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We report the low-temperature properties of $^{11}$B-enriched single-crystal CrB$_{2}$ as prepared from high-purity Cr and B powder by a solid-state reaction and optical float zoning. The electrical resistivity, $\rho_{\rm xx}$, Hall effect, $\rho_{\rm xy}$, and specific heat, $C$, are characteristic of an exchange-enhanced Fermi liquid ground state, which develops a slightly anisotropic spin gap $\Delta \approx 220\,{\rm K}$ below $T_{\rm N}=88\,{\rm K}$. This observation is corroborated by the absence of a Curie dependence in the magnetization for $T\to0$ reported in the literature. Comparison of $C$ with $d\rho_{\rm xx}/dT$, where we infer lattice contributions from measurements of VB$_2$, reveals strong antiferromagnetic spin fluctuations with a characteristic spin fluctuation temperature $T_{\rm sf}\approx 257\,{\rm K}$ in the paramagnetic state, followed by a pronounced second-order mean-field transition at $T_{\rm N}$, and unusual excitations around $\approx T_{\rm N}/2$. The pronounced anisotropy of $\rho_{\rm xx}$ above $T_{\rm N}$ is characteristic of an easy-plane anisotropy of the spin fluctuations consistent with the magnetization. The ratio of the Curie-Weiss to the N$\acute{\rm{e}}$el temperatures, $f=-\Theta_{\rm CW}/T_{\rm N}\approx 8.5$, inferred from the magnetization, implies strong geometric frustration. All physical properties are remarkably invariant under applied magnetic fields up to $14\,\,{\rm T}$, the highest field studied. In contrast to earlier suggestions of local-moment magnetism our study identifies CrB$_{2}$ as a weak itinerant antiferromagnet par excellence with strong geometric frustration., Comment: 15 pages, 9 figures

Using ab initio electronic structure calculations employing the LDA+U method we have investigated the exchange coupling in materials containing Eu in Eu2+ formal valency including: (i) insulating ferromagnets (EuO, EuS, EuSe, EuTe), and (ii) elemental metal BCC Eu. The total energies calculated with constrained magnetic structures were mapped on the Heisenberg Hamiltonian in order to investigate the thermodynamic properties. The mechanisms of inter-site exchange, which differ between the studied systems, are discussed.

We report the angular dependence of three distinct de Haas-van Alphen (dHvA) frequencies of the torque magnetization in the itinerant antiferromagnet CrB2 at temperatures down to 0.3K and magnetic fields up to 14T. Comparison with the calculated Fermi surface of nonmagnetic CrB2 suggests that two of the observed dHvA oscillations arise from electron-like Fermi surface sheets formed by bands with strong B-px,y character which should be rather insensitive to exchange splitting. The measured effective masses of these Fermi surface sheets display strong enhancements of up to a factor of two over the calculated band masses which we attribute to electron-phonon coupling and electronic correlations. For the temperature and field range studied, we do not observe signatures reminiscent of the heavy d-electron bands expected for antiferromagnetic CrB2. In view that the B-p bands are at the heart of conventional high-temperature superconductivity in the isostructural MgB2, we consider possible implications of our findings for nonmagnetic CrB2 and an interplay of itinerant antiferromagnetism with superconductivity., 8 pages, 4 figures

Particularly in Sr2IrO4, the interplay between spin-orbit coupling, bandwidth and on-site Coulomb repulsion stabilizes a J(eff) = 1/2 spin-orbital entangled insulating state at low temperatures. Whether this insulating phase is Mott- or Slater-type, has been under intense debate. We address this issue via spatially resolved imaging and spectroscopic studies of the Sr2IrO4 surface using scanning tunneling microscopy/spectroscopy (STM/S). STS results clearly illustrate the opening of an insulating gap (150 ~ 250 meV) below the Néel temperature (TN), in qualitative agreement with our density-functional theory (DFT) calculations. More importantly, the temperature dependence of the gap is qualitatively consistent with our DFT + dynamical mean field theory (DMFT) results, both showing a continuous transition from a gapped insulating ground state to a non-gap phase as temperatures approach TN. These results indicate a significant Slater character of gap formation, thus suggesting that Sr2IrO4 is a uniquely correlated system, where Slater and Mott-Hubbard-type behaviors coexist.

Hole and electron doped LaCoO3 is studied using dynamical mean-field theory. The one-particle spectra are analyzed and compared to the available experimental data, in particular the x-ray absorption spectra. Analyzing the temporal spin-spin correlation functions we find the atomic intermediate spin state is not important for the observed Curie-Weiss susceptibility. Contrary to the commonly held view about the roles played by the t2g and eg electrons we find narrow quasiparticle bands of t2g character crossing the Fermi level accompanied by strongly damped eg excitations., Comment: published version

Fifteen parameters characterizing the crystal field of rare-earth ions in the RMO$_3$ perovskites (R = Pr, Nd, M = Ga, Co) are calculated by expanding the local Hamiltonian expressed in the basis of Wannier functions into a series of spherical tensor operators. The method contains a single adjustable parameter that characterizes the hybridization of R($4f$) states with the states of oxygen ligands. Subsequently the energy levels and magnetic moments of trivalent R ion are determined by diagonalization of an effective Hamiltonian which, besides the crystal field, contains the $4f$ electron-electron repulsion, spin-orbit coupling and interaction with magnetic field. In the Ga compounds the energy levels of the ground multiplet agree within few meV with those determined experimentally by other authors. For all four compounds in question the temperature dependence of magnetic susceptibility is measured on polycrystalline samples and compared with the results of calculation. For NdGaO$_3$ theory is also compared with the magnetic measurements on a single crystal presented by Luis {\it et al.} Phys. Rev. B {\bf 58}, 798 (1998). A good agreement between the experiment and theory is found.

We present an algorithm for measurement of the Green's function in the hybridization expansion continuous-time quantum Monte-Carlo based on continuous estimators. Compared to the standard method, the present algorithm has similar or better accuracy with improvement notable especially at low perturbation orders and high frequencies. The resulting statistical noise is weakly correlated so high-accuracy data for the numerical analytical continuation can be produced., Comment: 22 pages, 10 figures

We employ the combination of the density functional and the dynamical mean-field theory (LDA+DMFT) to investigate the electronic structure and magnetic properties of SrCoO3, monocrystal of which were prepared recently. Our calculations lead to a ferromagnetic metal in agreement with experiment. We find that, contrary to some suggestions, the local moment in SrCoO3 does not arise from intermediate spin state, but is a result of coherent superposition of many different atomic states. We discuss how attribution of magnetic response to different atomic states in solids with local moments can be quantified., Comment: 5 pages, 5 figures

We develop a computational approach for calculating the optical conductivity in the augmented plane wave basis set of Wien2K and apply it for thoroughly comparing the full dipole matrix element calculation and the Peierls approximation. The results for SrVO3 and V2O3 show that the Peierls approximation, which is commonly used in model calculations, works well for optical transitions between the d orbitals. In a typical transition metal oxide, these transitions are solely responsible for the optical conductivity at low frequencies. The Peierls approximation does not work, on the other hand, for optical transitions between p- and d-orbitals which usually became important at frequencies of a few eVs, Comment: 11 pages, 4 figures

Ab initio analyses of ${A}_{2}{\mathrm{IrO}}_{4}$ ($A=\mathrm{Sr},\mathrm{Ba}$) are presented. Effective Hubbard-type models for Ir $5d$ ${t}_{2g}$ manifolds downfolded from the global band structure are solved based on the dynamical mean-field theory. The results for $A=\mathrm{Sr}$ and Ba correctly reproduce paramagnetic metals undergoing continuous transitions to insulators below the N\'eel temperature ${T}_{N}$. These compounds are classified not into Mott insulators but into Slater insulators. However, the insulating gap opens by a synergy of the N\'eel order and significant band renormalization, which is also manifested by a 2D bad metallic behavior in the paramagnetic phase near the quantum criticality.

We use the dynamical mean-field theory to study a p-d Hubbard Hamiltonian for LaCoO 3 derived from ab initio calculations in local density approximation (LDA+DMFT scheme). We address the origin of local moments observed above 100 K and discuss their attribution to a particular atomic multiplet in the presence of covalent Co-O bonding. We show that in solids such attribution, based on the single ion picture, is in general not possible. We explain when and how the single ion picture can be generalized to provide a useful approximation in solids. Our results demonstrate that the apparent magnitude of the local moment is not necessarily indicative of the underlying atomic multiplet. We conclude that the local moment behavior in LaCoO 3 arises from the high-spin state of Co and explain the precise meaning of this statement. © 2012 American Physical Society.

We present an efficient and numerically stable algorithm for calculation of two-particle response functions within the dynamical mean-field theory. The technique is based on inferring the high frequency asymptotic behavior of the irreducible vertex function from the local dynamical susceptibility. The algorithm is tested on several examples. In all cases rapid convergence of the vertex function towards its asymptotic form is observed., 8 pages, 9 figures, revised acknowledgement

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

We study the thermally driven spin state transition in a two-orbital Hubbard model with crystal field splitting, which provides a minimal description of the physics of LaCoO3. We employ the dynamical mean-field theory with quantum Monte-Carlo impurity solver. At intermediate temperatures we find a spin disproportionated phase characterized by checkerboard order of sites with small and large spin moments. The high temperature transition from the disproportionated to a homogeneous phase is accompanied by vanishing of the charge gap. With the increasing crystal-field splitting the temperature range of the disproportionated phase shrinks and eventually disappears completely., Comment: 4+ pages, 4 figures

Doping dependence of the spin fluctuations and the electron correlations in the effective five-band Hubbard model for iron pnictides is investigated using the fluctuation-exchange approximation. For a moderate hole doping, we find a dominant low-energy spin excitation at Q=(\pi,0), which becomes critical at low temperature. The low-energy spin excitations in the heavily hole-doped region are characterized by weak Q dependence. The electron doping leads to an appearance of a pseudogap in spin excitation spectrum. Correspondingly, the NMR-1/T1 relaxation rate is strongly enhanced on the hole-doped side and suppressed on the electron-doped side of the phase diagram. This behavior can be to large extent understood by systematic changes of the Fermi-surface topology., Comment: 4 pages, 5 figures

The origin of the gap in ${\text{NiS}}_{2}$ as well as the pressure- and doping-induced metal-insulator transition in the ${\text{NiS}}_{2\ensuremath{-}x}{\text{Se}}_{x}$ solid solutions are investigated both theoretically using the first-principles band structures combined with the dynamical mean-field approximation for the electronic correlations and experimentally by means of infrared and x-ray absorption spectroscopies. The bonding-antibonding splitting in the S-S (Se-Se) dimer is identified as the main parameter controlling the size of the charge gap. The implications for the metal-insulator transition driven by pressure and Se doping are discussed.

Using the fluctuation-exchange (FLEX) approximation we study an effective five-band Hubbard model for iron-pnictide superconductors obtained from the first-principles band structure. We preclude deformations of the Fermi surface due to electronic correlations by introducing a static potential, which mimics the effect of charge relaxation. Evaluating the Eliashberg equation for various dopings and interaction parameters, we find that superconductivity can sustain higher hole than electron doping. Analyzing the symmetry of the superconducting order parameter we observe clear differences between the hole and electron doped systems. We discuss the importance of the pnictogen height for superconductivity. Finally, we dissect the pairing interaction into various contributions, which allows us to clarify the relationship between the superconducting transition temperature and the proximity to the anti-ferromagnetic phase., Comment: 15 pages, 15 figures

We present an implementaion of interface between the full-potential linearized augmented plane wave package Wien2k and the wannier90 code for the construction of maximally localized Wannier functions. The FORTRAN code and a documentation is made available and results are discussed for SrVO$_3$, Sr$_2$IrO$_4$ (including spin-orbit coupling), LaFeAsO, and FeSb$_2$., 15 pages, 9 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"

We present the results of numerical studies for selected materials with strongly correlated electrons using a combination of the local-density approximation and dynamical mean-field theory (DMFT). For the solution of the DMFT equations a continuous-time quantum Monte-Carlo algorithm was employed. All simulations were performed on the supercomputer HLRB II at the Leibniz Rechenzentrum in Munich. Specifically we have analyzed the pressure induced metal-insulator transitions in Fe2O3 and NiS2, the charge susceptibility of the fluctuating-valence elemental metal Yb, and the spectral properties of a covalent band-insulator model which includes local electronic correlations.

We study theoretically the electronic states in a $5d$ transition metal oxide ${\mathrm{Na}}_{2}{\mathrm{IrO}}_{3}$, in which both the spin-orbit interaction and the electron correlation play crucial roles. A tight-binding model analysis together with the first-principles band structure calculation predicts that this material is a layered quantum spin Hall system. Because of the electron correlation, an antiferromagnetic order first develops at the edge, and later inside the bulk at low temperatures.

We present a dynamical mean field theory study of the valence transition ($f^{14} \to f^{13}$) in elemental, metallic Yb under pressure. Our calculations reproduce the observed valence transition as reflected in the volume dependence of the $4f$ occupation. The transition is accelerated by heating, and suggests quasiparticle or Kondo-like structure in the spectra of the trivalent end state, consistent with the early lanthanides. Results for the local charge fluctuations and susceptibility, however, show novel signatures uniquely associated with the valence transition itself, indicating that Yb is a fluctuating valence material in contrast to the intermediate valence behavior seen in the early trivalent lanthanides Ce, Pr, and Nd., 4 pages, 4 figures

We present an optical conductivity of ${\text{EuB}}_{6}$ over a wide range (2 meV--5.5 eV) from reflectivity and ellipsometry measurements. Upon the ferromagnetic transition at ${T}_{c}=15.5\text{ }\text{K}$, interband transition ${\ensuremath{\sigma}}_{1}(\ensuremath{\omega})$ decreases below $\ensuremath{\omega}=3.3\text{ }\text{eV}$ and the lost spectral weight is transferred to the Drude ${\ensuremath{\sigma}}_{1}(\ensuremath{\omega})$ at $\ensuremath{\omega}l0.33\text{ }\text{eV}$. Further a high-energy optical peak at around 4 eV exhibits a large splitting below ${T}_{c}$. We explain these high-energy ${\ensuremath{\sigma}}_{1}(\ensuremath{\omega})$ changes, that is, spectral-weight recovery and peak splitting, using local-density $\text{approximation}+U$ band-structure calculations. Our finding suggests strongly that: (1) ${\text{EuB}}_{6}$ is a semimetal and (2) the exchange-driven band splitting is the primary source of the drastic ${\ensuremath{\omega}}_{p}^{2}$ change at $Tl{T}_{c}$.

Motivated by the peculiar behavior of FeSi and ${\text{FeSb}}_{2}$, we study the effect of local electronic correlations on magnetic, transport, and optical properties in a specific type of band insulator, namely a covalent insulator. Investigating a minimum model of covalent insulator within a single-site dynamical mean-field approximation, we are able to obtain the crossover from low-temperature nonmagnetic insulator to high-temperature paramagnetic metal with parameters realistic for FeSi and ${\text{FeSb}}_{2}$ systems. Our results show that the behavior of FeSi does not imply microscopic description in terms of Kondo insulator (periodic Anderson model) as can be often found in the literature, but in fact reflects generic properties of a broader class of materials.

The metal-insulator transition in correlated electron systems, where electron states transform from itinerant to localized, has been one of the central themes of condensed matter physics for more than half a century. The persistence of this question has been a consequence both of the intricacy of the fundamental issues and the growing recognition of the complexities that arise in real materials, even when strong repulsive interactions play the primary role. The initial concept of Mott was based on the relative importance of kinetic hopping (measured by the bandwidth) and on-site repulsion of electrons. Real materials, however, have many additional degrees of freedom that, as is recently attracting note, give rise to a rich variety of scenarios for a ``Mott transition.'' Here we report results for the classic correlated insulator MnO which reproduce a simultaneous moment collapse, volume collapse, and metallization transition near the observed pressure, and identify the mechanism as collapse of the magnetic moment due to increase of crystal field splitting, rather than to variation in the bandwidth.

We present synchrotron-excited oxygen x-ray $K$-emission spectroscopy ($\text{O}\text{ }K\ensuremath{\alpha}$ XES) and oxygen x-ray absorption spectroscopy ($\text{O}\text{ }1s$ XAS) spectra of transition-metal (TM) oxides MnO, CoO, and NiO. The comparison of oxygen $K$-emission and absorption spectra to valence band photoemission and bremsstrahlung isochromat spectra measurements shows that $\text{O}\text{ }1s$ XAS is not strongly influenced by the core hole effect, whereas the $\text{TM}\text{ }2p$ XAS significantly shifts to a lower energy. New and effective methods for determining the band gap and anion-to-cation cation charge-transfer energies of the oxides from the measured spectra are presented and applied, and the combination of O XAS and XES is shown to agree well with the results of numerical electronic structure methods applied to strongly correlated oxides. For MnO, the charge-transfer energy is found to be 6.6 eV and the band gap is 4.1 eV; for CoO, the values are 6.1 and 2.6 eV and for NiO, the values are 5.4 and 4.0 eV.

Superconductivity in hole-doped ${\mathrm{Li}}_{x}\mathrm{Nb}{\mathrm{O}}_{2}$ has been reported with ${T}_{c}\ensuremath{\approx}5\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ in the range $0.45\ensuremath{\leqslant}xl0.8$. The electronic structure is based on a two-dimensional triangular Nb lattice. The strong trigonal crystal field results in a single Nb ${d}_{{z}^{2}}$ band isolated within a wide gap, leading to a single-band triangular lattice system. The isolated, partially filled band has a width $W=1.5\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ with dominant second neighbor hopping. To identify possible correlation effects, we apply dynamical mean field theory and quantum Monte Carlo (QMC) using on-site Coulomb repulsion $U=0--3\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ and check selected results using determinant QMC. For $U$ as small as $1\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$, the single particle spectrum displays a robust lower Hubbard band, suggesting the importance of correlation effects in ${\mathrm{Li}}_{x}\mathrm{Nb}{\mathrm{O}}_{2}$ even for $U\ensuremath{\leqslant}W$. At half-filling $(x=0)$, a Mott transition occurs at ${U}_{c}\ensuremath{\approx}1.5\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. Coupling between the O ${A}_{g}$ phonon displacement and correlation effects is assessed.

We present an x-ray spectromicroscopic investigation of single-crystalline magnetic $\mathrm{Fe}\mathrm{Mn}∕\mathrm{Co}$ bilayers on Cu(001), using x-ray magnetic circular dichroism and x-ray magnetic linear dichroism (XMLD) at the Co and Fe ${L}_{3}$ absorption edges in combination with photoelectron emission microscopy. Using the magnetic coupling between the ferromagnetic Co layer and the antiferromagnetic FeMn layer, we are able to produce magnetic domains with two different crystallographic orientations of the magnetic easy axis within the same sample at the same time. We find a huge difference in the XMLD contrast between the two types of magnetic domains, which we discuss in terms of intrinsic magnetocrystalline anisotropy of XMLD of the Co layer. We also demonstrate that due to the high sensitivity of the method, the small number of induced ferromagnetic Fe moments at the FeMn-Co interface is sufficient to obtain magnetic contrast from XMLD in a metallic system.

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

The idea of exciton condensation in solids was introduced in 1960's with the analogy to superconductivity in mind. While exciton supercurrents have been realized only in artificial quantum-well structures so far, the application of the concept of excitonic condensation to bulk solids leads to a rich spectrum of thermodynamic phases with diverse physical properties. In this review we discuss recent developments in the theory of exciton condensation in systems described by Hubbard-type models. In particular, we focus on the connections to their various strong-coupling limits that have been studied in other contexts, e.g., cold atoms physics. One of our goals is to provide a 'dictionary' which would allow the reader to efficiently combine results obtained in these different fields., Comment: 22 pages, 18 figures, review article

Starting from ab initio electronic structure results we study dynamics of potassium ions in the pyrochlore superconductor ${\mathrm{KOs}}_{2}{\mathrm{O}}_{6}$. An effective ionic Hamiltonian characterized by an on-site instability (rattler) and frustrated nearest-neighbor interaction is derived and various approximations are investigated. We show that qualitative differences in the properties of ${\mathrm{KOs}}_{2}{\mathrm{O}}_{6}$ and its analogs ${\mathrm{RbOs}}_{2}{\mathrm{O}}_{6}$ and ${\mathrm{CsOs}}_{2}{\mathrm{O}}_{6}$ arise from the relative weakness of intersite coupling, which is related to limited space for rattling in the latter compounds. We argue that the unusual physical properties of ${\mathrm{KOs}}_{2}{\mathrm{O}}_{6}$ are related to unconventional lattice dynamics rather than electronic correlations.

The electronic structure, magnetic moment, and volume collapse of MnO under pressure are obtained from four different correlated band theory methods; local density approximation + Hubbard U (LDA+U), pseudopotential self-interaction correction (pseudo-SIC), the hybrid functional (combined local exchange plus Hartree-Fock exchange), and the local spin density SIC (SIC-LSD) method. Each method treats correlation among the five Mn 3d orbitals (per spin), including their hybridization with three O $2p$ orbitals in the valence bands and their changes with pressure. The focus is on comparison of the methods for rocksalt MnO (neglecting the observed transition to the NiAs structure in the 90-100 GPa range). Each method predicts a first-order volume collapse, but with variation in the predicted volume and critical pressure. Accompanying the volume collapse is a moment collapse, which for all methods is from high-spin to low-spin (5/2 to 1/2), not to nonmagnetic as the simplest scenario would have. The specific manner in which the transition occurs varies considerably among the methods: pseudo-SIC and SIC-LSD give insulator-to-metal, while LDA+U gives insulator-to-insulator and the hybrid method gives an insulator-to-semimetal transition. Projected densities of states above and below the transition are presented for each of the methods and used to analyze the character of each transition. In some cases the rhombohedral symmetry of the antiferromagnetically ordered phase clearly influences the character of the transition., 14 pages, 9 figures. A 7 institute collaboration, Updated version

The goal to describe plutonium phases from ``first principles'' calculation methods is complicated by the problem of 5f-electrons localization. While for early actinides (Th, U,Np) standard DFT (Density Functional Theory) description with itinerant 5f-electrons works well for late actinides (Am, Cm) DFT calculations with completely localized (pseudocore) 5f-electrons give satisfactory results. However plutonium presents a border case of partial localization and both limits (itinerant and completely localized) are not valid. We present a review of the methods used to solve this problem and discuss what could be the reasons for their successes and failures., Comment: 19 pages, 18 figures, to be published in The Journal of Alloys and Compounds

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 electron microscope, already a powerful research instrument, could become even more powerful following the discovery that magnetic circular dichroism can be detected with a conventional transmission electron microscope. Materials display magnetic circular dichroism if the absorption of left and right circularly polarized light differs in the presence of an applied magnetic field. Application of this effect using synchrotron X-ray photons is a powerful tool for the investigation of magnetic phenomena. The new technique — EMCD or energy loss magnetic chiral dichroism — exploits the similarities between X-ray absorption and inelastic electron scattering to give a TEM capabilities normally associated with expensive synchrotrons. EMCD could be useful in many fields including spintronics and nanomagnetism. Comparison of measurements of electron energy-loss magnetic chiral dichroism with X-ray magnetic circular dichroism spectra obtained from the same specimen, together with theoretical calculations, show that chiral atomic transitions in a specimen are accessible with inelastic electron scattering under particular scattering conditions. A material is said to exhibit dichroism if its photon absorption spectrum depends on the polarization of the incident radiation. In the case of X-ray magnetic circular dichroism (XMCD), the absorption cross-section of a ferromagnet or a paramagnet in a magnetic field changes when the helicity of a circularly polarized photon is reversed relative to the magnetization direction. Although similarities between X-ray absorption and electron energy-loss spectroscopy in a transmission electron microscope (TEM) have long been recognized, it has been assumed that extending such equivalence to circular dichroism would require the electron beam in the TEM to be spin-polarized. Recently, it was argued on theoretical grounds that this assumption is probably wrong1. Here we report the direct experimental detection of magnetic circular dichroism in a TEM. We compare our measurements of electron energy-loss magnetic chiral dichroism (EMCD) with XMCD spectra obtained from the same specimen that, together with theoretical calculations, show that chiral atomic transitions in a specimen are accessible with inelastic electron scattering under particular scattering conditions. This finding could have important consequences for the study of magnetism on the nanometre and subnanometre scales, as EMCD offers the potential for such spatial resolution down to the nanometre scale while providing depth information—in contrast to X-ray methods, which are mainly surface-sensitive.

Charge disproportionation (CD) and spin differentiation in ${\mathrm{N}\mathrm{a}}_{1/2}{\mathrm{C}\mathrm{o}\mathrm{O}}_{2}$ are studied using the correlated band local-density approximation $+$ Hubbard U (LDA+U) approach. The simultaneous CD and gap opening seen previously is followed in detail through a first-order charge disproportionation transition $2{\mathrm{C}\mathrm{o}}^{3.5+}\ensuremath{\rightarrow}{\mathrm{C}\mathrm{o}}^{3+}+{\mathrm{C}\mathrm{o}}^{4+}$. Disproportionation in the Co ${a}_{g}$ orbital results in half of the ions (${\mathrm{C}\mathrm{o}}^{3+}$) becoming electronically and magnetically dead, transforming the quarter-filled ${a}_{g}$ system into a half-filled subsystem that subsequently undergoes the observed charge ordering or metal-insulator transition. Comparing with data in the $x\ensuremath{\approx}0.3$ regime suggests the system has moved into the multiband regime where the effective Coulomb repulsion $U\ensuremath{\rightarrow}{U}_{\mathrm{e}\mathrm{f}\mathrm{f}}=U/\sqrt{3}$ strongly lessens correlation effects.

The highest superconducting temperature T$_c$ observed in any elemental metal (Li with T$_c$ ~ 20 K at pressure P ~ 40 GPa) is shown to arise from critical (formally divergent) electron-phonon coupling to the transverse T$_1$ phonon branch along intersections of Kohn anomaly surfaces with the Fermi surface. First principles linear response calculations of the phonon spectrum and spectral function $\alpha^2 F(\omega)$ reveal (harmonic) instability already at 25 GPa. Our results imply that the fcc phase is anharmonically stabilized in the 25-38 GPa range., Comment: 4 pages, 3 embedded figures

Electronic, magnetic, and dynamical properties of superconducting $\ensuremath{\beta}$-pyrochlore $\mathrm{K}{\mathrm{Os}}_{2}{\mathrm{O}}_{6}$ and related $\mathrm{Rb}{\mathrm{Os}}_{2}{\mathrm{O}}_{6}$ and $\mathrm{Cs}{\mathrm{Os}}_{2}{\mathrm{O}}_{6}$ compounds are calculated and compared with experiment and contrasted with structurally related spinel pyrochlores. The calculated susceptibility Stoner enhancement (110%) and thermal mass enhancement $\ensuremath{\lambda}=2.5--3$ reflect moderate but perhaps important Coulomb correlations. The ${\mathrm{K}}^{+}$ ion optic mode is found to be unstable, allowing large excursions of $0.5--0.6\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$ from its ideal site of the K ion along ⟨111⟩ directions. This dynamical mode is much less anharmonic in the isostructural Rb and Cs compounds (with larger cations), perhaps accounting for their progressively lower values of ${T}_{c}$. Electron scattering from this very anharmonic mode may be the cause of the anomalous concave-downward resistivity that is seen only in $\mathrm{K}{\mathrm{Os}}_{2}{\mathrm{O}}_{6}$.

The strength and effect of Coulomb correlations in the (superconducting when hydrated) $x\ensuremath{\approx}1∕3$ and ``enhanced'' $x\ensuremath{\approx}2∕3$ regimes of ${\mathrm{Na}}_{x}{\mathrm{CoO}}_{2}$ are evaluated using the correlated band theory $\mathrm{LDA}+\mathrm{U}$ (local density approximation of Hubbard U) method. Our results, neglecting quantum fluctuations, are (1) allowing only ferromagnetic order, there is a critical ${U}_{c}=3\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$, above which charge disproportionation occurs for both $x=1∕3$ and $x=2∕3$, (2) allowing antiferromagnetic order at $x=1∕3$, ${U}_{c}$ drops to $1\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ for disproportionation, (3) disproportionation and gap opening occur simultaneously, and (4) in a ${\mathrm{Co}}^{3+}\ensuremath{-}{\mathrm{Co}}^{4+}$ ordered state, antiferromagnetic coupling is favored over ferromagnetic, while below ${U}_{c}$ ferromagnetism is favored. A comparison of the calculated Fermi level density of states compared to reported linear specific heat coefficients indicates enhancement of the order of five for $x\ensuremath{\sim}0.7$, but negligible enhancement for $x\ensuremath{\sim}0.3$. This trend is consistent with strong magnetic behavior and local moments (Curie-Weiss susceptibility) for $xg0.5$ while there is no magnetic behavior or local moments reported for $xl0.5$. We suggest that the phase diagram is characterized by a crossover from effective single-band character with $U⪢W$ for $xg0.5$ into a three-band regime for $xl0.5$, where $U\ensuremath{\rightarrow}{U}_{\mathrm{eff}}\ensuremath{\leqslant}U∕\sqrt{3}\ensuremath{\sim}W$ and correlation effects are substantially reduced.

${\mathrm{FeI}}_{2}$ is an insulating antiferromagnet with an unusual magnetic structure at ambient pressure. Recently, hyperfine field, electric field gradient, and other local quantities at the Fe site were measured over a wide range of pressures indicating a collapse of the orbital magnetic moment below the insulator-to-metal transition. In the present work we apply local density approximation (LDA) and LDA+U approaches to calculate the electronic structure and to investigate the Fe on-site quantities. We propose that the vanishing of the orbital moment is connected to the change of orbital order as reflected in the symmetry of the occupied minority-spin orbital. We find good agreement between the calculated and measured hyperfine fields and quadrupolar shifts, and make predictions which can serve to validate the proposed mechanism of orbital moment collapse experimentally.

The electronic and optical properties of DyP and DyBi are investigated both experimentally and computationally. The reflectivity spectra, which have been measured up to 12 eV on single crystals, display richly peaked spectral structures that are analogous for both pnictides. From the measured reflectivities the plasma frequencies, Drude relaxation times, and optical conductivity spectra are derived. The fitted Drude conductivity reveals that DyP and DyBi are semimetals with a number of free carriers of about 0.16 and 0.23 per formula unit, respectively. The very-structured experimental optical conductivity spectra are compared to calculated spectra, which are computed using two different approaches to the Dy 4f states: the open-core approach and the L(S)DA+U approach in three versions. These approaches to the 4f states lead to very similar optical spectra. There exists a reasonable agreement between calculation and experiment for a number of the spectral features, which are interpreted by specific interband transitions within the calculated band structure. The agreement between theory and experiment substantiates that the 4f electrons do not participate in the bonding. The differences that remain between theory and experiment for some of the spectral features do not appear to rest on aspects of the treatment of the 4f states, but rather to be intrinsic shortcomings in the description of the other band states.

The hyperfine field is a precise and essential probe of the magnetic state of a solid, and of the quality of theoretical core wave functions, but it's accurate evaluation has proven challenging from first principles. In this work, the self-interaction free potential, suggested recently by Lundin and Eriksson, is applied to the core states in the calculation of the hyperfine field for 3d transition metal ferromagnets Fe, Co, and Ni, and for three Fe compounds. Compared to the local spin density approximation and to its conventional self-interaction corrected form, the new potential functional is found to increase substantially the core contribution to the Fermi contact term, leading to good agreement with measurements for Fe and Co, and significantly better results for iron compounds. Our results strongly suggest that the new functional is more suitable for generating realistic core wave functions to high accuracy for a wide range of materials. The subtle effects resulting from the change of potential functional are also addressed.

It is shown that the spin-polarized electronic structure of antiferromagnetic (AFM) materials can be investigated on an element-selective level using a versatile, novel x-ray magneto-optical spectroscopy in reflection. We demonstrate this with spectra observed at the Ni ${L}_{3}$ absorption edge of two buried exchange-biased microstructures of current technological interest: NiO/Co and NiMn/Co, which contain the insulating antiferromagnet NiO, and the metallic antiferromagnet NiMn, respectively. The measured spectrum provides information about the exchange-split d density of states of the AFM atom.

Experimental and theoretical x-ray magnetic circular dichroism (XMCD) studies of the intermetallic compounds UCoAl and UPtAl at the uranium M 4 and M 5 edges are reported. UPtAI is a 5f-electron ferromagnet, whereas UCoAl exhibits, at low temperatures and in a magnetic field of 0.65 T, a metamagnetic transition from paramagnetic to ferromagnetic ordering of U moments. Applying the XMCD sum rules to the experimental spectra, expectation values of the orbital and spin magnetic moments of the uranium 5f electrons were calculated. The results show that the orbital-to-spin moment ratio is of comparable value, μ L μ/ S -2, for both compounds; however, the moments in UCoAl are strongly reduced with respect to UPtAl. Furthermore, the μ L /μ S ratio for UCoAl appears to be conserved both in the ferromagnetic and paramagnetic states, although the μ S value in the former case is at least five times larger. The theoretical counterparts of the experimental data were obtained from the electronic structure calculated using the FLAPW method within the local spin density approximation. The calculated x-ray absorption and XMCD spectra show a satisfactory agreement with the experimental data and reproduce well the shape and structure of the spectral lines at the M 4 and M5 edges.

Understanding of the damping of fast magnetization precession in ferromagnetic materials is important for technological applications, in particular for the development of fast magnetic memories. We use first-principles electronic structures together with the model of ``breathing Fermi surface'' to calculate the magnetic damping rates for ferromagnetic Fe, Co, and Ni. The numerical results are found to be in reasonable agreement with the experimental data at low temperatures; in particular, we find an order of magnitude difference between Fe and the other two studied materials.

We report a combined experimental-computational investigation of the electronic structure of ${\mathrm{CrO}}_{2}.$ We have measured the magneto-optical Kerr spectra of ${\mathrm{CrO}}_{2}$ at 10 K and 300 K. At 10 K the Kerr signal is significantly enhanced over that obtained at 300 K. We compare the measured Kerr spectra to first-principles theoretical spectra, which we computed using three different approximations to the exchange-correlation functional, i.e., the local spin-density approximation (LSDA), generalized gradient approximation (GGA), and $\mathrm{LSDA}+U.$ The experimental low-temperature magneto-optical Kerr spectra are best explained by calculations employing the GGA functional. The addition of an on-site Coulomb correlation U does not lead to reasonable Kerr spectra.