Electronic structure and X-ray magnetic circular dichroism in the quadruple perovskite CaCu3Re2Fe2O12

We study the electronic and magnetic properties of ferroelectric ${\mathrm{CaMnTi}}_{2}{\mathrm{O}}_{6}$ within density functional theory using the generalized gradient approximation (GGA) with the consideration of strong Coulomb correlations $(\mathrm{GGA}+U)$ in the framework of the fully relativistic spin-polarized Dirac linear muffin-tin orbital band structure method. The x-ray absorption spectra (XAS) and x-ray magnetic circular dichroism (XMCD) at the Mn, Ti ${L}_{2,3}$, and O $K$ edges have been investigated theoretically. The calculated results are in good agreement with experimental data. The core-hole effect in the final state as well as the effect of the electric quadrupole ${E}_{2}$ and magnetic dipole ${M}_{1}$ transitions have been investigated. The core-hole effect has improved the agreement with the experimental XAS and XMCD spectra at the Ti and Mn ${L}_{2,3}$ edges.

We have investigated the electronic structure of the transition metal oxide Ca3Ru2O7 within density-functional theory using the generalized gradient approximation while considering strong Coulomb correlations in the framework of the fully relativistic spin-polarized Dirac linear muffin-tin orbital band structure method. Ca3Ru2O7 can be classified as a Mott insulator since it was expected to be metallic from band structure calculations. We found that the magnetic ground state of Ca3Ru2O7 possesses an AFM−b magnetic structure with the Ru spin moments ordered antiferromagnetically along the b axis. We have investigated the resonant inelastic x-ray scattering (RIXS) spectra at the Ru and Ca K, L3, and M3 edges as well as at the OK edge. The experimentally measured RIXS spectrum of Ca3Ru2O7 at the Ru L3 edge possesses a sharp feature ≤2eV corresponding to transitions within the Ru t2g levels. The excitation located from 2 to 4 eV is due to t2g→ eg transitions. The third wide structure situated at 4.5−11eV appears due to transitions between the Ru 4dO states derived from the tails of oxygen 2p states and eg and t2g states. The RIXS spectra at the Ru L3 and M3 edges are quite similar. However, the corresponding RIXS spectra at the Ca site are quite different from each other due to the significant difference in the widths of core levels. The RIXS spectrum at the OK edge consists of three major inelastic excitations. We found that the first two low energy features ≤2.5eV are due to interband transitions between occupied and empty Ot2g states which appear due to the strong hybridization between oxygen 2p and Ru t2g states in close vicinity to the Fermi level. The next major peak between 2.5 and 8 eV reflects interband transitions from occupied O2p to empty Ot2g states. A wide energy interval between 4 and 11 eV is occupied by rather weak O2p→Oeg transitions. Published by the American Physical Society 2024

We studied the electronic structure of the $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{Li}}_{2}{\mathrm{IrO}}_{3}$ insulator within the density-functional theory using the generalized gradient approximation while taking into account strong Coulomb correlations in the framework of the fully relativistic spin-polarized Dirac linear muffin-tin orbital band structure method. The $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{Li}}_{2}{\mathrm{IrO}}_{3}$ undergoes a pressure-induced structural and magnetic phase transition at ${P}_{c}\phantom{\rule{4pt}{0ex}}\ensuremath{\sim}4$ GPa with symmetry lowering to the monoclinic $C2/c$. The structural phase transition is accompanied by the formation of ${\mathrm{Ir}}_{2}$ dimers on the zigzag chains, with an Ir-Ir distance of $\ensuremath{\sim}2.66$ \AA{}, even shorter than that of metallic Ir. The strong dimerization stabilizes the bonding molecular-orbital state, leads to the collapse of the magnetism, and opens the energy gap with a concomitant electronic phase transition from a Mott insulator to band insulator. The resonant inelastic x-ray scattering spectra (RIXS) at the Ir ${L}_{3}$ edge were investigated theoretically from first principles. The calculated results are in good agreement with the experimental data. We show that the drastic reconstruction of the RIXS spectral peak at 0.7 eV associated with the structural $Fddd\ensuremath{\rightarrow}C2/c$ phase transition at ${P}_{c}$ can be related to the disappearing of the Coulomb correlations in the high-pressure $C2/c$ phase.

We studied the electronic, magnetic, and optical properties of $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{Li}}_{2}{\mathrm{IrO}}_{3}$ insulator within the density-functional theory using the generalized gradient approximation taking into account strong Coulomb correlations (GGA+$U$) in the framework of the fully relativistic spin-polarized Dirac linear muffin-tin orbital band-structure method. The x-ray absorption spectra and x-ray magnetic circular dichroism (XMCD) at the Ir ${L}_{2,3}$ edges were investigated theoretically from first principles. The calculated results are in good agreement with experimental data. We found that the GGA+$U$ approach with Hubbard ${U}_{\text{eff}}=1.5\phantom{\rule{4pt}{0ex}}\mathrm{eV}$ well describes the optical spectra and XMCD spectra at the Ir ${L}_{2,3}$ edges. We investigated the electronic structure of $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{Li}}_{2}{\mathrm{IrO}}_{3}$ under pressure from first principles. The hyperhoneycomb iridate $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{Li}}_{2}{\mathrm{IrO}}_{3}$ is on the border of the magnetic ordering with relatively weak Coulomb electron-electron correlations. The $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{Li}}_{2}{\mathrm{IrO}}_{3}$ undergoes a pressure-induced structural and magnetic phase transitions at ${P}_{c}=4.4$ GPa with symmetry lowering to the monoclinic $C2/c$. The structural phase transition is accompanied by a dimerization of the previously equally long $x/y$ Ir-Ir bonds. We find remarkable nonmagnetic ground states of $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{Li}}_{2}{\mathrm{IrO}}_{3}$ at ${P}_{c}$, with a concomitant electronic phase transition from a Mott insulator to band insulators. We found that ${J}_{\text{eff}}=1/2$ states are still dominant in the low-energy region in high-pressure monoclinic $C2/c$ structure.

Resonant inelastic x-ray scattering in Ca3LiOsO6 from first-principles

The electronic structure of (Zn,V)O dilute magnetic semiconductors (DMSs) was investigated theoretically from first principles, using the fully relativistic Dirac linear muffin-tin orbital band structure method and the Korringa-Kohn-Rostoker Green's function approach within the local spin-density approximation. X-ray magnetic circular dichroism (XMCD) spectra at the V ${L}_{2,3}$ edges were calculated. Comparing the experimental results and the theoretical simulations we discuss possible crystal and magnetic structures of (Zn,V)O and reach the best agreement with the experimental XMCD spectra for the antiferromagnetically ordered DMSs in the presence of an oxygen vacancy. The corresponding structure was used to determine the electronic and magnetic properties of (Zn,V)O.

Hexagonal ferrites ($h\text{\ensuremath{-}}R{\mathrm{FeO}}_{3}$, $R$ = Sc, Y, Ho-Lu) have recently been identified as a new family of multiferroic complex oxides. We study electronic and magnetic properties of $h\text{\ensuremath{-}}{\mathrm{YbFeO}}_{3}$ ferrite within the density-functional theory using the generalized gradient approximation (GGA) with consideration of strong Coulomb correlations $(\mathrm{GGA}+U)$ in the framework of the fully relativistic spin-polarized Dirac linear muffin-tin orbital band-structure method. The $4f$ electrons of Yb are explicitly treated as valence electrons. The x-ray absorption spectra and x-ray magnetic circular dichroism (XMCD) at the Yb ${M}_{4,5}$, Fe ${L}_{2,3}$, and O $K$ edges were investigated theoretically. The calculated results are in good agreement with experimental data. We found that the $\mathrm{GGA}+U$ approach with Hubbard ${U}_{\mathrm{eff}}=6.1\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$ and 3.3 eV for Yb and Fe, respectively, well describes the XMCD spectra at the Yb ${M}_{4,5}$, Fe ${L}_{2,3}$, and O $K$ edges.

A systematic electronic structure study of ${A}_{2}{\mathrm{FeReO}}_{6}(A=\mathrm{Ba},\mathrm{Sr},\text{and}\mathrm{Ca})$ has been performed by employing the local-spin-density approximation (LSDA) and $\mathrm{LSDA}+U$ methods using the fully relativistic spin-polarized Dirac linear muffin-tin orbital band-structure method. We investigated the effects of the subtle interplay between spin-orbit coupling, electron correlations, and lattice distortion on the electronic structure of double perovskites. ${\mathrm{Ca}}_{2}{\mathrm{FeReO}}_{6}$ has a large distortion in the Fe-O-Re bond, and the electronic structure is mainly determined by electron correlations and lattice distortion. In the $\mathrm{Ba}\ensuremath{-}\mathrm{Sr}\ensuremath{-}\mathrm{Ca}$ row, the correlation effects at the Fe site are increased. The correlations at the Re site are small in the Ba- and Sr-based compounds but significant in ${\mathrm{Ca}}_{2}{\mathrm{FeReO}}_{6}$. ${\mathrm{Ca}}_{2}{\mathrm{FeReO}}_{6}$ behaves like an insulator only if considered with a relatively large value of Coulomb repulsion ${U}_{\text{eff}}=2.3$ eV at the Re site in addition to ${U}_{\text{eff}}=3.1$ eV at the Fe site. ${\mathrm{Ca}}_{2}{\mathrm{FeReO}}_{6}$ possesses a phase transition at 140 K where the metal-insulator transition (MIT) occurs between metallic high-temperature and insulating low-temperature phases. The spin and orbital magnetic moments are linear functions of temperature before and after the MIT but change abruptly at the point of the phase transition. From theoretically calculated magnetocrystalline anisotropy energy (MAE), we found that the easy axis of magnetization for the low-temperature phase is along the $b$ direction, in agreement with experimental data. We found that the major contribution to the MAE is due to the orbital magnetic anisotropy at the Re site. X-ray-absorption spectra and x-ray magnetic circular dichroism at the Re, Fe, and Ba ${L}_{2,3}$ and Fe, Ca, and O $K$ edges were investigated theoretically in the frame of the $\mathrm{LSDA}+U$ method. A qualitative explanation of the x-ray magnetic circular dichroism spectra shape is provided by an analysis of the corresponding selection rules, orbital character, and occupation numbers of individual orbitals. The calculated results are compared with available experimental data.

The electronic structure, Fermi surface, angle dependence of the cyclotron masses and extremal cross sections of the Fermi surface as well as x-ray magnetic circular dichroism (XMCD) in the CeAgSb2 compound were investigated from first principles using the fully relativistic Dirac linear muffin-tin orbital method. In our calculations Ce 4f states have been considered as: 1) itinerant using the generalized gradient approximation (GGA), 2) fully localized, treating them as core states, and 3) partly localized using the GGA + U approximation. The effect of the spin-orbit (SO) interaction and Coulomb repulsion U in a frame of the GGA + U method on the Fermi surface, orbital dependence of the cyclotron masses, and extremal cross sections of the Fermi surface are examined in details. We show that the conventional GGA band calculations fail to describe the Fermi surface of the CeAgSb2 due to wrong position of Ce 4f states (too close to the EF). On the other hand, fully localized (4f states in core) and the GGA + U approach produce similar Fermi surfaces and dHvA frequencies in the CeAgSb2. A good agreement with the experimental data of XMCD spectra at the Ce M4.5 edges was achieved using the GGA + U approximation. The origin of the XMCD spectra in the compound is examined. The core hole effect in the final states has been investigated using a supercell approximation. It improves the agreement be-tween the theory and the experiment of the XAS and the XMCD spectra at the Ce M4.5 edges.

The optical and magneto-optical (MO) spectra of UGa2 are investigated theoretically from first principles, using the fully relativistic Dirac linear muffin-tin orbital band structure method. The electronic structure is obtained with the local spin-density approximation (LSDA), as well as with generalization of the LSDA+U method which takes into account that in the presence of spin-orbit coupling the occupation matrix of localized electrons becomes nondiagonal in spin indexes. Although, the magnetic moment at the U site of UGa2 is described better by LSDA+U than by LSDA, neither the localized LSDA+U model nor the spin-polarized LSDA 5f-itinerant band model can explain all the peculiarities of the experimental MO spectra. The LSDA gives a better description of the optical and MO spectra of UGa2 in the 0–1 eV energy interval, but for higher energies the experimental Kerr rotation curve is situated someplace in between the curves obtained by the LSDA and the LSDA+U approximations. The origin of the Kerr rotation in the compound is examined.

The electronic structure and x-ray magnetic circular dichroism (XMCD) spectra of the Heusler alloy Co2MnGe were investigated theoretically from first principles, using the fully relativistic Dirac linear muffin-tin orbital band structure method. Densities of valence states, orbital and spin magnetic moments, as well as polarization of the electronic states at the Fermi level are analyzed and discussed. The origin of the XMCD spectra in the Co2MnGe compound is examined. The calculated results are compared with available experimental data.

The electronic structure of the Co-doped indium tin oxide (ITO) diluted magnetic semiconductors (DMSs) were investigated theoretically from first principles, using the fully relativistic Dirac linear muffin-tin orbital band structure method. The X-ray absorption spectra (XAS) and X-ray magnetic circular dichroism (XMCD) spectra at the Co L3, In M2, Sn M2, and O K edges were investigated theoretically from first principles. The origin of the XMCD spectra in these compounds was examined. The calculated results are compared with available experimental data.

The electronic structure and x-ray magnetic circular dichroism (XMCD) spectra of the ferromagnetic superconductor UCoGe at the U N4,5, Ge and Co K and Co L2,3 edges were investigated theoretically from first principles, using the fully relativistic Dirac linear muffin-tin orbital band structure method. The electronic structure is obtained with the local spin-density approximation (LSDA), as well as with a generalization of the LSDA+U method which takes into account the non-diagonal occupation matrix (in spin indexes) of localized electrons. A stable ferromagnetic ground state was found. The uranium total magnetic moment is quite small (about −0.171μB) in the LSDA approximation as a result of almost complete cancellation between the spin magnetic moment of 0.657 μB and the opposite orbital magnetic moment of −0.828 μB, resulting from strong spin-orbit coupling at the uranium site. Valency of U ion in UCoGe is close to 3+. The ratio orbital and spin magnetic moments Ml/Ms ranged from 1.163 in the GGA approach up to 2.456 for the LSDA+U calculations is smaller than the corresponding ratio for the free ion U3+ value (2.60), it can indicate a significant delocalization of the 5f-electron states due to the hybridization of the U 5f electrons with the conduction band and Co 3d electrons. The line shape of the dichroic spectra at the U M5 and M4 edges predicted by considering the magneto-optical selection rules as well as the occupation and the energy sequence of the mj-projected partial densities of states. The theoretically calculated XMCD spectra at the U M4,5, Ge and Co K and Co L2,3 edges are in good agreement with the experimentally measured spectra.

The electronic structure of (Ti,Mn)O2 diluted magnetic semiconductors was investigated theoretically from first principles using the fully relativistic Dirac linear muffin-tin orbital band structure method. The electronic structure was obtained with the local spin-density approximation taking into account strong Coulomb correlations in the frame of the LSDA + U approximation. The x-ray absorption spectra and x-ray magnetic circular dichroism spectra at the Mn and Ti L2,3 and O K edges were investigated theoretically from first principles. The origin of the XMCD spectra in these compounds was examined. The calculated results are compared with available experimental data.

The x-ray magnetic circular dichroism (XMCD) spectra of ${\text{CeFe}}_{2}$ at the $\text{Ce}\text{ }{L}_{2,3}$, ${M}_{4,5}$, $\text{Fe}\text{ }K$, and ${L}_{2,3}$ edges are investigated theoretically from first principles, using the fully relativistic Dirac linear muffin-tin orbital band-structure method. The electronic structure is obtained with the local spin-density approximation. The origin of the XMCD spectra in the compound is examined. The core hole effect in the final states has been investigated using a supercell approximation. It improves the agreement between the theory and the experiment at the $\text{Ce}\text{ }{M}_{5}$ edge. However, it has a minor influence on the shape of the $\text{Ce}\text{ }{L}_{2,3}$ XMCD spectra.