Dynamical Screening Effects in Correlated Electron Materials -- A Progress Report on Combined Many-Body Perturbation and Dynamical Mean Field Theory: "GW+DMFT"

We give a summary of recent progress in the field of electronic structure calculations for materials with strong electronic Coulomb correlations. The discussion focuses on developments beyond the by now well established combination of density functional and dynamical mean field theory dubbed "LDA+DMFT". It is organized around the description of dynamical screening effects in the solid. Indeed, screening in the solid gives rise to dynamical local Coulomb interactions U(w) (Aryasetiawan et al 2004 Phys. Rev. B 70 195104), and this frequency-dependence leads to effects that cannot be neglected in a truly first principles description. We review the recently introduced extension of LDA+DMFT to dynamical local Coulomb interactions "LDA+U(w)+DMFT" (Casula et al. Phys. Rev. B 85 035115 (2012), Werner et al. Nature Phys. 8 331 (2012)). A reliable description of dynamical screening effects is also a central ingredient of the "GW+DMFT" scheme (Biermann et al. Phys. Rev. Lett. 90 086402 (2003)), a combination of many-body perturbation theory in Hedin's GW approximation and dynamical mean field theory. Recently, the first GW+DMFT calculations including dynamical screening effects for real materials have been achieved, with applications to SrVO3 (Tomczak et al. Europhys. Lett. 100 67001 (2012); Phys. Rev. B 90 165138 (2014)) and adatom systems on surfaces (Hansmann et al. Phys. Rev. Lett. 110 166401 (2013)). We review these and comment on further perspectives in the field. This review is an attempt to put elements of the original works (Refs. 1-11) into the broad perspective of the development of truly first principles techniques for correlated electron materials.Comment: 40 pages, 12 figures. First published as "Highlight of the Month" (June 2013), of the Psi-k Network on "Ab initio calculation of complex processes in materials", see http://www.psi-k.org/newsletters/News_117/Highlight_117.pd

Spectral Properties of Correlated Materials: Local Vertex and Non-Local Two-Particle Correlations from Combined GW and Dynamical Mean Field Theory

We present a fully self-consistent combined GW and dynamical mean field (GW+DMFT) study of the spectral properties of the extended two-dimensional Hubbard model. The inclusion of the local dynamical vertex stemming from the DMFT self-energy and polarization is shown to cure the problems of self-consistent GW in the description of spectral properties. We calculate the momentum-resolved spectral functions, the two-particle polarization and electron loss spectra, and show that the inclusion of GW in extended DMFT leads to a narrowing of the quasi-particle width and more pronounced Hubbard bands in the metallic regime as one approaches the charge-ordering transition. Finally, the momentum-dependence introduced by GW into the extended DMFT description of collective modes is found to affect their shape, giving rise to dispersive plasmon-like long-wavelength and stripe modes.Comment: 5 pages, 4 figure

Screened exchange dynamical mean field theory and its relation to density functional theory: SrVO3 and SrTiO3

We present the first application of a recently proposed electronic-structure scheme to transition metal oxides: screened exchange dynamical mean-field theory includes non-local exchange beyond the local density approximation and dynamical correlations beyond standard dynamical mean-field theory. Our results for the spectral function of SrVO3 are in agreement with the available experimental data, including photoemission spectroscopy and thermodynamics. Finally, the 3d0 compound SrTiO3 serves as a test case to illustrate how the theory reduces to the band structure of standard electronic-structure techniques for weakly correlated compounds.Comment: 6 pages, 4 figure

Neuartige Ansätze zur Behandlung des elektronischen Vielteilchenproblems in kondensierter Materie

The investigation of the properties of an interacting many-particle system isone of the key problems in modern solid state physics.In the first part of this work, a recently developed method for the descriptionof correlated electrons beyond the local density approximation is studied.The application to the $\textit{fcc}$-phase of Manganese ($\gamma$-Mn) shows that this material can be considered as a strongly correlated metal. This statement is inagreement with recent photoemission data.In the second part, the possibility of describing many-particle systemsby means of supersymmetric methods is investigated. A representation ofthe time evolution operator of an interacting many-particle system is derivedas a functional integral over supervectors. We discuss similarities to anddifferences from familiar functional integral representations as well as problemsthat hamper further developments of the theory

Interorbital charge transfers and Fermi-surface deformations in strongly correlated metals: models, BaVS3_3 and Nax_{x}CoO2_2

Fermi-surface deformations in strongly correlated metals, in comparison to results from band-structure calculations, are investigated. We show that correlation-induced interband charge transfers in multi-orbital systems may give rise to substantial modifications of the actual Fermi surface. Depending in particular on the relative strength of the crystal-field splitting and of the Hund's exchange coupling, correlations may either reinforce orbital polarization or tend to compensate differences in orbital occupancies, as demonstrated by investigating a 2-band Hubbard model in the framework of dynamical mean field theory (DMFT). The physical implications of such interorbital charge transfers are then explored in two case studies: BaVS$_3$ and Na$_x$CoO$_2$. By means of the DMFT in combination with the local density approximation (LDA) to density functional theory (DFT), new insights in the underlying mechanism of the metal-to-insulator transition (MIT) of BaVS$_3$ are obtained. A strong charge redistribution in comparison to LDA calculations, i.e., a depletion of the broader $A_{1g}$ band in favor of the narrower $E_g$ bands just above the MIT is found. In addition, the intriguing problem of determining the Fermi surface in the strongly correlated cobaltate system Na$_{x}$CoO$_2$ is discussed.Comment: final versio

Dynamical screening effects in correlated materials: plasmon satellites and spectral weight transfers from a Green's function ansatz to extended dynamical mean field theory

Dynamical screening of the Coulomb interactions in correlated electron systems results in a low-energy effective problem with a dynamical Hubbard interaction U(omega). We propose a Green's function ansatz for the Anderson impurity problem with retarded interactions, in which the Green's function factorizes into a contribution stemming from an effective static-U problem and a bosonic high-energy part introducing collective plasmon excitations. Our approach relies on the scale separation of the low-energy properties, related to the instantaneous static U, from the intermediate to high energy features originating from the retarded part of the interaction. We argue that for correlated materials where retarded interactions arise from downfolding higher-energy degrees of freedom, the characteristic frequencies are typically in the antiadiabatic regime. In this case, accurate approximations to the bosonic factor are relatively easy to construct, with the most simple being the boson factor of the dynamical atomic limit problem. We benchmark the quality of our method against numerically exact continuous time quantum Monte Carlo results for the Anderson-Holstein model both, at half- and quarter-filling. Furthermore we study the Mott transition within the Hubbard-Holstein model within extended dynamical mean field theory. Finally, we apply our technique to a realistic three-band Hamiltonian for SrVO3. We show that our approach reproduces both, the effective mass renormalization and the position of the lower Hubbard band by means of a dynamically screened U, previously determined ab initio within the constrained random phase approximation. Our approach could also be used within schemes beyond dynamical mean field theory, opening a quite general way of describing satellites and plasmon excitations in correlated materials.Comment: 13 pages, 11 figure