arXiv subjects

Jan M. Tomczak

Publications and source records attributed to Jan M. Tomczak.

At least 55 records · Page 3Linked to original sources

arXiv2014

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.

cond-mat.str-el↗

arXiv2014

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.

cond-mat.str-el↗

arXiv2013

Asymmetric band widening by screened exchange competing with local correlations in SrVO3: new surprises on an old compound from combined GW and dynamical mean field theory GW+DMFT

The very first dynamical implementation of the combined GW and dynamical mean field scheme "GW+DMFT" for a real material was achieved recently [J.M. Tomczak et al., Europhys. Lett. 100 67001 (2012)], and applied to the ternary transition metal oxide SrVO3. Here, we review and extend that work, giving not only a detailed account of full GW+DMFT calculations, but also discussing and testing simplified approximate schemes. We give insights into the nature of exchange and correlation effects: Dynamical renormalizations in the Fermi liquid regime of SrVO3 are essentially local, and nonlocal correlations mainly act to screen the Fock exchange term. The latter substantially widens the quasi-particle band structure, while the band narrowing induced by the former is accompanied by a spectral weight transfer to higher energies. Most interestingly, the exchange broadening is much more pronounced in the unoccupied part of spectrum. As a result, the GW+DMFT electronic structure of SrVO3 resembles the conventional density functional based dynamical mean field (DFT+DMFT) description for occupied states, but is profoundly modified in the empty part. Our work leads to a reinterpretation of inverse photoemission spectroscopy (IPES) data. Indeed, we assign a prominent peak at about 2.7 eV dominantly to eg states, rather than to an upper Hubbard band of t2g character. Similar surprises can be expected for other transition metal oxides, calling for more detailed investigations of the conduction band states.

cond-mat.str-el↗

arXiv2013

Highly Dispersive Electron Relaxation and Colossal Thermoelectricity in the Correlated Semiconductor FeSb$_2$

We show that the colossal thermoelectric power, $S(T)$, observed in the correlated semiconductor FeSb$_2$ below 30\,K is accompanied by a huge Nernst coefficient $ν(T)$ and magnetoresistance MR$(T)$. Markedly, the latter two quantities are enhanced in a strikingly similar manner. While in the same temperature range, $S(T)$ of the reference compound FeAs$_2$, which has a seven-times larger energy gap, amounts to nearly half of that of FeSb$_2$, its $ν(T)$ and MR$(T)$ are intrinsically different to FeSb$_2$: they are smaller by two orders of magnitude and have no common features. With the charge transport of FeAs$_2$ successfully captured by the density functional theory, we emphasize a significantly dispersive electron-relaxation time $τ(ε_k)$ due to electron-electron correlations to be at the heart of the peculiar thermoelectricity and magnetoresistance of FeSb$_2$.

cond-mat.str-el↗

arXiv2013

Electronics with correlated oxides: SrVO$_3$/SrTiO$_3$ as a Mott transistor

We employ density functional theory plus dynamical mean field theory and identify the physical origin of why two layers of SrVO$_3$ on a SrTiO$_3$ substrate are insulating: the thin film geometry lifts the orbital degeneracy which in turn triggers a Mott-Hubbard transition. Two layers of SrVO$_3$ are just at the verge of a Mott-Hubbard transition and hence ideally suited for technological applications of the Mott-Hubbard transition: the heterostructure is highly sensitive to strain, electric field, and temperature. A gate voltage can also turn the insulator into a metal, so that a transistor with ideal on-off (metal-insulator) switching properties is realized.

cond-mat.str-el↗

arXiv2013

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.

cond-mat.str-el↗

arXiv2012

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.

cond-mat.str-el↗

arXiv2012

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.

cond-mat.str-el↗

arXiv2012

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.

cond-mat.str-el↗

arXiv2012

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.

cond-mat.str-el↗

arXiv2010

Realistic many-body models for Manganese Monoxide under pressure

In materials like transition metals oxides where electronic Coulomb correlations impede a description in terms of standard band-theories, the application of genuine many-body techniques is inevitable. Interfacing the realism of density-functional based methods with the virtues of Hubbard-like Hamiltonians, requires the joint ab initio construction of transfer integrals and interaction matrix elements (like the Hubbard U) in a localized basis set. In this work, we employ the scheme of maximally localized Wannier functions and the constrained random phase approximation to create effective low-energy models for Manganese monoxide, and track their evolution under external pressure. We find that in the low pressure antiferromagnetic phase, the compression results in an increase of the bare Coulomb interaction for specific orbitals. As we rationalized in recent model considerations [Phys. Rev. B 79, 235133 (2009)], this seemingly counter-intuitive behavior is a consequence of the delocalization of the respective Wannier functions. The change of screening processes does not alter this tendency, and thus, the screened on-site component of the interaction - the Hubbard U of the effective low-energy system - increases with pressure as well. The orbital anisotropy of the effects originates from the orientation of the orbitals vis-a-vis the deformation of the unit-cell. Within the high pressure paramagnetic phase, on the other hand, we find the significant increase of the Hubbard U is insensitive to the orbital orientation and almost exclusively owing to a substantial weakening of screening channels upon compression.

cond-mat.str-el↗

arXiv2010

Thermopower of correlated semiconductors : application to FeAs2 and FeSb2

We investigate the effect of electronic correlations onto the thermoelectricity of semi-conductors and insulators. Appealing to model considerations, we study various many-body renormalizations that enter the thermoelectric response. We find that, contrary to the case of correlated metals, correlation effects do not per se enhance the Seebeck coefficient or the figure of merit, for the former of which we give an upper bound in the limit of vanishing vertex corrections. For two materials of current interest, FeAs2 and FeSb2, we compute the electronic structure and thermopower. We find FeAs2 to be well described within density functional theory, and the therefrom deduced Seebeck coefficient to be in quantitative agreement with experiment. The capturing of the insulating ground state of FeSb2, however, requires the inclusion of many-body effects, in which we succeed by applying the GW approximation. Yet, while we get qualitative agreement for the thermopower of FeSb2 at intermediate temperatures, the tremendously large Seebeck coefficient at low temperatures is found to violate our upper bound, suggesting the presence of decisive (e.g. phonon mediated) vertex corrections.

cond-mat.mtrl-sci↗

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

Effective Coulomb interactions in solids under pressure

Correlated materials are extremely sensitive to external stimuli, such as temperature or pressure. Describing the electronic properties of such systems often requires applying many-body techniques to effective low energy problems in the spirit of the Hubbard model, or extensions thereof. While the effect of pressure on structures and bands has been investigated extensively within density-functional based methods, the pressure dependence of electron-electron interactions has so far received little attention. As a step toward ab initio pressure studies for realistic systems within a setup of maximally localized Wannier functions and the constrained random phase approximation, we examine in this paper the paradigmatic pressure dependence of Coulomb interactions. While compression commonly causes the "extension" of Wannier functions, and thus transfer elements, to grow, we find the -- seemingly counter-intuitive -- tendency that the bare Coulomb interaction increases under compression as well. We reconcile these behaviors by appealing to a semi-analytical tight-binding model. We moreover argue that, for this model, the requirement of maximal Wannier localization is equivalent to maximizing the Coulomb interaction matrix elements. We then apply the above first principles techniques to fcc hydrogen under pressure. While we find our comprehension of the bare Coulomb interaction confirmed, the induced changes in screening strengths lead to an effective one band model with a Hubbard interaction that is non-monotonous under pressure.

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↗