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We present a computational study of PbCoO$_3$ at ambient and elevated pressure. We employ the static and dynamic treatment of local correlation in form of density functional theory + $U$ (DFT+$U$) and + dynamical mean-field theory (DFT+DMFT). Our results capture the experimentally observed crystal structures and identify the unsaturated Pb $6s$ - O $2p$ bonds as the driving force beyond the complex physics of PbCoO$_3$. We provide a geometrical analysis of the structural distortions and we discuss their implications, in particular, the internal doping, which triggers transition between phases with and without local moments and a site selective Mott transition in the low-pressure phase.

The anisotropic optical response of the layered, nodal-line semimetal ZrSiS at ambient and high pressure is investigated by frequency-dependent reflectivity measurements for the polarization along and perpendicular to the layers. The highly anisotropic optical conductivity is in very good agreement with results from density functional theory calculations and confirms the anisotropic character of ZrSiS. Whereas the in-plane optical conductivity shows only modest pressure-induced changes, we found strong effects on the out-of-plane optical conductivity spectrum of ZrSiS, with the appearance of two prominent excitations. These pronounced pressure-induced effects can neither be attributed to a structural phase transition according to our single-crystal x-ray diffraction measurements, nor can they be explained by electronic correlation and electron-hole pairing effects, as revealed by theoretical calculations. Our findings are discussed in the context of the recently proposed excitonic insulator phase in ZrSiS., 6 pages, 3 figures, accepted for publication in Phys. Rev. Lett

We present a dynamical mean-field study of antiferromagnetic magnons in one-, two- and three-orbital Hubbard model of square and bcc cubic lattice at intermediate coupling strength. Weinvestigate the effect of anisotropy introduced by an external magnetic field or single-ion anisotropy.For the latter we tune continuously between the easy-axis and easy-plane models. We also analyzea model with spin-orbit coupling in cubic site-symmetry setting. The ordered states as well as themagnetic excitations are sensitive to even a small breaking ofSU(2)symmetry of the model andfollow the expectations of spin-wave theory as well as general symmetry considerations., 8 pages, 6 figures

We study ferromagnetic ordering and microscopic inhomogeneity in tensile strained LaCoO$_3$ using numerical simulations. We argue that both phenomena originate from effective superexchange interactions between atoms in the high-spin (HS) state mediated by the intermediate-spin excitations. We derive a model of the HS excitation as a bare atomic state dressed by electron and electron-hole fluctuations on the neighbor atoms. We construct a series of approximations to account for electron correlation effects responsible for HS fluctuations and magnetic exchange. The obtained amplitudes and directional dependence of magnetic couplings between the "dressed" HS states show a qualitative agreement with experimental observations and provide a new physical picture of LaCoO$_3$ films., 23 pages, 9 figures, Submission to SciPost

Many-body calculations at the two-particle level require a compact representation of two-particle Green's functions. In this paper, we introduce a sparse sampling scheme in the Matsubara frequency domain as well as a tensor network representation for two-particle Green's functions. The sparse sampling is based on the intermediate representation basis and allows an accurate extraction of the generalized susceptibility from a reduced set of Matsubara frequencies. The tensor network representation provides a system independent way to compress the information carried by two-particle Green's functions. We demonstrate efficiency of the present scheme for calculations of static and dynamic susceptibilities in single- and two-band Hubbard models in the framework of dynamical mean-field theory., 27 pages in single column format, 12 pages (added missing references)

We study $1s$ and $2p$ hard x-ray photoemission spectra (XPS) in a series of late transition metal oxides: ${\mathrm{Fe}}_{2}{\mathrm{O}}_{3}$ ($3{d}^{5}$), ${\mathrm{FeTiO}}_{3}$ ($3{d}^{6}$), CoO ($3{d}^{7}$), and NiO ($3{d}^{8}$). The experimental spectra are analyzed with two theoretical approaches: ${\mathrm{MO}}_{6}$ cluster model and local density approximation (LDA) $+$ dynamical mean-field theory (DMFT). Owing to the absence of the core-valence multiplets and spin-orbit coupling, $1s$ XPS is found to be a sensitive probe of chemical bonding and nonlocal charge-transfer screening, providing complementary information to $2p$ XPS. The $1s$ XPS spectra are used to assess the accuracy of the ab initio $\mathrm{LDA}+\mathrm{DMFT}$ approach, developed recently to study the material-specific charge-transfer effects in core-level XPS.

We present a computational study of antiferromagnetic transition in ${\mathrm{RuO}}_{2}$. The rutile structure with the magnetic sublattices coupled with $\ensuremath{\pi}/2$ rotation leads to a spin-polarized band structure in the antiferromagnetic state, which gives rise to a $d$-wave modulation of the Fermi surface in the spin-triplet channel. We argue a finite spin conductivity that changes sign in the $ab$ plane is expected in ${\mathrm{RuO}}_{2}$ because of this band structure. We analyze the origin of the antiferromagnetic instability and link it to presence of a nodal line close to the Fermi level.

We study theoretically low-temperature phases of a recently synthesized compound ${\mathrm{Sr}}_{2}{\mathrm{CoO}}_{3}\mathrm{F}$ under pressure. The analysis combining LDA+DMFT and a strong-coupling effective model points to the existence of not only normal paramagnetic and antiferromagnetic regimes, but also a spin-state ordered phase in a certain range of applied pressure and low temperature. This order is characterized by a checkerboard arrangement of different spin states of cobalt atoms in the lattice.

We present a dynamical mean-field study of dynamical susceptibilities in two-band Hubbard model. Varying the model parameters we analyze the two-particle excitations in the normal as well as in the ordered phase, an excitonic condensate. The two-particle DMFT spectra in the ordered phase reveal the gapless Goldstone modes arising from spontaneous breaking of continuous symmetries. We also observe gapped Higgs mode, characterized by vanishing of the gap at the phase boundary. Qualitative changes observed in the spin susceptibility can be used as an experimental probe to identify the excitonic condensation., published version

We present a computational study of $L$-edge resonant inelastic x-ray scattering (RIXS) in correlated 3$d$ transition-metal oxides using an $ab$ $initio$ method based on local density approximation + dynamical mean-field theory (DMFT). The present method, building on Anderson impurity model with an optimized continuum bath within DMFT, is an extension of the cluster model to include unbound electron-hole pair excitations as well as material-specific charge-transfer excitations with less empirical parameters. We find a good agreement with available experimental data. The relationship between correlated bands and fluorescence-like feature in the RIXS spectra is discussed.

ZrSiS is a nodal-line semimetal, whose electronic band structure contains a diamond-shaped line of Dirac nodes. We carried out a comparative study on the optical conductivity of ZrSiS and the related compounds ZrSiSe, ZrSiTe, ZrGeS, and ZrGeTe by reflectivity measurements over a broad frequency range combined with density functional theory calculations. The optical conductivity exhibits a distinct U shape, ending at a sharp peak at around $10\phantom{\rule{0.16em}{0ex}}000\phantom{\rule{0.28em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ for all studied compounds except for ZrSiTe. The U shape of the optical conductivity is due to transitions between the linearly dispersing bands crossing each other along the nodal line. The sharp high-energy peak is related to transitions between almost parallel bands, and its energy position depends on the interlayer bonding correlated with the $c/a$ ratio, which can be tuned by either chemical or external pressure. For ZrSiTe, another pair of crossing bands appears in the vicinity of the Fermi level, corrugating the nodal-line electronic structure and leading to the observed difference in optical conductivity. The findings suggest that the Dirac physics in $\mathrm{Zr}XY$ compounds with $X=\mathrm{Si},\mathrm{Ge}$ and $Y=\mathrm{S},\mathrm{Se},\mathrm{Te}$ is closely connected to the interlayer bonding.

We present a theoretical investigation of the origin of Raman-like and fluorescencelike (FL) features of resonant inelastic x-ray scattering (RIXS) spectra. Using a combination of local-density approximation + dynamical mean-field theory and a configuration interaction solver for Anderson impurity model, we calculate the $L$-edge RIXS and x-ray absorption spectra of high-valence transition-metal oxides LaCuO$_3$ and NaCuO$_2$. We analyze in detail the behavior of the FL feature and show how it is connected to the details of electronic and crystal structure. On the studied compounds we demonstrate how material details determine whether the electron-hole continuum can be excited in the $L$-edge RIXS process.

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., 34 pages, 11 figures

We investigate spin-triplet exciton condensation in the two-orbital Hubbard model close to half filling by means of dynamical mean-field theory. Employing an impurity solver that handles complex off-diagonal hybridization functions, we study the behavior of excitonic condensate in stoichiometric and doped systems subject to external magnetic field. We find a general tendency of the triplet order parameter to lay perpendicular with the applied field and identify exceptions from this rule. For solutions exhibiting k-odd spin textures, we discuss the Bloch theorem which, in the absence of spin-orbit coupling, forbids the appearance of spontaneous net spin current. We demonstrate that the Bloch theorem is not obeyed by the dynamical mean-field theory., Comment: 10 pages, 10 figures

By means of dynamical mean-field theory allowing for complete account of SU(2) rotational symmetry of interactions between spin-1/2 particles, we observe a strong effect of suppression of ferromagnetic order in the multiorbital Fermi-Hubbard model in comparison with a widely used restriction to density-density interactions. In the case of orbital degeneracy, we show that the suppression effect is the strongest in the two-orbital model (with effective spin $S_{\rm eff}=1$) and significantly decreases when considering three orbitals ($S_{\rm eff}=3/2$), thus magnetic ordering can effectively revive for the same range of parameters, in agreement with arguments based on vanishing of quantum fluctuations in the limit of classical spins ($S_{\rm eff}\to\infty$). We analyze a connection to the double-exchange model and observe high importance of spin-flip processes there as well., 7 pages, 6 figures, final version

We present a computational study of 2$p$ core-level X-ray photoemission spectra of transition metal monoxides MO (M=Ni, Co, Mn) and sesquioxides M$_2$O$_3$ (M=V, Cr, Fe) using a theoretical framework based on the local-density approximation (LDA) $+$ dynamical mean-field theory (DMFT). We find a very good description of the fine spectral features, which improves considerably over the conventional cluster model. We analyze the role of the non-local screening and its relationship to the long-range magnetic order and the lattice geometry. Our results reveal the potential of the present method for the analysis and interpretation of the modern high-energy-resolution experiments., Comment: 9 pages, 8 figures

Using Wannier functions to represent the density functional results we calculate the hybridization corrections to the orbital momentum operator in the Os 5d shell of Mott insulators Ba2NaOsO6 and Ba2YOsO6. The g-factors are obtained by evaluating the spin and orbital momentum operators in the atomic ground states of the Os ion. While the hybridization corrections play minor role in d3 ion of Ba2YOsO6 with dominant spin moment, they are instrumental for observation of non- zero g-factor of the d1 ions of Ba2NaOsO6. In addition we analyze the exchange interactions in Ba2YOsO6 and find them consistent with the reported magnetic structure.

Using x-ray magnetic circular and linear dichroism techniques, we demonstrate a collinear exchange coupling between an epitaxial antiferromagnet, tetragonal CuMnAs, and an Fe surface layer. A small uncompensated Mn magnetic moment is observed which is antiparallel to the Fe magnetization. The staggered magnetization of the 5nm thick CuMnAs layer is rotatable under small magnetic fields, due to the interlayer exchange coupling. This allows us to obtain the x-ray magnetic linear dichroism spectra for different crystalline orientations of CuMnAs in the (001) plane., 10 pages, 3 figures

Recent breakthroughs in electrical detection and manipulation of antiferromagnets have opened a new avenue in the research of non-volatile spintronic devices. Antiparallel spin sublattices in antiferromagnets, producing zero dipolar fields, lead to the insensitivity to magnetic field perturbations, multi-level stability, ultra-fast spin dynamics and other favorable characteristics which may find utility in fields ranging from magnetic memories to optical signal processing. However, the absence of a net magnetic moment and the ultra-short magnetization dynamics timescales make antiferromagnets notoriously difficult to study by common magnetometers or magnetic resonance techniques. In this paper we demonstrate the experimental determination of the Neel vector in a thin film of antiferromagnetic CuMnAs which is the prominent material used in the first realization of antiferromagnetic memory chips. We employ a femtosecond pump-probe magneto-optical experiment based on magnetic linear dichroism. This table-top optical method is considerably more accessible than the traditionally employed large scale facility techniques like neutron diffraction and X-ray magnetic dichroism measurements. This optical technique allows an unambiguous direct determination of the Neel vector orientation in thin antiferromagnetic films utilized in devices directly from measured data without fitting to a theoretical model., 10 pages, 4 figures

Motivated by recent observation of magnetic field induced transition in LaCoO3 we study the effect of external field in systems close to instabilities towards spin-state ordering and exciton condensation. We show that, while in both cases the transition can be induced by an external field, temperature dependencies of the critical field have opposite slopes. Based on this result we argue that the experimental observations select the exciton condensation scenario. We show that such condensation is possible due to high mobility of the intermediate spin excitations. The estimated width of the corresponding dispersion is large enough to overrule the order of atomic multiplets and to make the intermediate spin excitation propagating with a specific wave vector the lowest excitation of the system., Comment: 5 pages, 5 figures

We review our investigations of electronic properties of strongly correlated materials using the combination of first principles electronic band structures and the dynamical mean-field theory, so called LDA+DMFT method. Our investigations focus on two phenomena, the spin state transitions and their relationship to the metal-insulator transition, and the effect of hybridization between correlated and ligand orbitals in chargetransfer type materials. The pressure driven spin transitions are studied for a group of materials containing MnO, FeO and Fe2O3. To investigate the hybridization effects we focus on NiO and NiS(Se)2. We identify various mechanisms of the metal-insulator transition, which can take place in multi-band systems, in addition to the band-width control known from the single band Hubbard model. c

Within the linearized augmented plane-wave method for electronic structure calculations, a force expression was derived for such exchange–correlation energy functionals that lead to orbital-dependent potentials (e.g., LDA + U or hybrid methods). The forces were implemented into the WIEN2k code and were tested on systems containing strongly correlated d and f electrons. The results show that the expression leads to accurate atomic forces.

Constrained density functional theory scheme in Wannier functions formalism has been used to calculate Coulomb repulsion U and Hund’s exchange J parameters for Fe-3d electrons in LaFeAsO. Results strongly depend on the basis set. When O-2p, As-4p, and Fe-3d orbitals are included, computation results in U = 3–4 eV. With the basis set restricted to Fe-3d orbitals only, computation gives parameters corresponding to F0 = 0.8 eV, J = 0.5 eV. Local Density Approximation combined with Dynamical Mean-Field Theory calculation with these parameters results in weakly correlated electronic structure.

Spin textures in k-space arising from spin-orbit coupling in non-centrosymmetric crystals find numerous applications in spintronics. We present a mechanism that leads to appearance of k-space spin texture due to spontaneous symmetry breaking driven by electronic correlations. Using dynamical mean-field theory we show that doping a spin-triplet excitonic insulator provides a means of creating new thermodynamic phases with unique properties. The numerical results are interpreted using analytic calculations within a generalized double-exchange framework., the published version with Supplemental Material attached

We combine several numerical and semi-analytical methods to study the $5d$ double perovskites Sr2YIRO6 and Ba2YIRO6 which were recently proposed to exhibit excitonic magnetism. Starting from the density functional theory and constrained random phase approximation we construct effective multi-band Hubbard models. These are analyzed by means of static and dynamical mean-field theories and strong coupling expansion. We find both materials to be insulators, but, contrary to the experimental claims, with a large spin gap of several hundreds meV preventing formation of an ordered state at low temperature, Comment: 7 pages, 6 figures

Alexander Schnell: Einleitung Christoph Asmuth: Die Bedeutung J. G. Fichtes fur eine Theorie der Bildlichkeit Franziskus v. Heereman: Durch und durch ein "Durch". Grosse und Grenze des Fichteschen Bildbegriffs Alexander Schnell: Die drei Bildtypen in der transzendentalen Bildlehre J. G. Fichtes Alessandro Bertinetto: Das Bild als Durch-Einheit: Das Bild und die Wissenschaftslehre Jindrich Karasek: Ich und Nicht-Ich. Zur reinen Struktur der Welt als Bild bei Fichte Martin Vrabec: Verfugt das absolute Ich aus der 'Grundlage der gesamten Wissenschaftslehre ' uber ein Selbstbewusstsein? Marco Ivaldo: Die Rolle der Einbildungskraf in Fichtes Uberlegungen uber Geist und Buchstaben aus den Jahren Laurent Guyot: Le role de l'imagination productrice dans la genese de la conscience de soi Rainer Adolphi: Warum wir alle gleich ticken. Ent-Intellektualisierung der idealistischen Rationalitat: eine Schelling'sche Einsicht und eine Theorieentwicklung des spateren Fichte Hartmut Traub: "Lasst uns Menschen machen". Fichtes Lehre vom Bild: Zwischen Gottes-Ebenbildlichkeit und Bilderverbot Max Marcuzzi: L'image morale chez Fichte, entre esthetique et religion Marco Rampazzo Bazzan (Toulouse): La Doctrine de la science comme pratique reflexive de production d'images. Une relecture du Gesicht dans la Doctrine de la science et la Destination du savant de 1811 a partir de Michel Foucault et Louis Althusser Gunter Zoller: Fichtebilderverbot. Historische und systematische Uberlegungen zum philosophischen Umgang mit Fichtes Texten

In an attempt to provide a clearer understanding of the impressive increase in T_c under pressure in elemental Li, linear response calculation of the phonon dispersion curves, electron-phonon matrix elements, phonon linewidths and mode lambda's have been carried out on a finer mesh (24^3 in the Brillouin zone) than done previously, for the volume corresponding to 20 GPa pressure. The result illustrates the great need for a fine mesh (even finer than this) for converged results of lambda and the spectral function alpha^2 F. Although the initial pressure-induced transverse T_1 phonon instability (in harmonic approximation) near the symmetry point K has dominated attention, the current results show that the high value of T_c gets strong contributions from elsewhere in the zone, particularly from the longitudinal mode along (100)., Proceedings for M2S

We present an algorithm for the adaptive tetrahedral integration over the Brillouin zone of crystalline materials, and apply it to compute the optical conductivity, dc conductivity, and thermopower. For these quantities, whose contributions are often localized in small portions of the Brillouin zone, adaptive integration is especially relevant. Our implementation, the woptic package, is tied into the wien2wannier framework and allows including a many-body self energy, e.g. from dynamical mean-field theory (DMFT). Wannier functions and dipole matrix elements are computed with the DFT package Wien2k and Wannier90. For illustration, we show DFT results for fcc-Al and DMFT results for the correlated metal SrVO$_3$., 14 pages, 10 figures. Changes from v1: corrected prefactor of optical conductivity; minor changes for readability

We calculate the momentum dependent spectral function of the Bose-Hubbard model on a simple cubic lattice in three dimensions within the bosonic dynamical mean-field theory (B-DMFT). The continuous-time quantum Monte Carlo method is used to solve the self-consistent B-DMFT equations together with the maximum entropy method for the analytic continuation to real frequencies. Results for weak, intermediate, and strong interactions are presented. In the limit of weak and strong interactions very good agreement with results obtained by perturbation theory is found. By contrast, at intermediate interactions the results differ significantly, indicating that in this regime perturbative methods fail to describe the dynamics of interacting bosons.

We use the LDA+U approach to search for possible ordered ground states of LaSrCoO$_4$. We find a staggered arrangement of magnetic multipoles to be stable over a broad range of Co $3d$ interaction parameters. This ordered state can be described as a spin-denity-wave-type condensate of $d_{xy} \otimes d_{x^2-y^2}$ excitons carrying spin $S=1$. Further, we construct an effective strong-coupling model, calculate the exciton dispersion and investigate closing of the exciton gap, which marks the exciton condensation instability. Comparing the layered LaSrCoO$_4$ with its pseudo cubic analog LaCoO$_3$, we find that for the same interaction parameters the excitonic gap is smaller (possibly vanishing) in the layered cobaltite., 6 pages, 6 figures

Using the dynamical mean-field approximation we investigate formation of excitonic condensate in the two-band Hubbard model in the vicinity of the spin-state transition. With temperature and band filling as the control parameters we realize all symmetry allowed spin-triplet excitonic phases, some exhibiting a ferromagnetic polarization. While the transitions are first-order at low temperatures, at elevated temperatures continuous transitions are found that give rise to a multi-critical point. Rapid but continuous transition between ferromagnetic and non-magnetic excitonic phases allows switching of uniform magnetization by small changes of chemical potential., 8 pages, 6 figures

We use a combination of dynamical mean-field model calculations and LDA + U material specific calculations to investigate the low temperature phase transition in the compounds from the $({\mathrm{Pr}}_{1\ensuremath{-}y}R{}_{y}){}_{x}{\mathrm{Ca}}_{1\ensuremath{-}x}{\mathrm{CoO}}_{3}$ $(R=\text{Nd,}\phantom{\rule{4.pt}{0ex}}\text{Sm,}\phantom{\rule{4.pt}{0ex}}\text{Eu,}\phantom{\rule{4.pt}{0ex}}\text{Gd,}\phantom{\rule{4.pt}{0ex}}\text{Tb,}\phantom{\rule{4.pt}{0ex}}\text{Y})$ family (PCCO). The transition, marked by a sharp peak in the specific heat, leads to an exponential increase of dc resistivity and a drop of the magnetic susceptibility, but no order parameter has been identified yet. We show that condensation of spin-triplet, atomic-size excitons provides a consistent explanation of the observed physics. In particular, it explains the exchange splitting on the Pr sites and the simultaneous Pr valence transition. The excitonic condensation in PCCO is an example of a general behavior expected in certain systems in the proximity of a spin-state transition.

Ryotaro Arita1,2, Jan Kunes3, Pavel Augustinský3,4, Anton V. Kozhevnikov5, Adolfo G. Eguiluz6, Masatoshi Imada1 1Department of Applied Physics, University of Tokyo, Hongo, Bunkyo-ku, Tokyo, 113-8656, Japan 2JST-PRESTO, Kawaguchi, Saitama 332-0012, Japan 3Inst. Phys., Acad. of Sciences, Cukrovarnicka 10, Praha 6, 162 53, Czech Republic 4Theoretical Physics III, Center for Electronic Correlations and Magnetism, Institute of Physics, University of Augsburg, D-86135, Augsburg, Germany 5Institute for Theoretical Physics, ETH Zurich, CH-8093 Zurich, Switzerland 6Department of Physics and Astronomy, Univ. Tennessee, Knoxville, Tennessee 37996, USA

We present experimental magneto-optical (MO) Faraday rotation data obtained with linearly polarised soft X-ray synchrotron radiation at the 2p–3d resonance in Fe films. The dichroism of the dispersive and absorptive parts of the refraction index are compared with ab initio calculations based on the local approximation to spin-density functional theory.

Thin film magnetic systems have recently attracted much attention of experimental and computational physicists. In the present paper magnetic, optical and magneto-optical properties of a free standing Fe-monolayer are studied using the full-potential linearized-augmented-plane wave method (FLAPW) within the local spin-density approximation (LSDA).

Using a many-body formulation we show that the integral over the imaginary part of the off-diagonal optical conductivity gives---at zero temperature---exactly the many-electron center of mass angular moment in the volume, i.e., ${\ensuremath{\int}}_{0}^{\ensuremath{\infty}}\mathrm{Im}[{\ensuremath{\sigma}}_{\mathrm{xy}}(\ensuremath{\omega})]d\ensuremath{\omega}\ensuremath{\propto}〈(\mathit{R}\ifmmode\times\else\texttimes\fi{}\mathit{P}{)}_{z}〉,$ with $\mathit{R}$ the center of mass coordinate and $\mathit{P}$ the total relativistic momentum. At elevated temperatures the sum rule is no longer exactly fulfilled, but the temperature-related correction is estimated to be very small. In a single-particle description the sum rule leads to the total orbital moment in the volume plus an additional term.

Using linear response theory with the dynamical mean-field approximation we investigate the particle-hole instabilities of the two-band Hubbard model in the vicinity of the spin-state transition. Besides the previously reported high-spin--low-spin order we find an instability towards triplet excitonic condensate. We discuss the strong and weak coupling limits of the model, in particular, a connection to the spinful hard-core bosons with a nearest-neighbor interaction. Possible realization in LaCoO3 at intermediate temperatures is briefly discussed., 9 pages, 6 figures, revised extended version

We investigate the electronic structure of (Sr$_{1-x}$La$_x$)$_2$RhO$_4$ using a combination of the density functional and dynamical mean-field theories. Unlike the earlier local density approximation plus Hubbard $U$ (LDA+U) studies, we find no sizable enhancement of the spin-orbit splitting due to electronic correlations and show that such an enhancement is a spurious effect of the static mean-field approximation of the LDA+U method. The electron doping suppresses the importance of electronic correlations, which is reflected in quasi-particle bandwidth increasing with $x$. (Sr$_{1-x}$La$_x$)$_2$RhO$_4$ can be classified as weakly correlated metal, which becomes an itinerant in-plane ferromagnet (but possibly A-type antiferromagnet) due to Stoner instability around $x=0.2$., Comment: Corrected a factor of 2 error in our definition of doping x