arXiv subjects

Silke Biermann

Publications and source records attributed to Silke Biermann.

At least 73 records · Page 4Linked to original sources

arXiv2012

Screening and Non-local Correlations in the Extended Hubbard Model from Self-Consistent Combined GW and Dynamical Mean Field Theory

We describe a recent implementation of the combined GW and dynamical mean field (DMFT) method "GW+DMFT" for the two-dimensional Hubbard model with on-site and nearest-neighbor repulsion. We clarify the relation of the GW+DMFT scheme to alternative approaches in the literature, and discuss the corresponding approximations to the free energy functional of the model. Furthermore, we describe a numerically exact technique for the self-consistent solution of the GW+DMFT equations, namely the hybridization expansion continuous-time algorithm for solving the dynamical impurity model that arises within the GW+DMFT scheme. We compute the low-temperature phase diagram of the extended Hubbard model, addressing the metal-insulator transition at small intersite interactions and the transition to a charge-ordered state for stronger intersite repulsions. Within the GW+DMFT framework, as in extended DMFT, the intersite repulsion translates into a frequency-dependence of the local effective interaction. We analyze this dependence by extracting a charateristic plasma frequency and show how it affects the local spectral function.

cond-mat.str-el↗

arXiv2012

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.

cond-mat.str-el↗

arXiv2011

Satellites and large doping- and temperature-dependence of electronic properties in hole-doped BaFe2As2

Over the last years, superconductivity has been discovered in several families of iron-based compounds. Despite intense research, even basic electronic properties of these materials, such as Fermi surfaces, effective electron masses, or orbital characters are still subject to debate. Here, we address an issue that has not been considered before, namely the consequences of dynamical screening of the Coulomb interactions among Fe-d electrons. We demonstrate its importance not only for correlation satellites seen in photoemission spectroscopy, but also for the low-energy electronic structure. From our analysis of the normal phase of BaFe2As2 emerges the picture of a strongly correlated compound with strongly doping- and temperature-dependent properties. In the hole overdoped regime, an incoherent metal is found, while Fermi-liquid behavior is recovered in the undoped compound. At optimal doping, the self-energy exhibits an unusual square-root energy dependence which leads to strong band renormalizations near the Fermi level.

cond-mat.str-el↗

arXiv2011

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.

cond-mat.str-el↗

arXiv2011

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

We discuss the notions of spin-orbital polarization and ordering in paramagnetic materials, and address their consequences in transition metal oxides. Extending the combined density functional and dynamical mean field theory scheme to the case of materials with large spin-orbit interactions, we investigate the electronic excitations of the paramagnetic phases of Sr2IrO4 and Sr2RhO4. We show that the interplay of spin-orbit interactions, structural distortions and Coulomb interactions suppresses spin-orbital fluctuations. As a result, the room temperature phase of Sr2IrO4 is a paramagnetic spin-orbitally ordered Mott insulator. In Sr2RhO4, the effective spin-orbital degeneracy is reduced, but the material remains metallic, due to both, smaller spin-orbit and smaller Coulomb interactions. We find excellent agreement of our ab-initio calculations for Sr2RhO4 with angle-resolved photoemission, and make predictions for spectra of the paramagnetic phase of Sr2IrO4.

cond-mat.str-el↗

arXiv2010

Theoretical evidence for strong correlations and incoherent metallic state in FeSe

The role of electronic Coulomb correlations in iron-based superconductors is an important open question. We provide theoretical evidence for strong correlation effects in the FeSe compound, based on dynamical mean field calculations. A lower Hubbard band is found in the spectral properties. Moreover, together with significant orbital-dependent mass enhancements, we find that the normal state is a bad metal over an extended temperature range, implying a non-Fermi liquid. Predictions for angle-resolved photoemission spectroscopy are made.

cond-mat.str-el↗

arXiv2009

Dynamical Mean-Field Theory within an Augmented Plane-Wave Framework: Assessing Electronic Correlations in the Iron Pnictide LaFeAsO

We present an approach that combines the local density approximation (LDA) and the dynamical mean-field theory (DMFT) in the framework of the full-potential linear augmented plane waves (FLAPW) method. Wannier-like functions for the correlated shell are constructed by projecting local orbitals onto a set of Bloch eigenstates located within a certain energy window. The screened Coulomb interaction and Hund's coupling are calculated from a first-principle constrained RPA scheme. We apply this LDA+DMFT implementation, in conjunction with continuous-time quantum Monte-Carlo, to study the electronic correlations in LaFeAsO. Our findings support the physical picture of a metal with intermediate correlations. The average value of the mass renormalization of the Fe 3d bands is about 1.6, in reasonable agreement with the picture inferred from photoemission experiments. The discrepancies between different LDA+DMFT calculations (all technically correct) which have been reported in the literature are shown to have two causes: i) the specific value of the interaction parameters used in these calculations and ii) the degree of localization of the Wannier orbitals chosen to represent the Fe 3d states, to which many-body terms are applied. The latter is a fundamental issue in the application of many-body calculations, such as DMFT, in a realistic setting. We provide strong evidence that the DMFT approximation is more accurate and more straightforward to implement when well-localized orbitals are constructed from a large energy window encompassing Fe-3d, As-4p and O-2p, and point out several difficulties associated with the use of extended Wannier functions associated with the low-energy iron bands. Some of these issues have important physical consequences, regarding in particular the sensitivity to the Hund's coupling.

cond-mat.str-el↗

arXiv2009

Optical Properties of Correlated Materials -- or Why Intelligent Windows may look Dirty

Materials with strong electronic Coulomb correlations play an increasing role in modern materials applications. "Thermochromic" systems, which exhibit thermally induced changes in their optical response, provide a particularly interesting case. The optical switching associated with the metal-insulator transition of vanadium dioxide, for example, has been proposed for use in numerous applications, ranging from anti-laser shields to "intelligent" windows, which selectively filter radiative heat in hot weather conditions. Are present-day electronic structure techniques able to describe, or -- eventually even predict -- such a kind of behavior ? How far are we from materials design using correlated oxides ? These are the central questions we try to address in this article. We review recent attempts of calculating optical properties of correlated materials within dynamical mean field theory, and summarize results for vanadium dioxide obtained within a novel scheme aiming at particularly simple and efficient calculations of optical transition matrix elements within localized basis sets. Finally, by optimizing the geometry of "intelligent windows", we argue that this kind of technique can in principle be used to provide guidance for experiments, thus giving a rather optimistic answer to the above questions.

cond-mat.str-el↗

arXiv2009

Optical properties of correlated materials -- Generalized Peierls approach and its application to VO2

The aim of the present paper is to present a versatile scheme for the computation of optical properties of solids, with particular emphasis on realistic many-body calculations for correlated materials. Geared at the use with localized basis sets, we extend the commonly known lattice "Peierls substitution" approach to the case of multi-atomic unit cells. We show in how far this generalization can be deployed as an approximation to the full Fermi velocity matrix elements that enter the continuum description of the response of a solid to incident light. We further devise an upfolding scheme to incorporate optical transitions, that involve high energy orbitals that had been downfolded in the underlying many-body calculation of the electronic structure. As an application of the scheme, we present results on a material of longstanding interest, vanadium dioxide, VO2. Using dynamical mean-field data of both, the metallic and the insulating phase, we calculate the corresponding optical conductivities, elucidate optical transitions and find good agreement with experimental results.

cond-mat.str-el↗

arXiv2008

Effective Band Structure of Correlated Materials - The Case of VO2

Vanadium dioxide, VO2, and its metal-insulator transition at T=340K continues to receive considerable interest. The question whether the physics of the insulating low-temperature phase is dominated by the Mott or the Peierls scenario, i.e. by correlation or band effects, is still under debate. A recent cluster dynamical mean field theory calculation [Biermann et al, Phys. Rev. Lett., 94, 026404 (2005)] suggests a combination of both effects, characterizing the transition as of a correlation assisted Peierls type. In this paper we present a detailed analysis of the excitation spectrum of the insulating M1 phase of VO2, based on this calculation. We implement a scheme to analytically continue self-energies from Matsubara to real frequencies, and study the physics of the strong interactions, as well as the corresponding changes with respect to the density functional theory (LDA) band structure. We find that in the M1 phase life-time effects are rather negligible, indeed allowing for an effective band structure description. A frequency independent but orbital dependent potential, constructed as an approximation to the full cluster dynamical mean field self-energy, turns out to satisfactory reproduce the fully interacting one-particle spectrum, acting as a scissors operator which pushes the a1g bonding and egp bands apart and, thus, opening the gap.

cond-mat.str-el↗

arXiv2008

Multi-orbital Effects in Optical Properties of Vanadium Sesquioxide

Vanadium sesquioxide, V2O3, boasts a rich phase diagram whose description necessitates the accounting for many-body Coulomb correlations. Spectral properties of this compound have been successfully addressed within dynamical mean field theory to an extent that results of recent angle resolved photoemission experiments have been correctly predicted. While photoemission spectroscopy probes the occupied part of the one-particle spectrum, optical experiments measure transitions into empty states and thus provide complementary information. In this work, we focus on the optical properties of V2O3 in its paramagnetic phases by employing our recently developed "generalized Peierls approach". Compared to experiments, we obtain results in overall satisfactory agreement. Further, we rationalize that the experimentally observed temperature dependence stems from the different coherence scales of the involved charge carriers.

cond-mat.str-el↗

arXiv2008

Materials Design using Correlated Oxides: Optical Properties of Vanadium Dioxide

Materials with strong electronic Coulomb interactions play an increasing role in modern materials applications. "Thermochromic" systems, which exhibit thermally induced changes in their optical response, provide a particularly interesting case. The optical switching associated with the metal-insulator transition of vanadium dioxide (VO2), for example, has been proposed for use in "intelligent" windows, which selectively filter radiative heat in hot weather conditions. In this work, we develop the theoretical tools for describing such a behavior. Using a novel scheme for the calculation of the optical conductivity of correlated materials, we obtain quantitative agreement with experiments for both phases of VO2. On the example of an optimized energy-saving window setup, we further demonstrate that theoretical materials design has now come into reach, even for the particularly challenging class of correlated electron systems.

cond-mat.str-el↗

arXiv2008

Bandwidth and Fermi surface of Iron-Oxypnictides: covalency and sensitivity to structural changes

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 LaOFeAs and LaOFeP. The width of the Fe-bands is significantly larger for LaOFeP, indicating a better metal and weaker electronic correlations. When calculated at their experimental crystal structure these two materials have significantly different low-energy band structure. The topology of the Fermi surface changes when going from LaOFeP to LaOFeAs, 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 LaOFeAs evolves towards that of LaOFeP 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 ReOFeAs and discuss the physical origin of this effect.

cond-mat.supr-con↗

arXiv2008

Effective band-structure in the insulating phase versus strong dynamical correlations in metallic VO2

Using a general analytical continuation scheme for cluster dynamical mean field calculations, we analyze real-frequency self-energies, momentum-resolved spectral functions, and one-particle excitations of the metallic and insulating phases of VO2. While for the former dynamical correlations and lifetime effects prevent a description in terms of quasi-particles, the excitations of the latter allow for an effective band-structure. We construct an orbital-dependent, but static one-particle potential that reproduces the full many-body spectrum. Yet, the ground state is well beyond a static one-particle description. The emerging picture gives a non-trivial answer to the decade-old question of the nature of the insulator, which we characterize as a ``many-body Peierls'' state.

cond-mat.str-el↗

arXiv2007

Spin-dependent Hedin's equations

Hedin's equations for the electron self-energy and the vertex were originally derived for a many-electron system with Coulomb interaction. In recent years it has been increasingly recognized that spin interactions can play a major role in determining physical properties of systems such as nanoscale magnets or of interfaces and surfaces. We derive a generalized set of Hedin's equations for quantum many-body systems containing spin interactions, e.g. spin-orbit and spin-spin interactions. The corresponding spin-dependent GW approximation is constructed.

cond-mat.str-el↗

arXiv2007

Competing itinerant and localized states in strongly correlated BaVS$_3$

The electronic structure of the quasi-lowdimensional vanadium sulfide \bavs3 is investigated for the different phases above the magnetic ordering temperature. By means of density functional theory and its combination with dynamical-mean field theory, we follow the evolution of the relevant low-energy electronic states on cooling. Hence we go in the metallic regime from the room temperature hexagonal phase to the orthorhombic phase after the first structural transition, and close with the monoclinic insulating phase below the metal-insulator transition. Due to the low symmetry and expected intersite correlations, the latter phase is treated within cellular dynamical mean-field theory. It is generally discussed how the intriguing interplay between band-structure and strong-correlation effects leads to the stabilization of the various electronic phases with decreasing temperature.

cond-mat.str-el↗

arXiv2006

Structure and optical properties of alpha- and gamma-cerium sesquisulfide

Structural and electronic properties of the alpha- and gamma-phases of cerium sesquisulfide, Ce2S3, are examined by first-principles calculations using the GGA+U extension of density functional theory. The strongly correlated f-electrons of Ce are described by a Hubbard-type on-site Coulomb repulsion parameter. A single parameter of $U^/prime$=4 eV yields excellent results for crystal structures, band gaps, and thermodynamic stability for both Ce2S3 allotropes. This approach gives insights in the difference in color of brownish-black alpha-Ce2S3 and dark red gamma-Ce2S3. The calculations predict that both Ce2S3 modifications are insulators with optical gaps of 0.8 eV (alpha-phase) and 1.8 eV (gamma-phase). The optical gaps are determined by direct electronic excitations at k=Gamma from localized and occupied Ce 4f-orbitals into empty Ce 5d-states. The f-states are situated between the valence and conduction bands. The difference of 1 eV between the optical gaps of the two Ce2S3 modifications is explained by different coordinations of the cerium cations by sulfur anions. For both Ce2S3 modifications the calculations yield an effective local magnetic moment of 2.6 $mu_B$ per cerium cation, which is in agreement with measurements. The electronic energy of the alpha-phase is computed to be 6 kJ/mol lower than that of the gamma-phase, which is consistent with the thermodynamic stability of the two allotropes.

cond-mat.mtrl-sci↗

arXiv2005

Interorbital charge transfers and Fermi-surface deformations in strongly correlated metals: models, BaVS$_3$ and Na$_{x}$CoO$_2$

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.

cond-mat.str-el↗