On the separability of dynamical and non-local correlations in three dimensions

While second-order phase transitions always cause strong non-local fluctuations, their effect on spectral properties crucially depends on the dimensionality. For the important case of three dimensions, we show that the electron self-energy is well separable into a local dynamical part and static non-local contributions. In particular, our non-perturbative many-body calculations for the 3D Hubbard model at different fillings demonstrate that the quasi-particle weight remains essentially momentum-independent, also in the presence of overall large non-local corrections to the self-energy. Relying on this insight we propose a space-time-separated scheme for many-body perturbation theory that is up to ten times more efficient than current implementations. Besides these far-reaching implications for state-of-the-art electronic structure schemes, our analysis will also provide guidance to the quest of going beyond them.

QSGW+DMFT: an electronic structure scheme for the iron pnictides and beyond

While in strongly correlated materials one often focuses on local electronic correlations, the influence of non-local exchange and correlation effects beyond band-theory can be pertinent in systems with more extended orbitals. Thus in many compounds an adequate theoretical description requires the joint treatment of local and non-local self-energies. Here, I will argue that this is the case for the iron pnictide and chalcogenide superconductors. As an approach to tackle their electronic structure, I will detail the implementation of the recently proposed scheme that combines the quasi-particle self-consistent GW approach with dynamical mean-field theory: QSGW+DMFT. I will showcase the possibilities of QSGW+DMFT with an application on BaFe2As2. Further, I will discuss the empirical finding that in pnictides dynamical and non-local correlation effects separate within the quasi-particle band-width.

Rare-Earth vs. Heavy Metal Pigments and their Colors from First Principles

Many inorganic pigments contain heavy metals hazardous to health and environment. Much attention has been devoted to the quest for non-toxic alternatives based on rare-earth elements. The computation of colors from first principles is a challenge to electronic structure methods however, especially for materials with localized f-orbitals. Here, starting from atomic positions only, we compute the color of the red pigment cerium fluorosulfide CeSF, as well as of mercury sulfide HgS (classic vermilion). Our methodology employs many-body theories to compute the optical absorption, combined with an intermediate length-scale modelization to assess how coloration depends on film thickness, pigment concentration and granularity. We introduce a quantitative criterion for the performance of a pigment. While for HgS this criterion is satisfied due to large transition matrix elements between wide bands, CeSF presents an alternative paradigm: the bright red color is shown to stem from the combined effect of the quasi two-dimensionality and the localized nature of 4f-states. Our work demonstrates the power of modern computational methods, with implications for the theoretical design of materials with specific optical properties.

Combined GW and dynamical mean field theory: Dynamical screening effects in transition metal oxides

We present the first dynamical implementation of the combined GW and dynamical mean field scheme (GW+DMFT) for first principles calculations of the electronic properties of correlated materials. The application to the ternary transition metal oxide SrVO3 demonstrates that this schemes inherits the virtues of its two parent theories: a good description of the local low energy correlation physics encoded in a renormalized quasi-particle band structure, spectral weight transfer to Hubbard bands, and the physics of screening driven by long-range Coulomb interactions. Our data is in good agreement with available photoemission and inverse photoemission spectra; our analysis leads to a reinterpretation of the commonly accepted three-peak structure as originating from orbital effects rather than from the electron addition peak within the t2g manifold.

Thermopower of the Correlated Narrow Gap Semiconductor FeSi and Comparison to RuSi

Iron based narrow gap semiconductors such as FeSi, FeSb2, or FeGa3 have received a lot of attention because they exhibit a large thermopower, as well as striking similarities to heavy fermion Kondo insulators. Many proposals have been advanced, however, lacking quantitative methodologies applied to this problem, a consensus remained elusive to date. Here, we employ realistic many-body calculations to elucidate the impact of electronic correlation effects on FeSi. Our methodology accounts for all substantial anomalies observed in FeSi: the metallization, the lack of conservation of spectral weight in optical spectroscopy, and the Curie susceptibility. In particular we find a very good agreement for the anomalous thermoelectric power. Validated by this congruence with experiment, we further discuss a new physical picture of the microscopic nature of the insulator-to-metal crossover. Indeed, we find the suppression of the Seebeck coefficient to be driven by correlation induced incoherence. Finally, we compare FeSi to its iso-structural and iso-electronic homologue RuSi, and predict that partially substituted Fe(1-x)Ru(x)Si will exhibit an increased thermopower at intermediate temperatures.

Many-body effects in iron pnictides and chalcogenides -- non-local vs dynamic origin of effective masses

We apply the quasi-particle self-consistent GW (QSGW) approximation to some of the iron pnictide and chalcogenide superconductors. We compute Fermi surfaces and density of states, and find excellent agreement with experiment, substantially improving over standard band-structure methods. Analyzing the QSGW self-energy we discuss non-local and dynamic contributions to effective masses. We present evidence that the two contributions are mostly separable, since the quasi-particle weight is found to be essentially independent of momentum. The main effect of non locality is captured by the static but non-local QSGW effective potential. Moreover, these non-local self-energy corrections, absent in e.g. dynamical mean field theory (DMFT), can be relatively large. We show, on the other hand, that QSGW only partially accounts for dynamic renormalizations at low energies. These findings suggest that QSGW combined with DMFT will capture most of the many-body physics in the iron pnictides and chalcogenides.

Signatures of electronic correlations in iron silicide

The intermetallic FeSi exhibits an unusual temperature dependence in its electronic and magnetic degrees of freedom, epitomized by the crossover from a low temperature non-magnetic semiconductor to a high temperature paramagnetic metal with a Curie-Weiss like susceptibility. Many proposals for this unconventional behavior have been advanced, yet a consensus remains elusive. Using realistic many-body calculations, we here reproduce the signatures of the metal-insulator crossover in various observables: the spectral function, the optical conductivity, the spin susceptibility, and the Seebeck coefficient. Validated by quantitative agreement with experiment, we then address the underlying microscopic picture. We propose a new scenario in which FeSi is a band-insulator at low temperatures and is metalized with increasing temperature through correlation induced incoherence. We explain that the emergent incoherence is linked to the unlocking of iron fluctuating moments which are almost temperature independent at short time scales. Finally, we make explicit suggestions for improving the thermoelectric performance of FeSi based systems.