Correlated electron systems as such in transition metals and their alloys show a wide variety and richness of physical properties which become not only apparent in their band structures, but also in wave function-dependent quantitites as the electron momentum density (EMD) and the momentum density of annihilating electron-positron pairs (MDAP). Both quantities that provide valuable informations about the nature of many-body interactions in these materials offer direct connections to experiments based on Compton spectroscopy and on positron annihilation spectroscopy, the latter especially in form of angular correlation of annihilation radiation (ACAR) measurements. Although conceptionally clean and beautiful, the theoretical simplifications contained both in the functional theory (DFT) and the frequently used local density (LD) approximation turned out to be inadequate for a proper description of electron-electron correlations. Consequently, calculated momentum density distributions for correlated systems show only - if at all - a qualitative agreement with corresponding experiments. For the project proposed in the following, theoretical state-of-the-art techniques such as the GW approximation (GWA) and the newly developed dynamical mean-field theory (DMFT) shall be used to calculate EMD and MDAP distributions for paramagnetic and ferromagnetic transition metals, and half-metallic ferromagnets. For simple metals and elementary transition metals as Fe, Co, Ni, and Cu, there already exist encouraging theoretical results based on the combination of DFT-LD with the perturbative GWA. More recently, calculations with the emerged non-perturbative DMFT showed an improvement in the description of electronic spectra, especially for TM and their alloys. In both cases, these calculations focused on quasiparticle electron band structures in comparison with angel-resolved photoemission spectroscopy, while, in the present project, GW and DMFT calculations will be used to compute the wave function-dependent quantities EMD and MDAP. Methodologically, we plan to implement the quasiparticle equation as a reliable solver of the many-body problem in combination with the calculation of Compton and ACAR profiles. Because of their peculiar electronic structure, metallic for one spin direction and semiconducting for the other one, half-metallic ferromagnets are considered as promising spintronic materials. We plan to investigate three types of half metals: the semi-Heusler compounds NiMnSb (ferromagnetic) and FeMnSb (ferrimagnetic), and the zincblende VAs. At finite temperatures, DMFT results predict the existence of incoherent spectral weight in the gap of the minority spin channel with an expected change in the Fermi surface (FS). One well-proved way to investigate the properties of a FS are calculations of Compton and ACAR profiles which can be directly compared with experimental data. Therefore, we are confident that our calculations will help to clarify the FS modification in such materials, and to answer the question concerning the role played by half metals in designing future spintronic devices at finite temperatures.
|Effective start/end date||1/01/08 → 31/12/10|
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