It has been recently shown that under pressure trilayer Ruddlesden-Popper nickelate La$_4$Ni$_3$O$_{10}$ (LNO) becomes superconducting below a critical temperature $\sim$20 K, in addition to the infinite-layer and bilayer systems. Motivated by this observation, we explore the effects of electron correlations on its electronic structure and magnetic properties using the advanced DFT+dynamical mean-field theory approach. Our results for the normal-state electronic structure and correlation effects in LNO have much in common with the infinite-layer and bilayer nickelates, with a remarkable site- and orbital-dependent renormalizations of the Ni $3d$ bands and notable incoherence of the Ni $d_{3z^2-r^2}$ states, caused by correlation effects. Our analysis of the Fermi surface and magnetic correlations suggests the emergence of competing spin and charge stripe states, implying the importance of in-plane spin fluctuations to explain superconductivity in this material.

Using the density functional theory plus dynamical mean-field theory method, we revisit the pressuretemperature phase diagram of the prototypical correlated insulator NiO. We study the pressure-induced evolution of the electronic structure, magnetic state, and exchange couplings of the antiferromagnetic (AFM) phase of NiO. We calculate the ordered magnetic moments and static magnetic spin susceptibility of the Ni 3d states of NiO, which allow us to determine the pressure dependence of the N & eacute;el temperature TN. We note that the long-range magnetism has no significant effects on the valence band photoemission spectra of NiO under moderate compressions, implying the importance of correlation effects to explain the insulating state of NiO. Upon compression, we observe a crossover from a charge transfer to the Mott-Hubbard insulating character of the insulating gap, with a predominant contribution from the majority Ni eg states near the Fermi level. The insulating state of AFM NiO is found to be stable up to a high compression similar to 0.4V0 (assuming the cubic B1 crystal structure of NiO). It is categorized as a correlation-assisted Slater-type insulator, implying the importance of long-range magnetic ordering. In fact, the paramagnetic phase of NiO at such high compression is found to be metallic, characterized by strong delocalization of the Ni 3d states. The calculated TN exhibits a nonmonotonic behavior upon compression, with a maximum associated with the crossover from Mott localized (strong coupling) to itinerant moment regimes, in qualitative agreement with the phase diagram of the half-filled single-band Hubbard model. We point out the importance of the nonlocal correlation effects to explain the magnetic properties of NiO.

We report a theoretical study of the effects of electronic correlations, magnetic properties, and chemical bonding in the recently synthesized high-pressure orthorhombic phase of FeN2 using the density functional theory plus dynamical mean-field theory approach. Our analysis documents a complex crystal-chemical behavior of FeN2 characterized by the formation of a strongly covalent N-N bond with an unexpected valence state of Fe ions 3+ (paramagnetic ferric Fe3+ ions in the low-spin state), in agreement with available experimental data. Our results reveal weak (orbital-dependent) correlation effects, which are complicated by the possible emergence of multiple spin density wave states on a microscopic level. This suggests the importance of antiferromagnetic spin fluctuations to explain the properties of FeN2 under pressure.

Iron-bearing oxides undergo a series of pressure-induced electronic, spin, and structural transitions that can cause seismic anomalies and dynamic instabilities in Earth's mantle and outer core. We employ x-ray diffraction and x-ray emission spectroscopy along with density-functional theory+dynamical mean-field theory calculations to characterize the electronic structure and spin states, and crystal-structural properties of wustite (Fe1-xO)-a basic oxide component of Earth's interior-at high pressure-temperature conditions up to 140 GPa and 2100 K. We find that FeO exhibits complex polymorphism under pressure, with abnormal compression behavior associated with electron-spin and crystallographic phase transitions, and resulting in a substantial change of bulk modulus. Our results reveal the existence of a high-pressure phase characterized by a metallic high-spin state of iron at the pressure-temperature conditions near to those of Earth's core-mantle boundary. The presence of high-spin metallic iron near the mantle can significantly influence the geophysical and geochemical properties of Earth's deep interior.

We report a theoretical study of the effects of electron correlations and structural confinement on the electronic properties and magnetic state of LaNiO 3 (LNO) thin films epitaxially deposited on the (001) LaAlO 3 (LAO) substrate. Using the DFT + U method we compute the electronic band structure, magnetic properties, and phase stability of the 1.5 unit-cell-thick NiO 2 -terminated LNO thin films. Our results reveal complex diversity of the electronic states caused by the effects of structural confinement, interfacial charge transfer and electronic correlations. Our calculations suggest the appearance of in-plane (110) charge disproportionation of the Ni ions in the interface NiO 2 layer of the antiferromagnetically ordered LNO thin films. Moreover, the electronic states of both the antiferromagnetic and ferromagnetic LNO/LAO show a large orbital polarization of the Ni ions in the surface NiO 2 layers. Our results suggest the crucial importance of oxygen defects to explain the metal-to-insulator phase transition experimentally observed in a few-unit-cell-thick LNO/LAO thin films.

Using ab initio band structure and DFT + dynamical mean-field theory methods we examine the effects of electron-electron interactions on the normal state electronic structure, Fermi surface, and magnetic correlations of the recently discovered double-layer perovskite superconductor La3Ni2O7 under pressure. Our results suggest the formation of a negative charge transfer mixed-valence state with the Ni valence close to 1.75+. We find a remarkable orbital-selective renormalization of the Ni 3d bands, with m*/m similar to 3 and 2.3 for the Ni 3z(2) -r(2) and x(2) -y(2) orbitals, respectively, in agreement with experimental estimates. Our results for the k-dependent spectral functions and Fermi surfaces show significant incoherence of the Ni 3z(2) - r(2) states, implying the proximity of the Ni 3d states to orbital-dependent localization. Based on our analysis of the static magnetic susceptibility, we propose the possible formation of the spin and charge (or bond) density wave stripe states in high-pressure La3Ni2O7.

We perform a theoretical study of the electronic structure and magnetic properties of the prototypical magnetic MAX-phase Mn$_2$GaC with the main focus given to the origin of magnetic interactions in this system. Using the density functional theory+dynamical mean-field theory (DFT+DMFT) method we explore the effects of electron-electron interactions and magnetic correlations on the electronic properties, magnetic state, and spectral weight coherence of paramagnetic and magnetically-ordered phases of Mn$_2$GaC. We also benchmark the DFT-based disordered local moment approach for this system by comparing the obtained electronic and magnetic properties with that of the DFT+DMFT method. Our results reveal a complex magnetic behavior characterized by a near degeneracy of the ferro- and antiferromagnetic configurations of Mn$_2$GaC, implying a high sensitivity of its magnetic state to fine details of the crystal structure and unit-cell volume, consistent with experimental observations. We observe robust local-moment behavior and orbital-selective incoherence of the spectral properties of Mn$_2$GaC, implying the importance of orbital-dependent localization of the Mn $3d$ states. We find that Mn$_2$GaC can be described in terms of local magnetic moments, which may be modeled by DFT with disordered local moments. However, the magnetic properties are dictated by the proximity to the regime of formation of local magnetic moments, in which the localization is in fact driven by the Hund's exchange interaction, and not the Coulomb interaction.

AbstractThe pressure-induced Mott insulator-to-metal transitions are often accompanied by a collapse of magnetic interactions associated with delocalization of 3delectrons and high-spin to low-spin (HS-LS) state transition. Here, we address a long-standing controversy regarding the high-pressure behavior of an archetypal Mott insulator FeBO3and show the insufficiency of a standard theoretical approach assuming a conventional HS-LS transition for the description of the electronic properties of the Mott insulators at high pressures. Using high-resolution x-ray diffraction measurements supplemented by Mössbauer spectroscopy up to pressures ~ 150 GPa, we document an unusual electronic state characterized by a “mixed” HS/LS state with a stable abundance ratio realized in the$$R\overline{3 }c$$R3¯ccrystal structure with a single Fe site within a wide pressure range of ~ 50–106 GPa. Our results imply an unconventional cooperative (and probably dynamical) nature of the ordering of the HS/LS Fe sites randomly distributed over the lattice, resulting in frustration of magnetic moments.

The metal–insulator transition driven by electronic correlations is one of the most fundamental concepts in condensed matter. In mixed-valence compounds, this transition is often accompanied by charge ordering (CO), resulting in the emergence of complex phases and unusual behaviors. The famous example is the archetypal mixed-valence mineral magnetite, Fe3O4, exhibiting a complex charge-ordering below the Verwey transition, whose nature has been a subject of long-time debates. In our study, using high-resolution X-ray diffraction supplemented by resistance measurements and DFT+DMFT calculations, the electronic, magnetic, and structural properties of recently synthesized mixed-valence Fe4O5 are investigated under pressure to ∼100 GPa. Our calculations, consistent with experiment, reveal that at ambient conditions Fe4O5 is a narrow-gap insulator characterized by the original Verwey-type CO. Under pressure Fe4O5 undergoes a series of electronic and magnetic-state transitions with an unusual compressional behavior above ∼50 GPa. A site-dependent collapse of local magnetic moments is followed by the site-selective insulator-to-metal transition at ∼84 GPa, occurring at the octahedral Fe sites. This phase transition is accompanied by a 2+ to 3+ valence change of the prismatic Fe ions and collapse of CO. We provide a microscopic explanation of the complex charge ordering in Fe4O5 which “unifies” it with the behavior of two archetypal examples of charge- or bond-ordered materials, magnetite and rare-earth nickelates (RNiO3). We find that at low temperatures the Verwey-type CO competes with the “trimeron”/“dimeron” charge ordered states, allowing for pressure/temperature tuning of charge ordering. Summing up the available data, we present the pressure–temperature phase diagram of Fe4O5.

Using ab initio band structure methods and DFT-Pdynamical mean-field theory approach we explore the possible formation of spin and charge stripes in the Ni-O plane of hole-doped infinite-layer nickelates, RNiO2. Our results reveal a remarkable instability of the C-type (110) spin state with undistorted lattice towards the formation of the spin density, charge and bond disproportionation stripe phases accompanied by in-plane"breathinglike" distortions of the crystal structure. Our work gives a comprehensive picture of competing charge and spin stripe states, with possible frustration of different stripe patterns upon doping. It suggests that the spin and charge stripe state likely arises from strong magnetic correlations (with concomitant lattice distortions), which play a key role for understanding the anomalous properties of hole-doped layered nickelates.

An entry from the Inorganic Crystal Structure Database, the world’s repository for inorganic crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the joint CCDC and FIZ Karlsruhe Access Structures service and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

We compute the electronic structure, spin and charge state of Fe ions, and the structural phase stability of paramagnetic CaFeO3 under pressure using a fully self-consistent in charge density DFT + dynamical mean-field theory method. We show that at ambient pressure CaFeO3 is a negative charge transfer insulator characterized by strong localization of the Fe 3d electrons. It crystallizes in the monoclinic P21/n crystal structure with a cooperative breathing mode distortion of the lattice. While the Fe 3d Wannier occupations and local moments are consistent with robust charge disproportionation of Fe ions in the insulating P21/n phase, the physical charge density difference around the structurally distinct Fe A and Fe B ions with the "contracted" and "expanded" oxygen octahedra, respectively, is rather weak, of similar to 0.04. This implies the importance of the Fe 3d and O 2p negative charge transfer and supports the formation of a bond-disproportionated state characterized by the Fe A 3d5-8L2-8 and Fe B 3d5 valence configurations with 8 << 1, in agreement with strong hybridization between the Fe 3d and O 2p states. This complex interplay between electronic correlations, strong covalency, and lattice effects, resulting in bond disproportionation, is in many ways reminiscent of the behavior of rare-earth nickelates, RNiO3 (R = rare earth). Upon compression, CaFeO3 undergoes the metal-to-insulator phase transition (MIT) which is accompanied by a structural transformation into the orthorhombic Pbnm phase. The phase transition is accompanied by suppression of the cooperative breathing mode distortion of the lattice and, hence, results in the melting of bond disproportionation of the Fe ions. Our analysis suggests that the MIT transition is associated with orbital-dependent delocalization of the Fe 3d electrons and leads to a remarkable collapse of the local magnetic moments. Our results imply the crucial importance of the interplay of electronic correlations and structural effects to explain the properties of CaFeO3.

Charge and orbital ordering, magnetic state, and exchange couplings in quasi-one-dimensional vanadate V 6 O 13 , a potential cathode material for Li-ion batteries, are investigated using the density functional theory with Coulomb interaction correction method (DFT + U ). While the difference between t_2g orbital occupancies of V 4+ (with a nominal 3d^1 electronic configuration) and V 5+ ions is large and gives direct evidence for charge and orbital ordering, the screening is so effective that the total 3d charge disproportionation is rather small. Our results show that the occupied t_2g states of V 4+ ions in the single V–V layer form a spin-singlet molecular orbital, while the rest half of V 4+ ions in the structurally distinct double V–V layers order antiferromagnetically in the low-temperature insulating phase of V 6 O 13 . We conclude that the metal-insulator transition and low-temperature magnetic properties of V 6 O 13 involve the spin-Peierls transition assisted by orbital ordering and concomitant distortions of the crystal structure.

The main disadvantage of existing rescue devices for the emergency evacuation of people from high-rise buildings in the event of a fire hazard is that they are not intended for mass evacuation, but are rather the devices for individual use. Evacuations are usually carried out by controlled descent with an irrevocable loss of braking energy. Therefore, these devices cannot be used due to the limitations of their capabilities, as they are not adapted to the specific conditions of offshore drilling platforms. A review of scientific papers and patents devoted to this issue showed that there are gaps in their coverage of automation issues that improve the reliability of rescue devices. A team of Bauman Moscow State Technical University developed basic requirements for the design of modern rescue devices for offshore drilling platforms. The design of a mechanical rescue device, patented in the Russian Federation, meets these requirements.

We study the effects of electron-electron interactions and hole doping on the electronic structure of Cu-doped NaFeAs using the density functional theory plus dynamical mean-field theory (DFT+DMFT) method. In particular, we employ an effective multi-orbital Hubbard model with a realistic bandstructure of NaFeAs in which Cu-doping was modeled within a rigid band approximation and compute the evolution of the spectral properties, orbital-dependent electronic mass renormalizations, and magnetic properties of NaFeAs upon doping with Cu. In addition, we perform fully charge self-consistent DFT+DMFT calculations for the long-range antiferromagnetically ordered Na(Fe,Cu)As with Cu $x=0.5$ with a real-space ordering of Fe and Cu ions. Our results reveal a crucial importance of strong electron-electron correlations and local potential difference between the Cu and Fe ions for understanding the \textbf{k}-resolved spectra of Na(Fe,Cu)As. Upon Cu-doping, we observe a strong orbital-dependent localization of the Fe $3d$ states accompanied by a large renormalization of the Fe $xy$ and $xz$/$yz$ orbitals. Na(Fe,Cu)As exhibits bad metal behavior associated with a coherence-to-incoherence crossover of the Fe $3d$ electronic states and local moments formation near a Mott metal-insulator transition (MIT). For heavily doped NaFeAs with Cu $x \sim 0.5$ we obtain a Mott insulator with a band gap of $\sim$0.3 eV characterized by divergence of the quasiparticle effective mass of the Fe $xy$ states. In contrast to this, the quasiparticle weights of the Fe $xz$/$yz$ and $e_g$ states remain finite at the MIT. The MIT occurs via an orbital-selective Mott phase to appear at Cu $x\simeq0.375$ with the Fe $xy$ states being Mott localized. We propose the possible importance of Fe/Cu disorder to explain the magnetic properties of Cu-doped NaFeAs.

We report a theoretical study of the effect of electron-electron interactions and Sr doping on the electronic structure of infinite-layer (Nd,Sr)NiO2 using the DFT+dynamical mean-field theory approach (DFT+DMFT). Here, we explore the effect of lattice strain that experience (Nd,Sr)NiO2 films upon growing on the SrTiO3 substrate on the electronic properties, magnetic correlations, and exchange couplings of (Nd,Sr)NiO2. For both strained and unstrained Sr-doped NdNiO2 our results reveal the crucial importance of orbital-de-pendent correlation effects in the Ni 3d shell. Upon doping with Sr, it undergoes a Lifshitz transition which is accompanied by a reconstruction of magnetic correlations. For Sr x < 0.2 (Nd,Sr)NiO2 adopts the Neel (111) antiferromagnetic (AFM) order, while for x > 0.3 the C-type (110) AFM sets in the unstrained (Nd,Sr) NiO2, with a highly frustrated region at x similar or equal to 0.2, all within DFT+DMFT at T = 290 K. Our results for the Neel AFM at Sr x = 0 suggest that AFM NdNiO2 appears at the verge of a Mott-Hubbard transition, providing a plausible explanation for the experimentally observed weakly insulating behavior of NdNiO2 for Sr x < 0.1. We observe that the Lifshitz transition makes a change of the band structure character from electron- to hole-like with Sr x, in agreement with recent experiments. We conclude that the in-plane strain adjusts a bandwidth of the Ni x(2)-y(2) band, i.e., controls the effect of electron correlations in the Ni x(2)-y(2) orbitals. It leads to a suppression of the static C-type (110) ordering in (Nd,Sr)NiO2 for Sr x > 0.3. Our results for the electronic structure and magnetic correlations of (Nd,Sr)NiO2 reveal an anomalous sensitivity upon a change of the crystal structure parameters. (C) 2021 Elsevier B.V. All rights reserved.

Extreme pressures and temperatures are known to drastically affect the chemistry of iron oxides, resulting in numerous compounds forming homologous series nFeOmFe_{2}O_{3} and the appearance of FeO_{2}. Here, based on the results of in situ single-crystal x-ray diffraction, Mössbauer spectroscopy, x-ray absorption spectroscopy, and density-functional theory+dynamical mean-field theory calculations, we demonstrate that iron in high-pressure cubic FeO_{2} and isostructural FeO_{2}H_{0.5} is ferric (Fe^{3+}), and oxygen has a formal valence less than 2. Reduction of oxygen valence from 2, common for oxides, down to 1.5 can be explained by a formation of a localized hole at oxygen sites.

The discovery of superconductivity in Nd${}_{1\ensuremath{-}x}$Sr${}_{x}$NiO${}_{2}$ has resulted in a flurry of experimental and theoretical work to understand the nature of nickelate superconductivity as compared to that of the cuprates and pnictides. Here, the authors use a combination of LDA+U and linear-response methods to study the magnetic exchange interactions in Nd${}_{1\ensuremath{-}x}$Sr${}_{x}$NiO${}_{2}$. The analysis reveals an underlying Mott insulating state comprised of Ni-3${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$/Ni-3${d}_{{z}^{2}\ensuremath{-}{r}^{2}}$ orbitals with either an $S$=0 or $S$=1 two-hole ground state depending on the precise value of intra-atomic Hund's coupling.

We explore the interplay of electron-electron correlations and surface effects in the prototypical correlated insulating material, NiO. In particular, we compute the electronic structure, magnetic properties, and surface energies of the $(001)$ and $(110)$ surfaces of paramagnetic NiO using a fully charge self-consistent DFT+DMFT method. Our results reveal a complex interplay between electronic correlations and surface effects in NiO, with the electronic structure of the $(001)$ and $(110)$ NiO surfaces being significantly different from that in bulk NiO. We obtain a sizeable reduction of the band gap at the surface of NiO, which is most significant for the $(110)$ NiO surface. This suggests a higher catalytic activity of the $(110)$ NiO surface than that of the $(001)$ NiO one. Our results reveal a charge-transfer character of the $(001)$ and $(110)$ surfaces of NiO. Most notably, for the $(110)$ NiO surface we observe a remarkable electronic state characterized by an alternating charge-transfer and Mott-Hubbard character of the band gap in the surface and subsurface NiO layers, respectively. This novel form of electronic order stabilized by strong correlations is not driven by lattice reconstructions but of purely electronic origin. We notice the importance of orbital-differentiation of the Ni $e_g$ states to characterize the Mott-Hubbard insulating state of the $(001)$ and $(110)$ NiO surfaces. The unoccupied Ni $e_g$ surface states are seen to split from the lower edge of the conduction band to form strongly localized states in the fundamental gap of bulk NiO. Our results for the surface energies of the $(001)$ and $(110)$ NiO surfaces show that the $(001)$ facet of NiO has significantly lower energy. This implies that the relative stability of different surfaces, at least from a purely energetic point of view, does not depend on the presence or absence of magnetic order in NiO.