Elektronische und exzitonische Anregungen in magnetischen Isolatoren

Im Zuge der Entwicklung von elektronischen Bauelementen, die neben den räumlichen auch die Spinfreiheitsgrade der Elektronen ausnutzen, rücken magnetische Materialien verstärkt ins Blickfeld der Forschung. Die vorrangigen Ziele der theoretischen Festkörperphysik sind dabei das grundlegende Verständnis und die Vorhersage der Eigenschaften solcher spinpolarisierter Systeme. Die vorgelegte Dissertation befasst sich mit der Berechnung der elektronischen und optischen Anregungsspektren magnetischer Halbleiter und Isolatoren. Basierend auf dem Formalismus der Green'schen Funktionen bietet die Vielteilchen-Störungstheorie einen eleganten Zugang zu den Anregungseigenschaften von Festkörpern. Unerlässlich ist zunächst ein fundiertes Verständnis der elektronischen Struktur. Im Rahmen der Arbeit wird gezeigt, dass der auf der Hedin'schen GW-Approximation basierende Quasiteilchen-Ansatz auch auf magnetische Systeme mit stark lokalisierten d-Elektronen, wie die antiferromagnetischen Übergangsmetalloxide Mangan-, Eisen-, Cobalt- und Nickeloxid oder den ferromagnetischen Isolator Chrom-III-Bromid, anwendbar ist. Die Polarisationsfunktion und damit das optische Absorptionsspektrum kann durch Lösung einer Bethe-Salpeter-Gleichung berechnet werden. Dieser Ansatz berücksichtigt insbesondere auch exzitonische Effekte, welche aus der Coulomb-Anziehung von Elektron und Loch resultieren und einen bedeutenden Einfluss auf die optischen Absorptionseigenschaften haben. In der vorgelegten Arbeit wird der Formalismus derart verallgemeinert, dass auch magnetische Materialien behandelt werden können. Anschließend werden für die oben genannten prototypischen magnetischen Isolatoren die optischen Absorptionsspektren berechnet sowie die gebundenen exzitonischen Zustände innerhalb der fundamentalen Bandlücke untersucht

Low-energy electronic excitations and band-gap renormalization in CuO

Combining nonresonant inelastic x-ray scattering experiments with state-of-the-art ab initio many-body calculations, we investigate the electronic screening mechanisms in strongly correlated CuO in a large range of energy and momentum transfers. The excellent agreement between theory and experiment, including the low-energy charge excitations, allows us to use the calculated dynamical screening as a safe building block for many-body perturbation theory and to elucidate the crucial role played by d-d excitations in renormalizing the band gap of CuO. In this way we can dissect the contributions of different excitations to the electronic self-energy which is illuminating concerning both the general theory and this prototypical material.Combining nonresonant inelastic x-ray scattering experiments with state-of-the-art ab initio many-body calculations, we investigate the electronic screening mechanisms in strongly correlated CuO in a large range of energy and momentum transfers. The excellent agreement between theory and experiment, including the low-energy charge excitations, allows us to use the calculated dynamical screening as a safe building block for many-body perturbation theory and to elucidate the crucial role played by d-d excitations in renormalizing the band gap of CuO. In this way we can dissect the contributions of different excitations to the electronic self-energy which is illuminating concerning both the general theory and this prototypical material.Combining nonresonant inelastic x-ray scattering experiments with state-of-the-art ab initio many-body calculations, we investigate the electronic screening mechanisms in strongly correlated CuO in a large range of energy and momentum transfers. The excellent agreement between theory and experiment, including the low-energy charge excitations, allows us to use the calculated dynamical screening as a safe building block for many-body perturbation theory and to elucidate the crucial role played by d-d excitations in renormalizing the band gap of CuO. In this way we can dissect the contributions of different excitations to the electronic self-energy which is illuminating concerning both the general theory and this prototypical material.Peer reviewe

Photoemission spectra and effective masses of n- and p-type oxide semiconductors from first principles: ZnO, CdO, SnO2, MnO, and NiO

International audienceWhile there is a persistent interest in oxides, e.g., for semiconductor technology or optoelectronics, it seems to be difficult to achieve n-type and p-type doping for one and the same material. At the same time, it is important to understand the electronic structure for both types of doping individually. In this work, we use modern electronic-structure calculations to compute the density of states as well as effective electron and hole masses for n-type (ZnO, CdO, SnO2) and p-type (MnO, NiO) oxide materials. We establish our ab initio electronic structures by comparison to photoemission experiments at various incident photon energies. Taking into account the photoionization cross-sections, we are able to analyze the contributions of different atomic states and to verify the results by comparison to measured data. Based on these electronic structures, we calculate free-electron and free-hole masses as well as their dependence on the concentration of free carriers in the system. For SnO2, we compare with experimental results from another article (see M. Feneberg et al., Phys. Status Solidi A, DOI 10.1002/pssa.201330147 (2013) ) in this special issue. (C) 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinhei

Crystalline and magnetic anisotropy of the 3d-transition metal monoxidesMnO, FeO, CoO, and NiO

International audienc

Quasiparticle excitations in the photoemission spectrum of CuO from first principles: A GW study

International audienceWe present ab initio quasiparticle calculations for electronic excitations and the fundamental band gap of the strongly correlated transition-metal oxide CuO using the GW approximation of many-body perturbation theory. Problems related to the suitability of the method for strongly correlated materials and issues of self-consistency are addressed. We explain why quasiparticle self-consistent GW strongly overestimates the band gap of CuO. Apart from the band gap, electron addition and removal spectra in the quasiparticle approximation including lifetime and matrix-element effects are found to be in excellent agreement with the quasiparticle excitations in direct and inverse photoemission data

Crystalline and magnetic anisotropy of the 3d-transition metal monoxidesMnO, FeO, CoO, and NiO

International audienc

Ensemble averages of ab initio optical, transport, and thermoelectric properties of hexagonal Six_xGe1−x_{1-x} alloys

We present a comprehensive first-principles investigation of optical, transport, and thermoelectric properties of pure and doped hexagonal Si$_x$Ge$_{1-x}$ alloys based on density-functional theory calculations, the Boltzmann transport equation, and the generalized quasi-chemical approximation to obtain alloy averages of electronic properties. At low temperature, phase decomposition into the hexagonal elementary crystals is thermodynamically favored, but around and above room temperature random alloys are predicted to be stable. While hexagonal Si has an indirect band gap, the gap of hexagonal Ge is direct with very weak optical transitions at the absorption edge. The alloy band gap remains direct for a Si content below 45\,\% and the oscillator strength of the lowest optical transitions is efficiently enhanced by alloying. The optical spectra show clear trends and both absorption edges and prominent peaks can be tuned with composition. The dependence of transport coefficients on carrier concentration and temperature is similar in cubic and hexagonal alloys. However, the latter display anisotropic response due to the reduced hexagonal symmetry. In particular, the transport mass exhibits a significant directional dependence. Seebeck coefficients and thermoelectric power factors of $n$-doped alloys show non-monotonous variations with the Si content independently of temperature