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Register a new userMagnetism of correlated systems: beyond LDA
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A novel approach to electronic correlations in magnetic crystals which takes into account a dynamical many-body effects is present. In order to to find a frequency dependence of the electron self energy, an effective quantum-impurity many-particle problem need to be solved within the dynamical mean-field theory. The numerically exact QMC-scheme and the spin-polarized fluctuation exchange approximation are used for the self-consistent solution of this single-site many-particle problem. The calculations of effective exchange interaction parameters based on the realistic electronic structure of correlated magnetic crystals have been discussed.
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Combining the density functional theory (DFT) and the Gutzwiller variational approach, a LDA+Gutzwiller method is developed to treat the correlated electron systems from {it ab-initio}. All variational parameters are self-consistently determined from total energy minimization. The method is computationally cheaper, yet the quasi-particle spectrum is well described through kinetic energy renormalization. It can be applied equally to the systems from weakly correlated metals to strongly correlated insulators. The calculated results for SrVO$_3$, Fe, Ni and NiO, show dramatic improvement over LDA and LDA+U.
We discuss a general approach to a realistic theory of the electronic structure in materials containing correlated d- or f- electrons. The main feature of this approach is the taking into account the energy dependence of the electron self-energy with the momentum dependence being neglected (local approximation). In the case of strong interactions (U/W>>1 - rare-earth system) the Hubbard-I approach is the most suitable. Starting from an exact atomic Green function with the constrained density matrix the band structure problem is formulated as the functional problem on Nmm for f-electrons and the standard LDA-functional for delocalized electrons. In the case of moderate correlations (U/W=1 metal-insulator regime) we start from the dynamical mean field iterative perturbation scheme (IPS) of G. Kotliar et. al. and also make use of our multiband atomic Green function. Finally for the weak interactions (U/W<1 -transition metals) the self-consistent diagrammatic fluctuation- exchange (FLEX)-approach of N. Bickers and D. Scalapino is generalized to the realistic multiband case. We presents two-band, two-dimensional model calculations for all three regimes. A realistic calculation in Hubbard-I scheme with the exact solution of the on-site multielectron problem for f(d)- shells was performed for mixed-valence 4f compound TmSe, and for the classical Mott insulator NiO.
We introduce in detail our newly developed textit{ab initio} LDA+Gutzwiller method, in which the Gutzwiller variational approach is naturally incorporated with the density functional theory (DFT) through the Gutzwiller density functional theory (GDFT) (which is a generalization of original Kohn-Sham formalism). This method can be used for ground state determination of electron systems ranging from weakly correlated metal to strongly correlated insulators with long-range ordering. We will show that its quality for ground state is as high as that by dynamic mean field theory (DMFT), and yet it is computationally much cheaper. In additions, the method is fully variational, the charge-density self-consistency can be naturally achieved, and the quantities, such as total energy, linear response, can be accurately obtained similar to LDA-type calculations. Applications on several typical systems are presented, and the characteristic aspects of this new method are clarified. The obtained results using LDA+Gutzwiller are in better agreement with existing experiments, suggesting significant improvements over LDA or LDA+U.
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.
A novel hybrid scheme is proposed. The {it ab initio} LDA calculation is used to construct the Wannier functions and obtain single electron and Coulomb parameters of the multiband Hubbard-type model. In strong correlation regime the electronic structure within multiband Hubbard model is calculated by the Generalized Tight-Binding (GTB) method, that combines the exact diagonalization of the model Hamiltonian for a small cluster (unit cell) with perturbation treatment of the intercluster hopping and interactions. For undoped La$_2$CuO$_4$ and Nd$_2$CuO$_4$ this scheme results in charge transfer insulators with correct values of gaps and dispersions of bands in agreement to the ARPES data.
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