Itinerant magnetism of chromium under pressure: a DFT+DMFT study.

We consider electronic and magnetic properties of chromium, a well-known itinerant antiferromagnet, by a combination of density functional theory (DFT) and dynamical mean-field theory (DMFT). We find that electronic correlation effects in chromium, in contrast to its neighbors in the periodic table, are weak, leading to the quasiparticle mass enhancement factorm*/m≈ 1.2. Our results for local spin-spin correlation functions and distribution of weights of atomic configurations indicate that the local magnetic moments are not formed. Similarly to previous results of DFT at ambient pressure, the non-uniform magnetic susceptibility as a function of momentum possesses close to the wave vectorQH= (0, 0, 2π/a) (ais the lattice constant) sharp maxima, corresponding to Kohn anomalies. We find that these maxima are preserved by the interaction and are not destroyed by pressure. Our calculations qualitatively capture a decrease of the Néel temperature with pressure and a breakdown of itinerant antiferromagnetism at pressure of ∼9 GPa in agreement with experimental data, although the Néel temperature is significantly overestimated because of the mean-field nature of DMFT.

Electronic correlation effects and local magnetic moments in L10 phase of FeNi.

We study the electronic and magnetic properties of L10 phase of FeNi, a perspective rare-earth-free permanent magnet, by using a combination of density functional and dynamical mean-field theory. Although L10 FeNi has a slightly tetragonally distorted fcc lattice, we find that magnetic properties of its constituent Fe atoms resemble those in pure bcc Fe. In particular, our results indicate the presence of well-localized magnetic moments on Fe sites, which are formed due to Hund's exchange. At the same time, magnetism of Ni sites is much more itinerant. Similarly to pure bcc Fe, the self-energy of Fe 3d states is found to show the non-Fermi-liquid behavior. This can be explained by peculiarities of density of Fe 3d states, which has pronounced peaks near the Fermi level. Our study of local spin correlation function and momentum dependence of particle-hole bubble suggests that the magnetic exchange in this substance is expected to be of RKKY-type, with iron states providing local-moment contribution, and the states corresponding to nickel sites (including virtual hopping to iron sites) providing itinerant contribution.

Potential capabilities of aircraft laser landing systems.

We present calculations of the efficiency of the laser landing system (LLS), based on determining the minimum required fluxes of scattered radiation from fixed extended landmarks (FELs), which are LLS indicators in the case of visual FEL detection under real operation conditions. It is shown that, when the meteorological visibility range Sm=800 m, for reliable detection of laser beams from the glissade slope group at ranges L∼1.0-1.6 km under nighttime conditions, the minimum required powers are Pmin=0.5 W for λ=0.52 and 0.64 µm, given deviations from the glissade path by the angle ϕ=0°-5°. The green and red rays are visible at distances L=1-1.2 km under twilight conditions. Our calculations corroborated the possibility of creating a new-generation laser-based LLS capable of ensuring aircraft landing under the conditions of International Civil Aviation Organization category 1 (decision height of 60 m at the minimum visibility equal 800 m).

Structural γ-ε phase transition in Fe-Mn alloys from a CPA + DMFT approach.

We present a computational scheme for total energy calculations of disordered alloys with strong electronic correlations. It employs the coherent potential approximation combined with the dynamical mean-field theory and allows one to study the structural transformations. The material-specific Hamiltonians in the Wannier function basis are obtained by density functional theory. The proposed computational scheme is applied to study the γ-ε structural transition in paramagnetic Fe-Mn alloys for Mn content from 10 to 20 at.%. The electronic correlations are found to play a crucial role in this transition. The calculated transition temperature decreases with increasing Mn content and is in good agreement with experiment. We demonstrate that in contrast to the α-γ transition in pure iron, the γ-ε transition in Fe-Mn alloys is driven by a combination of kinetic and Coulomb energies. The latter is found to be responsible for the decrease of the γ-ε transition temperature with Mn content.

Correlation-Driven Topological Fermi Surface Transition in FeSe.

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, (π,π) to (π,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.

Hellmann-Feynman forces within the DFT + U in Wannier functions basis.

The most general way to describe localized atomic-like electronic states in strongly correlated compounds is to use Wannier functions. In the present paper we continue development of widely-used DFT + U method with the Wannier function basis set and propose a technique to calculate Hubbard contribution to atomic forces. The technique was implemented as a part of plane-waves pseudopotential code Quantum-ESPRESSO and tested on two compounds: charge transfer insulator NiO with cubic crystal structure and correlated metal SrVO3 with perovskite structure.

Mechanism of magnetic moment collapse under pressure in ferropericlase.

We propose a new scenario for the magnetic collapse under pressure in ferropericlase (FP) (Fe(1/4)Mg(3/4))O without the presence of intermediate spin state, which contradicts the mechanism proposed in (2013 Phys. Rev. B 87 165113). This scenario is supported by results of combined local density approximation + dynamical mean-field theory method calculations for the paramagnetic phase at ambient and high pressures. At ambient pressure, calculation gave (Fe(1/4)Mg(3/4))O as an insulator with Fe 3d-shell in high-spin state. Experimentally observed high-spin to low-spin state transition of the Fe(2+) ion in the pressure range of 35-75 GPa is successfully reproduced in our calculations. The spin crossover is characterized by coexistence of Fe(2+) ions in high and low spin state but intermediate spin state is absent in the whole pressure range. Moreover, the probability of Fe ion d(7) onfiguration with S = 1 grows with pressure due to shortening of metal-oxygen distance. Also, no metal-insulator transition was obtained up to the pressure 140 GPa in agreement with experiment.

Investigation of electronic structure and magnetic properties of CaCo1.86As2 within the CPA method.

Recently in iron free arsenide compound CaCo(2)As(2) a 7(1)% of vacancies on the Co sites was detected (Quirinale D G et al 2013 Phys. Rev. B 88 174420). Here we report the investigation of electronic structure and magnetic properties of CaCo(1.86)As(2) within the coherent potential approximation (CPA). First, the CPA calculations are performed on the base of the local spin density approximation. Second, the possible role of Coulomb correlations is checked within the CPA scheme developed recently for strongly correlated systems. Then the spin-orbit coupling, which could be essential for Co, is also taken into account within the CPA calculation. The A type antiferromagnetic ground state and the value of magnetic moment obtained within the CPA approximation are in good agreement with experiment.

Coulomb interaction parameters in bcc iron: an LDA+DMFT study.

We study the influence of Coulomb interaction parameters on electronic structure and magnetic properties of paramagnetic bcc Fe by means of the local density approximation plus dynamical mean-field theory approach. We consider the local Coulomb interaction in the density-density form as well as in the form with spin rotational invariance approximated by averaging over all directions of the quantization axis. Our results indicate that the magnetic properties of bcc Fe are mainly affected by the Hund's rule coupling J rather than by the Hubbard U. By employing the constrained density functional theory approach in the basis of Wannier functions of spd character, we obtain U = 4 eV and J = 0.9 eV. In spite of the widespread belief that U = 4 eV is too large for bcc Fe, our calculations with the obtained values of U and J result in a satisfactory agreement with the experiment. The correlation effects caused by U are found to be weak even for large U = 6 eV. The agreement between the calculated and experimental Curie temperatures is further improved if J is reduced to 0.8 eV. However, with the decrease of J, the effective local magnetic moment moves further away from the experimental value.

Electronic correlations determine the phase stability of iron up to the melting temperature.

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 (T1) mode and a dynamical instability of the bcc lattice in harmonic approximation are identified. We relate these features to the α-to-γ and γ-to-δ phase transformations in iron. The high-temperature bcc phase is found to be highly anharmonic and appears to be stabilized by the lattice entropy.

First-principles calculation of atomic forces and structural distortions in strongly correlated materials.

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 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, SrVO3, and KCuF3.

Correlation effects and phonon modes softening with doping in Ba1-xKxBiO3.

The monoclinic crystal structure of the undoped BaBiO3 can be described as a cubic perovskite which is distorted by the frozen breathing and tilting phonon modes of the BiO6 octahedra. The phonon mode softening is experimentally observed (Braden et al 1996 Europhys. Lett. 34 531) in Ba1-xKxBiO3 through potassium doping followed by a transition into an ideal cubic perovskite structure at x = 0.37, close to the emergence of superconductivity. In our previous paper (Korotin et al 2012 J. Phys.: Condens. Matter 24 415603) we demonstrated that it is necessary to take into account correlation effects using the DFT+U method in Wannier functions as a basis to obtain a good agreement between the calculated and experimental values of crystal structure distortion and the energy gap in BaBiO3. In the present work, using the same method, we calculated the breathing mode phonon frequencies as a function of the potassium doping level in Ba1-xKxBiO3. The obtained frequencies are in good agreement with experimental values and the breathing mode softening with doping is reproduced, contrary to calculations made without consideration of correlation effects. We show that the cubic crystal structure becomes stable at x = 0.30 in agreement with the experimental transition to cubic perovskite at x = 0.37. The possible connections between the correlation effects, phonon mode softening and superconductivity in Ba1-xKxBiO3 are discussed.

The coherent potential approximation for strongly correlated systems: electronic structure and magnetic properties of NiO-ZnO solid solutions.

A method of electronic structure calculations for strongly correlated disordered materials is developed employing the basic idea of the coherent potential approximation. The evolution of the electronic structure and spin magnetic moment value with the concentration x in strongly correlated Ni1-xZnxO solid solutions is investigated in the framework of this method. The values of the energy gap and magnetic moment obtained are in agreement with the available experimental data.

Investigation of real materials with strong electronic correlations by the LDA+DMFT method.

Materials with strong electronic correlations are at the cutting edge of experimental and theoretical studies, capturing the attention of researchers for a great variety of interesting phenomena: metal-insulator, phase and magnetic spin transitions, `heavy fermion' systems, interplay between magnetic order and superconductivity, appearance and disappearance of local magnetic moments, and transport property anomalies. It is clear that the richness of physical phenomena for these compounds is a result of partially filled 3d, 4f or 5f electron shells with local magnetic moments preserved in the solid state. Strong interactions of d and f electrons with each other and with itinerant electronic states of the material are responsible for its anomalous properties. Electronic structure calculations for strongly correlated materials should explicitly take into account Coulombic interactions between d or f electrons. Recent advances in this field are related to the development of the LDA+DMFT method, which combines local density approximation (LDA) with dynamical mean-field theory (DMFT) to account for electronic correlation effects. In recent years, LDA+DMFT has allowed the successful treatment not only of simple systems but also of complicated real compounds. Nowadays, the LDA+DMFT method is the state-of-the-art tool for investigating correlated metals and insulators, spin and metal-insulator transitions (MIT) in transition-metal compounds in paramagnetic and magnetically ordered phases.

Ab initio study on the rare-earth iron-pnictides RFeAsO (R = Pr, Nd, Sm, Gd) in the low-temperature Cmma phase.

We present density functional theory calculations on the iron-based pnictides RFeAsO (R = Pr, Nd, Sm, Gd). The calculations have been carried out using plane waves and the projector augmented wave (PAW) pseudopotential approach. Structural, magnetic and electronic properties are studied within the generalized gradient approximation (GGA) and also within GGA + U in order to investigate the influence of electron correlation effects. The low-temperature Cmma structure is fully optimized by the GGA considering both non-magnetic and magnetic cells. We have found that the spin-polarized structure improves the agreement with experiments on equilibrium lattice parameters, particularly the c lattice parameter and the Fe-As bond-lengths. The electronic band structure, total density of states, and spin-dependent orbital-resolved density of states are also analyzed and discussed in the frameworks of GGA and GGA + U. For all materials, by including an on-site Coulomb correction, the rare-earth 4f states move away from the Fermi level and the Fermi level features of the systems are found to be mostly defined by the 3d electron-electron correlations in Fe.

Electronic correlations and crystal structure distortions in BaBiO3.

BaBiO3 is a material where Bi4+ ions with half-filled 6s-states form an alternating set of Bi3+ and Bi5+ ions resulting in a charge ordered insulator. The charge ordering is accompanied by breathing distortion of the BiO6 octahedra (extension and contraction of the Bi-O bond lengths). Standard density functional theory (DFT) calculations fail to obtain the crystal structure instability caused by the pure breathing distortions. Combining effects of the breathing distortions and tilting of the BiO6 octahedra allows DFT to reproduce qualitatively an experimentally observed insulator with monoclinic crystal structure but strongly underestimates the breathing distortion parameter and energy gap values. In the present work we reexamine the BaBiO3 problem within the GGA + U method using a Wannier function basis set for the Bi 6s-band. Due to the high oxidation state of bismuth in this material, the Bi 6s-symmetry Wannier function is predominantly extended spatially on surrounding oxygen ions and hence differs strongly from a pure atomic 6s-orbital. That is in sharp contrast to transition metal oxides (with exclusion of high oxidation state compounds) where the major part of the d-band Wannier function is concentrated on the metal ion and a pure atomic d-orbital can serve as a good approximation. The GGA + U calculation results agree well with experimental data, in particular with experimental crystal structure parameters and energy gap values. Moreover, the GGA + U method allows one to reproduce the crystal structure instability due to the pure breathing distortions without octahedra tilting.

Electronic correlations at the α-γ structural phase transition in paramagnetic iron.

We compute the equilibrium crystal structure and phase stability of iron at the α(bcc)-γ(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 α-γ structural phase transition in paramagnetic iron.

Linear-temperature dependence of static magnetic susceptibility in LaFeAsO from dynamical mean-field theory.

In this Letter we report the local density approximation with dynamical mean field theory results for magnetic properties of the parent superconductor LaFeAsO in the paramagnetic phase. Calculated uniform magnetic susceptibility shows linear dependence at intermediate temperatures in agreement with experimental data. Contributions to the temperature dependence of the uniform susceptibility are strongly orbitally dependent. For high temperatures (>1000 K) susceptibility first saturates and then decreases with temperature. Our results demonstrate that linear-temperature dependence of static magnetic susceptibility in pnictide superconductors can be reproduced without invoking antiferromagnetic fluctuations.

Electronic structure and magnetic state of transuranium metals under pressure.

The electronic structures of bcc Np, fcc Pu, Am, and Cm pure metals under pressure have been investigated employing the LDA + U method with spin-orbit coupling (LDA + U + SO). The magnetic state of the actinide ions was analyzed in both LS and jj coupling schemes to reveal the applicability of corresponding coupling bases. It was demonstrated that whereas Pu and Am are well described within the jj coupling scheme, Np and Cm can be described appropriately neither in a {mσ}, nor in a {jmj} basis, due to intermediate coupling scheme realization in these metals that requires some finer treatment. The LDA + U + SO results for the considered transuranium metals reveal band broadening and gradual 5f electron delocalization under pressure.

Pressure-driven metal-insulator transition in hematite from dynamical mean-field theory.

The local density approximation combined with dynamical mean-field theory is applied to study the paramagnetic and magnetically ordered phases of hematite Fe2O3 as a function of volume. As the volume is decreased, a simultaneous first-order insulator-metal and high-spin to low-spin transition occurs close to the experimental value of the critical volume. The high-spin insulating phase is destroyed by a progressive reduction of the spectral gap with increasing pressure, upon closing of which the high-spin phase becomes unstable. We conclude that the transition in Fe2O3 at approximately 50 GPa can be described as an electronically driven volume collapse.