[en] We present an ab initio quantum theory of the finite-temperature magnetism of iron and nickel. A recently developed technique which combines dynamical mean-field theory with realistic electronic structure methods successfully describes the many-body features of the one electron spectra and the observed magnetic moments below and above the Curie temperature

[en] Starting from an exact expression for the dynamical spin susceptibility in the time-dependent density functional theory, a controversial issue regarding exchange interaction parameters and spin-wave excitation spectra of itinerant electron ferromagnets is reconsidered. It is shown that the original expressions for exchange integrals based on the magnetic force theorem (Liechtenstein et al 1984 J. Phys. F: Met. Phys. 14 L125) are optimal for calculations of the magnon spectrum, whereas the static response function is better described using the 'renormalized' magnetic force theorem given by Bruno (2003 Phys. Rev. Lett. 90 087205). This conclusion is confirmed by ab initio calculations for Fe and Ni

[en] The realistic description of correlated electron systems took an important step forward a few years ago as the combination of density functional methods and dynamical mean-field theory was conceived. This framework allows access to both high and low energy physics and is capable of the description of the specific physics of strongly correlated materials, like the Mott metal-insulator transition. A very important step in the procedure is the interface between the band structure method and the dynamical mean-field theory and its impurity solver. We present a general interface between a projector augmented-wave-based density functional code and many-body methods based on Wannier functions obtained from a projection on local orbitals. The implementation is very flexible and allows for various applications. Quantities like the momentum-resolved spectral function are accessible. We present applications to SrVO₃ and the metal-insulator transition in Ca_2-xSr_xRuO₄.

[en] A general multiband formulation of the linear and nonlinear optical response functions for realistic models of correlated crystals is presented. Dipole-forbidden d-d optical transitions originate from vertex functions, which we consider assuming the locality of an irreducible four-leg vertex. The unified formulation for second- and third-order response functions in terms of the three-leg vertex is suitable for practical calculations in solids. We illustrate the general approach by consideration of intra-atomic spin-flip contributions, with an energy of 2J, where J is a Hund exchange, in the simplest two-orbital model. (fast track communication)

[en] The unusual electronic structure of the half-metallic ferromagnets (HMF) is analysed taking account of correlation effects (electron-magnon interaction, in particular spin-polaron effects). Special attention is paid to the so-called non-quasiparticle (NQP) (incoherent) states which arise in the minority- (majority-) spin gap above (below) the Fermi level and which may make considerable contributions to the electronic properties. First-principles calculations of the NQP states in HMF within the local-density approximation plus dynamical mean field theory (LDA+DMFT) are reviewed. These states can be probed, in particular, by spin-polarized scanning tunnelling microscopy. They also lead to observable effects in core-hole spectroscopy, nuclear magnetic relaxation and contribute to the temperature dependence of impurity resistivity, etc. The peculiarities of the transport properties of three- and two-dimensional HMF are discussed, which are connected with the absence of one-magnon spin-flip scattering processes

[en] In this paper we review recent developments towards a realistic description of the electronic structure and magnetism of correlated nanosystems. A new class of so-called continuous-time solvers for the quantum impurity problem is discussed, which provides a numerically exact solution without systematic errors due to imaginary time discretization. These solvers are able to handle general interactions, like the full Coulomb vertex. We further show how four-point or higher-order correlation functions of the impurity problem can be computed. This allows the calculation of dynamical susceptibilities which provide information about spin excitations. Moreover, we discuss a principally new many-body scheme recently proposed for the description of non-local correlations in strongly correlated systems. This approach provides a basis for a many-body description of extended correlated nanostructures on a substrate.

[en] The spectroscopic properties of elemental terbium, dysprosium, holmium, and erbium are investigated using first-principles calculations taking into account intra-atomic correlation effects. In order to describe the strongly localized f electrons together with the conduction bands, we have used the multiband Hubbard-I approximation to reproduce the multiplet features present in the experimental spectra. A comparison with available experimental data is made and the overall agreement is found to be good

[en] The pure Fano effect in angle-integrated valence-band photoemission of ferromagnets has been observed for the first time. A contribution of the intrinsic spin polarization to the spin polarization of the photoelectrons has been avoided by an appropriate choice of the experimental parameters. The theoretical description of the resulting spectra reveals a complete analogy to the Fano effect observed before for paramagnetic transition metals. While the theoretical photocurrent and spin-difference spectra are found in good quantitative agreement with experiment in the case of Fe and Co, only a qualitative agreement could be achieved in the case of Ni by calculations on the basis of plain local spin-density approximation. Agreement with experimental data could be improved in this case in a very substantial way by a treatment of correlation effects on the basis of dynamical mean field theory

[en] An intrinsic issue of the LDA + DMFT approach is the so called double counting of interaction terms. How to choose the double-counting potential in a manner that is both physically sound and consistent is unknown. We have conducted an extensive study of the charge-transfer system NiO in the LDA + DMFT framework using quantum Monte Carlo and exact diagonalization as impurity solvers. By explicitly treating the double-counting correction as an adjustable parameter we systematically investigated the effects of different choices for the double counting on the spectral function. Different methods for fixing the double counting can drive the result from Mott insulating to almost metallic. We propose a reasonable scheme for the determination of double-counting corrections for insulating systems.

[en] The influence of intrinsic defects in half-metals is calculated in the case of NiMnSb. Of the 14 cases of intrinsic defects, five affect the half-metallic properties. They are energetically very unlikely to occur. Circumstances are discussed under which defects may even have a beneficial effect on the spin polarization of the conduction electrons. Non-intrinsic defects, like deliberate doping by rare-earth atoms, as well as the effect of nano-structured contacts may influence the magnon spectrum, improving the behaviour at finite temperature