Seminarios del IFLP y del Departamento

SEMINARIOS DEL IFLP y DEL DEPARTAMENTO

Martes 16 de septiembre, 11hs – AULA CHICA

PARTE I TÍTULO: "First-principles study of uniaxial strained and bent ZnO wires"

EXPOSITOR: Wolfram Hergert GIPSA-Lab, Department of Physics, Martin Luther University Halle-Wittenberg, Halle

Recent photoluminescence experiments by Dietrich et al. (Ref. [1]) investigated the relationship of an uniaxial stress state of bent ZnO microwires and the energetic shift of the free A-exciton energy (EA). The experiments allowed to determine the uniaxial stress effect on the the band gap width. In the literature very different experimental and theoretical results of the deformation potential parameter can be found. We investigated the variation of the electronic band gap of ZnO bulk and of ZnO nanowires under unixial strain by means of density functional theory thoroughly with special emphasis on the choice of the exchange-correlation functional and correlation corrections. We find that LDA, GGA and meta-GGA approximations to the exchange-correlation functional result in deformation potentials, which are in good agreement with experiment.[2] We investigated also nanowires much thinner than the microwires studied in [1]. Such wires respond with stronger changes of the band gap to applied strain. Bulk behavior is approached with increasing thickness of the wire.

References: [1] C.P. Dietrich, M. Lange, F.J. Klupfel, H. Von Wenckstern, R. Schmidt- Grund, M. Grundmann, Appl. Phys. Lett. 98, 031105 (2011).

[2] W.A. Adeagbo, S. Thomas, S. K. Nayak, A. Ernst, W. Hergert, Phys. Rev. B 89, 195135 (2014).

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PARTE II TÍTULO: "Ab initio study of the magnetic properties of Sr2FeMoO6 with defects "

EXPOSITOR: Martin Hoffman GIPSA-Lab, Department of Physics, Martin Luther University Halle-Wittenberg, Halle

In spintronic applications, there is a need for thin ferromagnetic halfmetallic films, which have as high Curie temperature, TC, as possible, to be usable at room temperature. Double perovskite Sr2FeMoO6 (SFMO) with TC > 400K is one of the interesting compounds for this use. Unfortunately, preparing good SFMO thin _lms has proven to be difficult. The main reasons for the difficulties are mixing of the Fe and Mo atoms (antisite disorder), oxygen non-stoichiometry and strain effects. We used first-principle calculations with the Korringa-Kohn-Rostoker Green function method to systematically investigate the electronic and magnetic properties. The Curie temperature can be obtained from calculated magnetic exchange interactions which are used in a Monte Carlo simulation. We applied self-interaction correction (SIC) to take into account the correlation effects and obtain the half-metallic nature [1]. Unfortunately, with SIC, the band gap in the majority spin channel ΔE was too high in comparison to the measurements [2, 3]. Therefore, we considered also DFT+U. Already for small values of U, SFMO became a half metal [4]. For an continuously increasing U parameter, the TC decreased strongly below the measured values for bulk SFMO. So, we had either a larger ΔE and a half metal or a TC comparable with experimental measurements. This issue raised the question, how large is the spin polarization of SFMO and how strongly it is affected by defects. A possible reason for the low values of TC and the change in spin polarization might be the appearance of defects like antisite disorder and oxygen vacancies. Therefore, we applied the coherent potential approximation where we obtained changes in the density of states at the Fermi level and changes for the calculated Curie temperature (up to 100 K).

References:

1 Z. Szotek et al., Physical Review B 68, 104411 (2003).

2 Y. Tomioka et al., Physical Review B 61, 422 (2000).

3 T. Saitoh et al., Physical Review B 66, 035112 (2002).

4 Ana B. Muñoz-García et al., Chemistry of Materials 23, 4525 (2011).