Silke Biermann

Dynamical singlets and correlation-assisted Peierls transition in VO2.

A theory of the metal-insulator transition in vanadium dioxide from the high- temperature rutile to the low- temperature monoclinic phase is proposed on the basis of cluster dynamical mean-field theory, in conjunction with the density functional scheme. The interplay of strong electronic Coulomb interactions and structural distortions, in particular, the dimerization of vanadium atoms in the low-temperature phase, plays a crucial role. We find that VO2 is not a conventional Mott insulator, but that the formation of dynamical V-V singlet pairs due to strong Coulomb correlations is necessary to trigger the opening of a Peierls gap.

Frequency-dependent local interactions and low-energy effective models from electronic structure calculations

We propose a systematic procedure for constructing effective models of strongly correlated materials. The parameters, in particular the on-site screened Coulomb interaction

Mott transition and suppression of orbital fluctuations in orthorhombic 3d(1) perovskites

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Bandwidth and Fermi surface of iron oxypnictides: Covalency and sensitivity to structural changes

, are calculated from first principles, using the random-phase approximation. We derive an expression for the frequency-dependent

Theoretical evidence for strong correlations and incoherent metallic state in FeSe

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Dynamical mean-field theory within an augmented plane-wave framework: Assessing electronic correlations in the iron pnictide LaFeAsO

and show, for the case of nickel, that its high-frequency part has significant influence on the spectral functions. We propose a scheme for taking into account the energy dependence of

Dynamical mean-field theory using Wannier functions: A flexible route to electronic structure calculations of strongly correlated materials

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Femtosecond dynamics of electronic states in the Mott insulator 1T-TaS2 by time resolved photoelectron spectroscopy

, so that a model with an energy-independent local interaction can still be used for low-energy properties.

Reduced effective spin-orbital degeneracy and spin-orbital ordering in paramagnetic transition-metal oxides: Sr2IrO4 versus Sr2RhO4.

Using t(2g) Wannier functions, a low-energy Hamiltonian is derived for orthorhombic 3d(1) transition-metal oxides. Electronic correlations are treated with a new implementation of dynamical mean-field theory for noncubic systems. Good agreement with photoemission data is obtained. The interplay of correlation effects and cation covalency (GdFeO3-type distortions) is found to suppress orbital fluctuations in LaTiO3 and even more in YTiO3, and to favor the transition to the insulating state.

The alpha-gamma transition of cerium is entropy driven.

Some important aspects of the electronic structure of the iron oxypnictides depend very sensitively on small changes in interatomic distances and bond angles within the iron-pnictogen subunit. Using first-principles full-potential electronic structure calculations, we investigate this sensitive dependence, contrasting in particular LaFeAsO and LaFePO. The width of the Fe bands is significantly larger for LaFePO, indicating a better metal and weaker electronic correlations. When calculated at their experimental crystal structures, these two materials have significantly different low-energy band structures. The topology of the Fermi surface changes when going from LaFePO to LaFeAsO, with a three-dimensional hole pocket present in the former case transforming into a tube with two-dimensional dispersion. We show that the low-energy band structure of LaFeAsO evolves toward that of LaFePO as the As atom is lowered closer to the Fe plane with respect to its experimental position. The physical origin of this sensitivity to the iron-pnictogen distance is the covalency of the iron-pnictogen bond, leading to strong hybridization effects. To illustrate this, we construct Wannier functions, which are found to have a large spatial extension when the energy window is restricted to the bands with dominant iron character. Finally, we show that the Fe bandwidth slightly increases as one moves along the rare-earth series in REFeAsO and we discuss the physical origin of this effect.