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To explore correlated electrons in the presence of local and non-local disorder, the Blackman-Esterling-Berk method for averaging over off-diagonal disorder is implemented into dynamical mean-field theory using tensor notation. The impurity model combining disorder and correlations is solved using the recently developed fork tensor-product state solver, which allows one to calculate the single particle spectral functions on the real-frequency axis. In the absence of off-diagonal hopping, we establish exact bounds of the spectral function of the non-interacting Bethe lattice with coordination number $Z$. In the presence of interaction, the Mott insulating paramagnetic phase of the one-band Hubbard model is computed at zero temperature in alloys with site- and off-diagonal disorder. When the Hubbard $U$ parameter is increased, transitions from an alloy band-insulator through a correlated metal into a Mott insulating phase are found to take place., Comment: 15 pages, 5 figures. See ancillary folder for numerical data and computational details

We compute the phonon dispersion, density of states, and the Gr\"uneisen parameters of bulk palladium in the combined density functional theory (DFT) and dynamical mean-field theory (DMFT). We find good agreement with experimental results for ground state properties (equilibrium lattice parameter and bulk modulus) and the experimentally measured phonon spectra. We demonstrate that at temperatures $T \lesssim 20~K$ the phonon frequency in the vicinity of the Kohn anomaly, $\omega_{T1}({\bf q}_{K})$, strongly decreases. This is in contrast to DFT where this frequency remains essentially constant in the whole temperature range. Apparently correlation effects reduce the restoring force of the ionic displacements at low temperatures, leading to a mode softening., Comment: minor revisions

Stable organic radicals integrated into molecular junctions represent a practical realization of the single-orbital Anderson impurity model. Motivated by recent experiments for perchlorotriphenylmethyl (PTM) molecules contacted to gold electrodes, we develop a method that combines density functional theory (DFT), quantum transport theory, numerical renormalization group (NRG) calculations and renormalized super-perturbation theory (rSPT) to compute both equilibrium and non-equilibrium properties of strongly correlated nanoscale systems at low temperatures effectively from first principles. We determine the possible atomic structures of the interfaces between the molecule and the electrodes, which allow us to estimate the Kondo temperature and the characteristic transport properties, which compare well with experiments. By using the non-equilibrium rSPT results we assess the range of validity of equilibrium DFT+NRG-based transmission calculations for the evaluation of the finite voltage conductance. The results demonstrate that our method can provide qualitative insights into the properties of molecular junctions when the molecule-metal contacts are amorphous or generally ill-defined, and that it can further give a fully quantitative description when the experimental contact structures are well characterized., Comment: Nanoscale, 2018

We formulate real-space dynamical mean-field theory within scattering theory. Thereby the Friedel sum rule is derived for interacting lattice fermions at zero temperature., Comment: 7 pages, no figures, extended and corrected version

We report a detailed theoretical study of the electronic structure, spectral properties, and lattice parameters of bulk FeSe under pressure using a fully charge self-consistent implementation of the density functional theory plus dynamical mean-field theory method (DFT+DMFT). In particular, we perform a structural optimization and compute the evolution of the lattice parameters (volume, $c/a$ ratio, and the internal $z$ position of Se) and the electronic structure of the tetragonal (space group $P4/nmm$) paramagnetic FeSe. Our results for the lattice parameters are in good quantitative agreement with experiment. The $c/a$ ratio is slightly overestimated by about $3$~\%, presumably due to the absence of the van der Waals interactions between the FeSe layers in our calculations. The lattice parameters determined within DFT are off the experimental values by a remarkable $\sim$$6$-$15$~\%, implying a crucial importance of electron correlations. Upon compression to $10$~GPa, the $c/a$ ratio and the lattice volume show a decrease by $2$ and $10$~\%, respectively, while the Se $z$ coordinate weakly increases by $\sim$$2$~\%. Most importantly, our results reveal a topological change of the Fermi surface (Lifshitz transition) which is accompanied by a two- to three-dimensional crossover. Our results indicate a small reduction of the quasiparticle mass renormalization $m^*/m$ by about $5$~\% for the $e$ and less than $1$~\% for the $t_2$ states, as compared to ambient pressure. The behavior of the momentum-resolved magnetic susceptibility $\chi({\bf q})$ shows no topological changes of magnetic correlations under pressure, but demonstrates a reduction of the degree of the in-plane $(\pi,\pi)$ stripe-type nesting. Our results for the electronic structure and lattice parameters of FeSe are in good qualitative agreement with recent experiments on its isoelectronic counterpart FeSe$_{1-x}$S$_x$., Comment: 10 pages, 6 figures

Materials with correlated electrons often respond very strongly to external or internal influences, leading to instabilities and states of matter with broken symmetry. This behavior can be studied theoretically either by evaluating the linear response characteristics, or by simulating the ordered phases of the materials under investigation. We developed the necessary tools within the dynamical mean-field theory (DMFT) to search for electronic instabilities in materials close to spin-state crossovers and to analyze the properties of the corresponding ordered states. This investigation, motivated by the physics of LaCoO$_3$, led to a discovery of condensation of spinful excitons in the two-orbital Hubbard model with a surprisingly rich phase diagram. The results are reviewed in the first part of the article. Electronic correlations can also be the driving force behind structural transformations of materials. To be able to investigate correlation-induced phase instabilities we developed and implemented a formalism for the computation of total energies and forces within a fully charge self-consistent combination of density functional theory and DMFT. Applications of this scheme to the study of structural instabilities of selected correlated electron materials such as Fe and FeSe are reviewed in the second part of the paper., Comment: 34 pages, 11 figures

We present results of a detailed theoretical study of the electronic, magnetic, and structural properties of the chalcogenide parent system FeSe using a fully charge self-consistent implementation of the density functional theory plus dynamical mean-field theory (DFT+DMFT) method. In particular, we predict a remarkable change of the electronic structure of FeSe which is accompanied by a complete reconstruction of the Fermi surface topology (Lifshitz transition) upon a moderate expansion of the lattice volume. The phase transition results in a change of the in-plane magnetic nesting wave vector from $(\pi,\pi)$ to $(\pi,0)$ and is associated with a transition from itinerant to orbital-selective localized magnetic moments. We attribute this behavior to a correlation-induced shift of the van Hove singularity of the Fe $t_{2}$ bands at the M-point across the Fermi level. Our results reveal a strong orbital-selective renormalization of the effective mass $m^*/m$ of the Fe $3d$ electrons upon expansion. The largest effect occurs in the Fe $xy$ orbital, which gives rise to a non-Fermi-liquid-like behavior above the transition. The behavior of the momentum-resolved magnetic susceptibility $\chi({\bf q})$ demonstrates that magnetic correlations are also characterized by a pronounced orbital selectivity, suggesting a spin-fluctuation origin of the nematic phase of paramagnetic FeSe. We conjecture that the anomalous behavior of FeSe upon expansion is associated with the proximity of the Fe $t_{2}$ van Hove singularity to the Fermi level and the sensitive dependence of its position on external conditions., Comment: 9 pages, 9 figures

The magnetic phases induced by the interplay between disorder acting only on particles with a given spin projection ("spin-dependent disorder") and a local repulsive interaction is explored. To this end the magnetic ground state phase diagram of the Hubbard model at half-filling is computed within dynamical mean-field theory combined with the geometric average over disorder, which is able to describe Anderson localization. Five distinct phases are identified: a ferromagnetically polarized metal, two types of insulators, and two types of spin-selective localized phases. The latter four phases possess different long-range order of the spins. The predicted phase diagram may be tested experimentally using cold fermions in optical lattices subject to spin-dependent random potentials., Comment: 8 pages, 9 figures, revised version

The interplay between local, repulsive interactions and disorder acting only on one spin orientation of lattice fermions ("spin-dependent disorder") is investigated. The nonmagnetic disorder vs. interaction phase diagram is computed using Dynamical Mean-Field Theory in combination with the geometric average over disorder. The latter determines the typical local density of states and is therefore sensitive to Anderson localization. The effect of spin-dependent disorder is found to be very different from that of conventional disorder. In particular, it destabilizes the metallic solution and leads to a novel spin-selective, localized phase at weak interactions and strong disorder.

The electronic structure and phase stability of paramagnetic FeSe is computed by using a combination of ab initio methods for calculating band structure and dynamical mean-field theory. Our results reveal a topological change (Lifshitz transition) of the Fermi surface upon a moderate expansion of the lattice. The Lifshitz transition is accompanied with a sharp increase of the local moments and results in an entire reconstruction of magnetic correlations from the in-plane magnetic wave vector $(\pi,\pi)$ to $(\pi,0)$. We attribute this behavior to a correlation-induced shift of the Van Hove singularity originating from the $d_{xy}$ and $d_{xz}/d_{yz}$ bands at the M-point across the Fermi level. We propose that superconductivity is strongly influenced, or even induced, by a Van Hove singularity., Comment: 5 pages, 4 figures

We determine the electronic structure and phase stability of paramagnetic V$_2$O$_3$ at the Mott-Hubbard metal-insulator phase transition, by employing a combination of an ab initio method for calculating band structures with dynamical mean-field theory. The structural transformation associated with the metal-insulator transition is found to occur upon a slight expansion of the lattice volume by $\sim 1.5$ %, in agreement with experiment. Our results show that the structural change precedes the metal-insulator transition, implying a complex interplay between electronic and lattice degrees of freedom at the transition. Electronic correlations and full charge self-consistency are found to be crucial for a correct description of the properties of V$_2$O$_3$., Comment: 5 pages, 4 figures

We present theoretical results on the high-temperature phase stability and phonon spectra of paramagnetic bcc iron which explicitly take into account many-body effects. Several peculiarities, including a pronounced softening of the [110] transverse (T$_1$) mode and a dynamical instability of the bcc lattice in harmonic approximation are identified. We relate these features to the ${\alpha}$-to-${\gamma}$ and ${\gamma}$-to-${\delta}$ phase transformations in iron. The high-temperature bcc phase is found to be highly anharmonic and appears to be stabilized by the lattice entropy., Comment: 8 pages, 3 figures

The temperature dependence of the paramagnetic susceptibility of the iron pnictide superconductor KFe2As2 and its connection with the spectral properties of that material is investigated by a combination of density functional theory (DFT) in the local density approximation and dynamical mean-field theory (DMFT). Unlike other iron pnictide parent compounds where the typical oxidation state of iron is 2, the formal valence of Fe in KFe2As2 is 2.5, corresponding to an effective doping with 0.5 hole per iron atom compared to, for example, BaFe2As2. This shifts the chemical potential and thereby reduces the distance between the peaks in the spectral functions of KFe2As2 and the Fermi energy as compared to BaFe2As2. The shift, which is clearly seen on the level of DFT as well as in DMFT, is further enhanced by the strong electronic correlations in KFe2As2. In BaFe2As2 the presence of these peaks results [Phys. Rev. B 86, 125124 (2012)] in a temperature increase of the susceptibility up to a maximum at ~1000 K. While the temperature increase was observed experimentally the decrease at even higher temperatures is outside the range of experimental observability. We predict that in KFe2As2 the situation is different. Namely, the reduction of the distance between the peaks and the Fermi level due to doping is expected to shift the maximum in the susceptibility to much lower temperatures, such that the decrease of the susceptibility should become visible in experiment., Comment: 5 pages, 3 figures

We introduce a novel computational approach for the investigation of complex correlated electron materials which makes it possible to evaluate interatomic forces and thereby determine atomic displacements and structural transformations induced by electronic correlations. It combines \textit{ab initio} band structure and dynamical mean-field theory and is implemented with the linear-response formalism regarding atomic displacements. We apply this new technique to explore structural transitions of prototypical correlated systems such as elemental hydrogen, SrVO$_3$, and KCuF$_3$., Comment: 5 pages, 3 figures

We report on a hierarchy of temperatures Tc < Tgap < Tmet in superconducting Rb1-xFe2-ySe2 observed by THz spectroscopy. Above Tmet = 90 K the material reveals semiconducting characteristics. Below Tmet a coherent metallic THz response emerges. This metal-to-insulator type, orbital selective transition is clearly indicated by an isosbestic point in the temperature dependence of the optical conductivity and dielectric constant at THz-frequencies. At Tgap = 61 K a gap opens in the THz regime and then the superconducting transition occurs at Tc = 32 K. This sequence of temperatures seems to reflect a corresponding hierarchy of the electronic correlations in the different bands.

Motivated by the rapidly growing possibilities for experiments with ultracold atoms in optical lattices we investigate the thermodynamic properties of correlated lattice fermions in the presence of an external spin-dependent random potential. The corresponding model, a Hubbard model with spin-dependent local random potentials, is solved within dynamical mean-field theory. This allows us to present a comprehensive picture of the thermodynamic properties of this system. In particular, we show that for a fixed total number of fermions spin-dependent disorder induces a magnetic polarization. The magnetic response of the polarized system differs from that of a system with conventional disorder., Comment: 20 pages, 11 figures, published version

We analyze the sharpness of crossing ("isosbestic") points of a family of curves which are observed in many quantities described by a function f(x,p), where x is a variable (e.g., the frequency) and p a parameter (e.g., the temperature). We show that if a narrow crossing region is observed near x* for a range of parameters p, then f(x,p) can be approximated by a perturbative expression in p for a wide range of x. This allows us, e.g., to extract the temperature dependence of several experimentally obtained quantities, such as the Raman response of HgBa2CuO4+delta, photoemission spectra of thin VO2 films, and the reflectivity of CaCu3Ti4O12, all of which exhibit narrow crossing regions near certain frequencies. We also explain the sharpness of isosbestic points in the optical conductivity of the Falicov-Kimball model and the spectral function of the Hubbard model., Comment: 12 pages, 11 figures

Employing a combination of \emph{ab initio} band structure theory and dynamical mean-field theory we explain the experimentally observed linear temperature increase of the magnetic susceptibility of the iron pnictide material BaFe$_{2}$As$_{2}$. The microscopic origin of this anomalous behaviour is traced to a sharp peak in the spectral function located approximately 100 meV below the Fermi level. This peak is due to the weak dispersion of two-dimensional bands associated with the layered crystal structure of pnictides., Comment: 8 pages, 10 figures

We introduce a well-defined and unbiased measure of the strength of correlations in quantum many-particle systems which is based on the relative von Neumann entropy computed from the density operator of correlated and uncorrelated states. The usefulness of this general concept is demonstrated by quantifying correlations of interacting electrons in the Hubbard model and in a series of transition-metal oxides using dynamical mean-field theory., Comment: 6 pages, 3 figures; Final version including correction of typographical errors and new reference

The dynamical mean-field theory (DMFT) is a widely applicable approximation scheme for the investigation of correlated quantum many-particle systems on a lattice, e.g., electrons in solids and cold atoms in optical lattices. In particular, the combination of the DMFT with conventional methods for the calculation of electronic band structures has led to a powerful numerical approach which allows one to explore the properties of correlated materials. In this introductory article we discuss the foundations of the DMFT, derive the underlying self-consistency equations, and present several applications which have provided important insights into the properties of correlated matter., Comment: Chapter in "Theoretical Methods for Strongly Correlated Systems", edited by A. Avella and F. Mancini, Springer (2011), 31 pages, 5 figures

We compute the equilibrium crystal structure and phase stability of iron at the alpha(bcc)-gamma(fcc) phase transition as a function of temperature, by employing a combination of ab initio methods for calculating electronic band structures and dynamical mean-field theory. The magnetic correlation energy is found to be an essential driving force behind the alpha-gamma structural phase transition in paramagnetic iron., Comment: 4 pages, 3 figures

In their preprint Anders et al. [arXiv:1004.0510] propose a crucial modification of the Bosonic Dynamical Mean-Field Theory (B-DMFT) derived by us [Phys. Rev. B 77, 235106 (2008)]. Here we show that the modification consists of two steps which, in fact, cancel each other. Consequently their self-consistency equations are identical to ours., Comment: 2 pages, no figures

We review recent results on the properties of materials with correlated electrons obtained within the LDA+DMFT approach, a combination of a conventional band structure approach based on the local density approximation (LDA) and the dynamical mean-field theory (DMFT). The application to four outstanding problems in this field is discussed: (i) we compute the full valence band structure of the charge-transfer insulator NiO by explicitly including the p-d hybridization, (ii) we explain the origin for the simultaneously occuring metal-insulator transition and collapse of the magnetic moment in MnO and Fe2O3, (iii) we describe a novel GGA+DMFT scheme in terms of plane-wave pseudopotentials which allows us to compute the orbital order and cooperative Jahn-Teller distortion in KCuF3 and LaMnO3, and (iv) we provide a general explanation for the appearance of kinks in the effective dispersion of correlated electrons in systems with a pronounced three-peak spectral function without having to resort to the coupling of electrons to bosonic excitations. These results provide a considerable progress in the fully microscopic investigations of correlated electron materials., Comment: 24 pages, 14 figures, final version, submitted to Eur. Phys. J. for publication in the Special Topics volume "Cooperative Phenomena in Solids: Metal-Insulator Transitions and Ordering of Microscopic Degrees of Freedom"

Several new aspects of the subtle interplay between electronic correlations and disorder are reviewed. First, the dynamical mean-field theory (DMFT)together with the geometrically averaged ("typical") local density of states is employed to compute the ground state phase diagram of the Anderson-Hubbard model at half-filling. This non-perturbative approach is sensitive to Anderson localization on the one-particle level and hence can detect correlated metallic, Mott insulating and Anderson insulating phases and can also describe the competition between Anderson localization and antiferromagnetism. Second, we investigate the effect of binary alloy disorder on ferromagnetism in materials with $f$-electrons described by the periodic Anderson model. A drastic enhancement of the Curie temperature $T_c$ caused by an increase of the local $f$-moments in the presence of disordered conduction electrons is discovered and explained., Comment: 17 pages, 7 figures, final version, typos corrected, references updated, submitted to Eur. Phys. J. for publication in the Special Topics volume "Cooperative Phenomena in Solids: Metal-Insulator Transitions and Ordering of Microscopic Degrees of Freedom"

We review recent progress in our theoretical understanding of strongly correlated fermion systems in the presence of disorder. Results were obtained by the application of a powerful nonperturbative approach, the Dynamical Mean-Field Theory (DMFT), to interacting disordered lattice fermions. In particular, we demonstrate that DMFT combined with geometric averaging over disorder can capture Anderson localization and Mott insulating phases on the level of one-particle correlation functions. Results are presented for the ground-state phase diagram of the Anderson-Hubbard model at half filling, both in the paramagnetic phase and in the presence of antiferromagnetic order. We find a new antiferromagnetic metal which is stabilized by disorder. Possible realizations of these quantum phases with ultracold fermions in optical lattices are discussed., Comment: 25 pages, 5 figures, typos corrected, references updated

We present a computational scheme for ab initio total-energy calculations of materials with strongly interacting electrons using a plane-wave basis set. It combines ab initio band structure and dynamical mean-field theory and is implemented in terms of plane-wave pseudopotentials. The present approach allows us to investigate complex materials with strongly interacting electrons and is able to treat atomic displacements, and hence structural transformations, caused by electronic correlations. Here it is employed to investigate two prototypical Jahn-Teller materials, KCuF3 and LaMnO3, in their paramagnetic phases. The computed equilibrium Jahn-Teller distortion and antiferro-orbital order agree well with experiment, and the structural optimization performed for paramagnetic KCuF3 yields the correct lattice constant, equilibrium Jahn-Teller distortion and tetragonal compression of the unit cell. Most importantly, the present approach is able to determine correlation-induced structural transformations, equilibrium atomic positions and lattice structure in both strongly and weakly correlated solids in their paramagnetic phases as well as in phases with long-range magnetic order., Comment: 27 pages, 11 figures

Electronic correlations in the Fe-pnictide BaFe2As2 are explored within LDA+DMFT, the combination of density functional theory with dynamical mean-field theory. While the correlated band structure is substantially renormalized there is only little transfer of spectral weight. The computed k-integrated and k-resolved spectral functions are in good agreement with photoemission spectroscopy (PES) and angular resolved PES experiments. Making use of a general classification scheme for the strength of electronic correlations we conclude that BaFe2As2 is a moderately correlated system., Comment: 4 pages, 3 figures

A computational scheme for the investigation of complex materials with strongly interacting electrons is formulated which is able to treat atomic displacements, and hence structural relaxation, caused by electronic correlations. It combines ab initio band structure and dynamical mean-field theory and is implemented in terms of plane-wave pseudopotentials. The equilibrium Jahn-Teller distortion and antiferro-orbital order found for paramagnetic KCuF3 agree well with experiment., Comment: 4 pages, 3 figures

The bandstructure of the prototypical charge-transfer insulator NiO is computed by using a combination of an {\it ab initio} bandstructure method and the dynamical mean-field theory with a quantum Monte-Carlo impurity solver. Employing a Hamiltonian which includes both Ni-d and O-p orbitals we find excellent agreement with the energy bands determined from angle-resolved photoemission spectroscopy. This solves a long-standing problem in solid state theory. Most notably we obtain the low-energy Zhang-Rice bands with strongly k-dependent orbital character discussed previously in the context of low-energy model theories., Comment: 4 pages, 3 figure

Using a combination of {\it ab initio} bandstructure methods and dynamical mean-field theory we study the single-particle spectrum of the prototypical charge-transfer insulator NiO. Good agreement with photoemission and inverse-photoemission spectra is obtained for both stoichiometric and hole-doped systems. In spite of a large Ni-$d$ spectral weight at the top of the valence band the doped holes are found to occupy mainly the ligand $p$ orbitals. Moreover, high hole doping leads to a significant reconstruction of the single-particle spectrum accompanied by a filling of the correlation gap., Comment: 4 pages, 4 figures

The paramagnetic phase diagram of the Hubbard model with nearest-neighbor (NN) and next-nearest-neighbor (NNN) hopping on the Bethe lattice is computed at half-filling and in the weakly doped regime using the self-energy functional approach for dynamical mean-field theory. NNN hopping breaks the particle-hole symmetry and leads to a strong asymmetry of the electron-doped and hole-doped regimes. Phase separation occurs at and near half-filling, and the critical temperature of the Mott transition is strongly suppressed., Comment: 8 pages, 8 figures

The properties of condensed matter are determined by single-particle and collective excitations and their interactions. These quantum-mechanical excitations are characterized by an energy E and a momentum \hbar k which are related through their dispersion E_k. The coupling of two excitations may lead to abrupt changes (kinks) in the slope of the dispersion. Such kinks thus carry important information about interactions in a many-body system. For example, kinks detected at 40-70 meV below the Fermi level in the electronic dispersion of high-temperature superconductors are taken as evidence for phonon or spin-fluctuation based pairing mechanisms. Kinks in the electronic dispersion at binding energies ranging from 30 to 800 meV are also found in various other metals posing questions about their origins. Here we report a novel, purely electronic mechanism yielding kinks in the electron dispersions. It applies to strongly correlated metals whose spectral function shows well separated Hubbard subbands and central peak as, for example, in transition metal-oxides. The position of the kinks and the energy range of validity of Fermi-liquid (FL) theory is determined solely by the FL renormalization factor and the bare, uncorrelated band structure. Angle-resolved photoemission spectroscopy (ARPES) experiments at binding energies outside the FL regime can thus provide new, previously unexpected information about strongly correlated electronic systems., Comment: 8 pages, 5 figures

We investigate the properties of the spectral function A(omega,U) of correlated electrons within the Hubbard model and dynamical mean-field theory. Curves of A(omega,U) vs. omega for different values of the interaction U are found to intersect near the band-edges of the non-interacting system. For a wide range of U the crossing points are located within a sharply confined region. The precise location of these 'isosbestic points' depends on details of the non-interacting band structure. Isosbestic points of dynamic quantities therefore provide valuable insights into microscopic energy scales of correlated systems., Comment: 16 pages, 5 figures

The electronic spectrum, energy gap and local magnetic moment of paramagnetic NiO are computed by using the local density approximation plus dynamical mean-field theory (LDA+DMFT). To this end the noninteracting Hamiltonian obtained within the local density approximation (LDA) is expressed in Wannier functions basis, with only the five anti-bonding bands with mainly Ni 3d character taken into account. Complementing it by local Coulomb interactions one arrives at a material-specific many-body Hamiltonian which is solved by DMFT together with quantum Monte-Carlo (QMC) simulations. The large insulating gap in NiO is found to be a result of the strong electronic correlations in the paramagnetic state. In the vicinity of the gap region, the shape of the electronic spectrum calculated in this way is in good agreement with the experimental x-ray-photoemission and bremsstrahlung-isochromat-spectroscopy results of Sawatzky and Allen. The value of the local magnetic moment computed in the paramagnetic phase (PM) agrees well with that measured in the antiferromagnetic (AFM) phase. Our results for the electronic spectrum and the local magnetic moment in the PM phase are in accordance with the experimental finding that AFM long-range order has no significant influence on the electronic structure of NiO., Comment: 15 pages, 6 figures, 1 table; published version

The importance of electronic correlation effects in the layered perovskite Sr$_2$RuO$_4$ is evidenced. To this end we use state-of-the-art LDA+DMFT (Local Density Approximation + Dynamical Mean-Field Theory) in the basis of Wannier functions to compute spectral functions and the quasiparticle dispersion of Sr$_2$RuO$_4$. The spectra are found to be in good agreement with various spectroscopic experiments. We also calculate the $\textbf{k}$-dependence of the quasiparticle bands and compare the results with new angle resolved photoemission (ARPES) data. Two typical manifestations of strong Coulomb correlations are revealed: (i) the calculated quasiparticle mass enhancement of $m^*/m \approx2.5$ agrees with various experimental results, and (ii) the satellite structure at about 3 eV binding energy observed in photoemission experiments is shown to be the lower Hubbard band. For these reasons Sr$_2$RuO$_4$ is identified as a strongly correlated 4$d$ electron material., Comment: 14 pages, 11 figures, 1 table

LDA+DMFT, the merger of density functional theory in the local density approximation and dynamical mean-field theory, has been mostly employed to calculate k-integrated spectra accessible by photoemission spectroscopy. In this paper, we calculate k-resolved spectral functions by LDA+DMFT. To this end, we employ the Nth order muffin-tin (NMTO) downfolding to set up an effective low-energy Hamiltonian with three t_2g orbitals. This downfolded Hamiltonian is solved by DMFT yielding k-dependent spectra. Our results show renormalized quasiparticle bands over a broad energy range from -0.7 eV to +0.9 eV with small ``kinks'', discernible in the dispersion below the Fermi energy., Comment: 21 pages, 8 figures

We calculate the local Green function for a quantum-mechanical particle with hopping between nearest and next-nearest neighbors on the Bethe lattice, where the on-site energies may alternate on sublattices. For infinite connectivity the renormalized perturbation expansion is carried out by counting all non-self-intersecting paths, leading to an implicit equation for the local Green function. By integrating out branches of the Bethe lattice the same equation is obtained from a path integral approach for the partition function. This also provides the local Green function for finite connectivity. Finally, a recently developed topological approach is extended to derive an operator identity which maps the problem onto the case of only nearest-neighbor hopping. We find in particular that hopping between next-nearest neighbors leads to an asymmetric spectrum with additional van-Hove singularities., Comment: 16 pages, 7 figures; minor changes

We present parameter-free LDA+DMFT (local density approximation + dynamical mean field theory) results for the many-body spectra of cubic SrVO3 and orthorhombic CaVO3. Both systems are found to be strongly correlated metals, but not on the verge of a metal-insulator transition. In spite of the considerably smaller V-O-V bond angle in CaVO3 the LDA+DMFT spectra of the two systems for energies EE_F shows more pronounced, albeit still small, differences. This is in contrast to earlier theoretical and experimental conclusions, but in good agreement with recent bulk-sensitive photoemission and x-ray absorption experiments., Comment: 15 pages, 6 figures

Dynamical mean-field theory (DMFT) is a non-perturbative technique for the investigation of correlated electron systems. Its combination with the local density approximation (LDA) has recently led to a material-specific computational scheme for the ab initio investigation of correlated electron materials. The set-up of this approach and its application to materials such as (Sr,Ca)VO_3, V_2O_3, and Cerium is discussed. The calculated spectra are compared with the spectroscopically measured electronic excitation spectra. The surprising similarity between the spectra of the single-impurity Anderson model and of correlated bulk materials is also addressed., Comment: 20 pages, 9 figures, invited paper for the JPSJ Special Issue "Kondo Effect - 40 Years after the Discovery"; final version, references added

The combination of bandstructure theory in the local density approximation with dynamical mean field theory was recently successfully applied to V$_2$O$_3$ -- a material which undergoes the f amous Mott-Hubbard metal-insulator transition upon Cr doping. The aim of this sh ort paper is to emphasize two aspects of our recent results: (i) the filling of the Mott-Hubbard gap with increasing temperature, and (ii) the peculiarities of the Mott-Hubbard transition in this system which is not characterized by a diver gence of the effective mass for the $a_{1g}$-orbital., Comment: 2 pages, 3 figures, SCES'04 conference proceedings

We propose a computational scheme for the ab initio calculation of Wannier functions (WFs) for correlated electronic materials. The full-orbital Hamiltonian H is projected into the WF subspace defined by the physically most relevant partially filled bands. The Hamiltonian H^{WF} obtained in this way, with interaction parameters calculated by constrained LDA for the Wannier orbitals, is used as an ab initio setup of the correlation problem, which can then be solved by many-body techniques, e.g., dynamical mean-field theory (DMFT). In such calculations the self-energy operator \Sigma(e) is defined in WF basis which then can be converted back into the full-orbital Hilbert space to compute the full-orbital interacting Green function G(r,r',e). Using G(r,r',e) one can evaluate the charge density, modified by correlations, together with a new set of WFs, thus defining a fully self-consistent scheme. The Green function can also be used for the calculation of spectral, magnetic and electronic properties of the system. Here we report the results obtained with this method for SrVO3 and V2O3. Comparisons are made with previous results obtained by the LDA+DMFT approach where the LDA DOS was used as input, and with new bulk-sensitive experimental spectra., Comment: 36 pages, 14 figures

LDA+DMFT, the computation scheme merging the local density approximation and the dynamical mean-field theory, is employed to calculate spectra both below and above the Fermi energy and spin and orbital occupations in the correlated paramagnetic metallic and Mott insulating phase of V_2O_3. The self-consistent DMFT equations are solved by quantum Monte Carlo simulations. Room temperature calculations provide direct comparison with experiment. They show a significant increase of the quasiparticle height in comparison with the results at 1160 K. We also obtain new insights into the nature of the Mott-Hubbard transition in V_2O_3. Namely, it is found to be strikingly different from that in the one-band Hubbard model due to the orbital degrees of freedom. Furthermore we resolve the puzzle of the unexpectedly small Mott gap in Cr-doped V_2O_3., Comment: 14 pages, 22 figures

We report high-resolution high-energy photoemission spectra together with parameter-free LDA+DMFT (local density approximation + dynamical mean-field theory) results for Sr1-xCaxVO3, a prototype 3d1 system. In contrast to earlier investigations the bulk spectra are found to be insensitive to x. The good agreement between experiment and theory confirms the bulk-sensitivity of the high-energy photoemission spectra., Comment: 4 pages, 3 figures, supercedes cond-mat/0206471 and cond-mat/0211508, to appear in Phys. Rev. Lett

We present the first observation of a prominent quasi-particle peak in the photoemission spectrum of the metallic phase of V_2O_3 and report new spectral calculations that combine the local density approximation with the dynamical mean-field theory (using quantum Monte Carlo simulations) to show the development of such a distinct peak with decreasing temperature. The experimental peak width and weight are significantly larger than in the theory., Comment: 4 pages, 3 figures, supercedes cond-mat/0108044

We present parameter-free LDA+DMFT results for the many-particle density of states of cubic SrVO3 and orthorhombic CaVO3. Both systems are found to be strongly correlated metals, but em not on the verge of a metal-insulator transition. In spite of the considerably smaller V-O-V bonding angle in CaVO3 the photoemission spectra of the two systems are very similar, their quasiparticle parts being almost identical. This is in contrast to earlier theoretical and experimental conclusions, but in agreement with recent bulk-sensitive photoemission experiments., Comment: 4 pages, 4 figures. Supercedes cond-mat/0312429

LiV_2 O_4 is one of the most puzzling compounds among transition metal oxides because of its heavy fermion like behavior at low temperatures. In this paper we present results for the orbital state and magnetic properties of LiV_2 O_4 obtained from a combination of density functional theory within the local density approximation and dynamical mean-field theory (DMFT). The DMFT equations are solved by quantum Monte Carlo simulations. The trigonal crystal field splits the V 3d orbitals such that the a_{1g} and e_{g}^{pi} orbitals cross the Fermi level, with the former being slightly lower in energy and narrower in bandwidth. In this situation, the d-d Coulomb interaction leads to an almost localization of one electron per V ion in the a_{1g} orbital, while the e_{g}^{pi} orbitals form relatively broad bands with 1/8 filling. 2The theoretical high-temperature paramagnetic susceptibility chi(T) follows a Curie-Weiss law with an effective paramagnetic moment p_{eff}=1.65 in agreement with the experimental results., Comment: 11 pages, 10 figures, 2 tables

LDA+DMFT is a novel computational technique for ab initio investigations of real materials with strongly correlated electrons, such as transition metals and their oxides. It combines the strength of conventional band structure theory in the local density approximation (LDA) with a modern many-body approach, the dynamical mean-field theory (DMFT). In the last few years LDA+DMFT has proved to be a powerful tool for the realistic modeling of strongly correlated electronic systems. In this paper the basic ideas and the set-up of the LDA+DMFT(X) approach, where X is the method used to solve the DMFT equations, are discussed. Results obtained with X=QMC (quantum Monte Carlo) and X=NCA (non-crossing approximation) are presented and compared. By means of the model system La$_{1-x}$Sr$_{x}$TiO$_{3}$ we show that the method X matters qualitatively and quantitatively. Furthermore, we discuss recent results on the Mott-Hubbard metal-insulator transition in the transition metal oxide V$_2$O$_3$ and the $\alpha$-$\gamma$ transition in the 4f-electron system Ce., Comment: 35 pages, 15 figures including 4 flow diagrams, To be published in the Proceedings of the Winter School on "Quantum Simulations of Complex Many-Body Systems: From Theory to Algorithms", February 25 - March 1, 2002, Rolduc/Kerkrade (NL), organized by the John von Neumann Institute of Computing at the Forschungszentrum Juelich

The unphysical solutions of the periodic Anderson model obtained by H. Keiter and T. Leuders [Europhys. Lett. 49, 801(2000)] in dynamical mean-field theory (DMFT) are shown to result from the author's restricted choice of the functional form of the solution, leading to a violation of the analytic properties of the exact solution. By contrast, iterative solutions of the self-consistency condition within the DMFT obtained by techniques which preserve the correct analytic properties of the exact solution (e.g., quantum Monte-Carlo simulations or the numerical renormalization group) always lead to physical solutions., Comment: 4 pages, 1 figure

Wilson's numerical renormalization group (NRG) method for the calculation of dynamic properties of impurity models is generalized to investigate the effective impurity model of the dynamical mean field theory at finite temperatures. We calculate the spectral function and self-energy for the Hubbard model on a Bethe lattice with infinite coordination number directly on the real frequency axis and investigate the phase diagram for the Mott-Hubbard metal-insulator transition. While for TT_c there is a smooth crossover from metallic-like to insulating-like solutions., Comment: 10 pages, 9 eps-figures

New insights into the microscopic origin of itinerant ferromagnetism were recently gained from investigations of electronic lattice models within dynamical mean-field theory (DMFT). In particular, it is now established that even in the one-band Hubbard model metallic ferromagnetism is stable at intermediate values of the interaction U and density n on regular, frustrated lattices. Furthermore, band degeneracy along with Hund's rule couplings is very effective in stabilizing metallic ferromagnetism in a broad range of electron fillings. DMFT also permits one to investigate more complicated correlation models, e.g., the ferromagnetic Kondo lattice model with Coulomb interaction, describing electrons in manganites with perovskite structure. Here we review recent results obtained with DMFT which help to clarify the origin of band-ferromagnetism as a correlation phenomenon., Comment: 17 pages, 12 figures; to be published in the Proceedings of the Workshop on "Ground-State and Finite-Temperature Bandferromagnetism", Berlin/Wandlitz, 4-6 October 2000 [Lecture Notes in Physics, Springer Verlag]