases, the forward model can be learned and revised during game-play, improving the accuracy of the agent's state predictions over time. This allows the application of simulation-based search algorithms and belief revision techniques to previously unknown settings. We show that the propose…