Self-interaction Corrected Local Spin Density Research Articles

Electronic structure and ionicity of actinide oxides from first principles

The ground state electronic structures of the actinide oxides AO, A2O3 and AO2 (A=U, Np, Pu, Am, Cm, Bk, Cf) are determined from first-principles calculations, using the self-interaction corrected local spin-density (SIC-LSD) approximation. Emphasis is put on the degree of f-electron localization, which for AO2 and A2O3 is found to follow the stoichiometry, namely corresponding to A(4+) ions in the dioxide and A(3+) ions in the sesquioxides. In contrast, the A(2+) ionic configuration is not favorable in the monoxides, which therefore become metallic. The energetics of the oxidation and reduction of the actinide dioxides is discussed, and it is found that the dioxide is the most stable oxide for the actinides from Np onwards. Our study reveals a strong link between preferred oxidation number and degree of localization which is confirmed by comparing to the ground state configurations of the corresponding lanthanide oxides. The ionic nature of the actinide oxides emerges from the fact that only those compounds will form where the calculated ground state valency agrees with the nominal valency expected from a simple charge counting.

Nature of Stoner condition for metallic ferromagnetism

For proper applications of the Stoner condition, we describe the assumptions in its underlying theory and their implications that need to be considered in practice. The nature of Stoner exchange parameters is examined based on spin-polarization perturbational orbital theory and a connection has been made in understanding the parameters with both local spin density and Hartree-Fock approximations via a self-interaction-corrected local spin density (SIC-LSD) exchange-correlation functional.

Pressure induced valence transitions in f -electron systems

A review is given of pressure induced valence transitions in f-electron systems calculated with the self-interaction corrected local spin density (SIC-LSD) approximation. These calculations show that the SIC-LSD is able to describe valence changes as a function of pressure or chemical composition. An important finding is the dual character of the f-electrons as either localized or band-like. A finite temperature generalisation is presented and applied to the study of the p-T phase diagram of the α → γ phase transition in Ce.

Pseudo‐SIC study on the ferromagnetism induced by carbon in AO‐based DMS (A = Mg, Ca, Ba, Sr)

The origin of the ferromagnetism being induced by Carbon in alkaline earth metal oxides is discussed through the calculation of the electronic structure and exchange coupling constant by using the pseudopotential- like self-interaction corrected local spin density method. The Curie temperature is also estimated by employing the Monte Carlo simulation. It is found that the ferromagnetism in MgO-based DMS is stabilized due to the exchange interaction of the first nearest neighbor pairs and it might be suppressed by the anti-ferromagnetic super-exchange interaction at higher substitutional concentrations, while the ferromagnetism in the remaining compounds is stabilized by both the first and second nearest neighbor pairs and Tc monotonously increases with Carbon concentration. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Electronic structure and spectroscopic properties of thulium monochalcogenides

The electronic structure of the thulium monochalcogenides TmS, TmSe, and TmTe is studied with several theoretical approaches. The total energy is evaluated with the self-interaction corrected local-spin density approximation, whereby the Tm ions are described with either twelve or thirteen localized f electrons with the remaining electrons forming bands. The comparisons of these two scenarios reveal the valency shift of the Tm ion through the series. The spectral functions of TmX compounds are calculated including multiplet effects, and they are compared to experimental x-ray photoemission spectra. The basic tool is the Hubbard-I approximation in which the embedding of an isolated f(n) ion in a solid is performed by modifying the crystal Hamiltonian as obtained from the local-density approximation with the atomic self-energy of the ion. The parameters of the model are obtained from the self-consistent band structure calculation. The agreement with experiment is excellent, reproducing all significant multiplet structures.

Bulk modulus of CeO 2 and PrO 2—An experimental and theoretical study

The high-pressure structural behaviour of CeO 2 and PrO 2 has been investigated by synchrotron X-ray diffraction at pressures up to 20 and 35 GPa, respectively. The experiments are accompanied by first principles calculations using the self-interaction corrected local spin density (SIC-LSD) approximation. The experimental values for the zero-pressure bulk modulus of CeO 2 and PrO 2 are 220(9) and 187(8) GPa, respectively. Our calculations reproduce the lattice constants with good accuracy, but find identical bulk modulii for CeO 2 (176.9 GPa) and PrO 2 (176.8 GPa).

Self-interaction correction in multiple scattering theory

We propose a simplified version of self-interaction corrected local spin-density (SIC-LSD) approximation, based on multiple scattering theory, which implements self-interaction correction locally, within the KKR method. The multiple scattering aspect of this new SIC-LSD method allows for the description of crystal potentials which vary from site to site in a random fashion and the calculation of physical quantities averaged over ensembles of such potentials using the coherent potential approximation (CPA). This facilitates applications of the SIC to alloys and pseudoalloys which could describe disordered local moment systems, as well as intermediate valences. As a demonstration of the method, we study the well-known $\alpha$-$\gamma$ phase transition in Ce, where we also explain how SIC operates in terms of multiple scattering theory.

Electronic and magnetic properties of the (001) surface of hole-doped manganites

The electronic and magnetic properties of ferromagnetic doped manganites are investigated by means of model tight-binding and \textit{ab initio} self-interaction corrected local spin density approximation calculations. It is found that the surface alone by breaking the cubic symmetry induces a difference in the occupation of the two $e_{g}$ orbitals at the surface. With \textit{ab initio} calculations we found surface localisation of one orbital and hence a change in the Mn valency from four in the bulk to three at the sub-surface. Different surface or disordered interface induced localisation of the orbitals are considered too with respect to the nature and the strength of the magnetic exchange coupling between the surface/interface and the bulk-like region.

Electronic structure of samarium monopnictides and monochalcogenides

The electronic structures of $\mathrm{Sm}X$ ($X=\mathrm{N}$, P, As, Sb, Bi, O, S, Se, Te, Po) compounds are calculated using the self-interaction corrected local-spin density approximation. The Sm ion is described with either five or six localized $f$ electrons while the remaining electrons form bands, and the total energies of these scenarios are compared. With five localized $f$ electrons a narrow $f$ band is formed in the vicinity of the Fermi level leading to an effective intermediate valence. This scenario is the ground state of all the pnictides as well as SmO. With six localized $f$ electrons, the chalcogenides are semiconductors, which is the ground state of SmS, SmSe, and SmTe. Under compression the Sm chalcogenides undergo first order transitions with destabilization of the $f$ states into the intermediate valence state, the bonding properties of which are well reproduced by the present theory.

Electronic structure of half-metallic ferromagnets and spinel ferromagnetic insulators

We discuss an application of the self-interaction-corrected local spin density (SIC-LSD)approximation to study electronic structure of some half-metallic ferromagnets andferromagnetic insulators of current interest in spintronics. Both d- and f-electron materialsare considered, and we concentrate on the nominal valence and ground state properties ofthese systems.

Electronic structure of half-metallic double perovskites

We present the self-interaction corrected local spin density (SIC-LSD) electronic structure and total energy calculations, leading also to valencies of the ground state configurations, for the half-metallic double perovskites such as Sr$_{2}$FeMoO$_{6}$, Ba$_{2}$FeMoO$_{6}$, Ca$_{2}$FeMoO$_{6}$, and Ca$_{2}$FeReO$_{6}$. We conclude that the Fe and Mo (or Re) spin magnetic moments are anti-parallel aligned, and the magnitude of the hybridization induced moment on Mo does not vary much between the different compounds. The hybridization spin magnetic moment on Re is of the order of -1.1 $\mu_{B}$, while that on Mo is about -0.4 $\mu_{B}$, independently of the alkaline earth element. Also the electronic structure of all the compounds studied is very similar, with a well defined gap in the majority spin component and metallic density of states for the minority spin component, with highly hybridized Fe, Mo (or Re), and oxygen bands.

Theory of the electronic and magnetic structure of cerium pnictides under pressure

The electronic structures of the cerium pnictides CeN, CeP, CeAs, CeSb and CeBi are calculated within the self-interaction corrected local-spin density approximation. This method allows a description of the Ce f-electrons as either localized or delocalized. We find that the isostructural phase transition in CeP is associated with the delocalization of the f-electron under pressure. Similar structural phase transitions in CeAs, CeSb and CeBi are also well described by this theory. Moreover, in agreement with recent experiments we obtain a very low carrier type-I antiferromagnetic groundstate for CeP and CeSb.

Three-particle approximation for transition-metal oxides

Quasiparticle spectra are calculated for NiO and CuO on the basis of band structures obtained within the ab initio self-interaction-corrected local-spin density (SIC-LSD) and LSD+U approximations. On-site Coulomb correlations are described by a multiband Hubbard model, which is treated within Igarashi's three-particle approximation. The transition-metal d-state spectral weight is split into a main ${\mathrm{d}}^{\mathrm{n}}$L peak and a ${\mathrm{d}}^{\mathrm{n}\mathrm{\ensuremath{-}}1}$ satellite. We show that mean-field band structures in this way can lead to a good description of the experimental photoemission spectra of these compounds. The validity of the three-particle approach is investigated, and it is concluded that the method is best suited for a system which is well orbitally polarized on the mean-field level.

Theory of the electronic states and absorption spectrum of the LiCl:Ag+ impurity system.

The impurity absorption spectra of ${\mathrm{Ag}}^{+}$ and ${\mathrm{Cu}}^{+}$ impurities in alkali halide hosts show characteristically different features, despite the similar nature of the corresponding free ions. We use the self-interaction-corrected local-spin-density (SIC-LSD) theory to calculate the electronic structure of the ground state (4d) and the 5s and 5p excited states of the LiCl:${\mathrm{Ag}}^{+}$ impurity ion. The method of linear combinations of atomic orbitals is used to determine the wave functions and energy levels. By comparing with previous calculations for LiCl:${\mathrm{Cu}}^{+}$, we are able to attribute the differences in the d\ensuremath{\rightarrow}s and d\ensuremath{\rightarrow}p transitions in the ultraviolet spectra of these systems to the increased bonding between host crystal and impurity orbitals in LiCl:${\mathrm{Ag}}^{+}$, due to the more extensive nature of the ${\mathrm{Ag}}^{+}$ 4d orbitals. A modification of the earlier SIC-LSD impurity-crystal procedure is introduced to treat the strongly mixed impurity states.

Density functional calculations of the ionic radii of Be −, Mg −, Ca −, Sr −, Ba − and some metastable mononegative ions

Using the criterion that for the negative monoatomic ions, the location of the minimum in the electrostatic potential V( r) represents the ionic radius r m, the latter have been calculated for the negative ions H − (2s 2 1S), H − (3s 2 1S), H − (1s ↓ 12s ↓ 12p ↓ 1 4P), Li − (1s ↓ 12s ↓ 12p ↓ 2 5P), C − (2p ↓ 22p ↓ 1 2D), N − (2p ↓ 22p ↓ 2 1S), Be − (2p ↓ 1 2P), Mg − (3p ↓ 1 2P), Ca − (4p ↓ 1 2P), Sr − (5p ↓ 1 2P), and Ba − (6p ↓ 1 2P), respectively. Quasi-relativistic self-interaction-corrected local-spin-density (SIC-LSD) functional with correlation energy has been used to calculated V( r) for all the ions.

Ground and excited states of the NaCl:Cu+ impurity system.

The absorption spectra of transition-metal-doped alkali halide systems are distinguished by characteristic features identified with transitions between electron states localized on the impurity ion. The theoretical description of these features depends on a realistic treatment of the host crystal electronic structure, as well as on the treatment of the impurity ion. The success of recent calculations using the self-interaction-corrected local-spin-density (SIC-LSD) form of density-functional theory in determining the band structure of alkali halide crystals makes SIC-LSD a good candidate for use in the impurity problem. In this work SIC-LSD is applied to the NaCl:${\mathrm{Cu}}^{+}$ impurity system, using an embedded-cluster technique explicitly including seven near-neighbor shells of the host crystal ions around the ${\mathrm{Cu}}^{+}$ site, allowing a full characterization of the low-lying impurity excited states. The calculated values for the fundamental 3d\ensuremath{\rightarrow}4s and 3d\ensuremath{\rightarrow}4p impurity-ion transitions are in close agreement with corresponding features of the NaCl:${\mathrm{Cu}}^{+}$ absorption spectrum. We compare and contrast our SIC-LSD results with those of other recent calculations for NaCl:${\mathrm{Cu}}^{+}$.

Localized and canonical atomic orbitals in self-interaction corrected local density functional approximation

The self-interaction corrected (SIC) version of the local spin density (LSD) approximation has been applied to the first two rows of the periodic table with particular emphasis on local orbital choice. These are the first SIC–LSD calculations for atomic systems that account for all nonspherical corrections and are based on a rigorous variational theory. The resulting total energies and orbital energies are improved in comparison to experiment and restore a desirable trend which is found in Hartree–Fock theory. We demonstrate that with a proper treatment of the SIC–LSD off-diagonal Lagrange multipliers, the viral theorem is satisfied at self-consistency.

A new density functional for fractionally occupied orbital systems with application to ionization and transition energies

The use of fractional occupation of energy levels in the self-interaction-corrected local spin density (SIC-LSD) theory of electronic energy structure is studied with reference to calculation of ionization and excitation energies. With the original form of the SIC-LSD energy functional for fractional occupation, the one-electron eigenvalues exhibit nonlinear dependence on the occupation number. A new SIC-LSD density functional for fractional occupation based on the general behavior of the universal functional is proposed. The one-electron eigenvalues derived from this new functional vary quite linearly with the occupation number. This makes it possible to obtain ionization and excitation energies by a simple numerical integration over the occupation number. A one-point integration gives ionization energies that agree on the average with the results on taking the difference in self-consistent-field total energy between the atom and the ion to within 0.4% for the atoms He, Li, Be,...,Ar. Improvement can be made by using a three-point integration over the occupation number. This method is also applied to calculate excitation energies of selected transitions for He through Ar and the results are in good agreement with the corresponding total-energy differences.

Calculation of the spin-polarized electronic structure of an interstitial iron impurity in silicon.

We apply our self-consistent, all-electron, spin-polarized Green's-function method within an impurity-centered, dynamic basis set to study the interstitial iron impurity in silicon. We use two different formulations of the interelectron interactions: the local-spin-density (LSD) formalism and the self-interaction-corrected (SIC) local-spin-density (SIC-LSD) formalism. We find that the SIC-LSD approach is needed to obtain the correct high-spin ground state of Si:${\mathrm{Fe}}^{+}$. We propose a quantitative explanation to the observed donor ionization energy and the high-spin ground states for Si:${\mathrm{Fe}}^{+}$ within the SIC-LSD approach. For both Si:${\mathrm{Fe}}^{0}$ and Si:${\mathrm{Fe}}^{+}$, this approach leads to a hyperfine field, contact spin density, and ionization energy in better agreement with experiments than the simple LSD approach. The apparent dichotomy between the covalently delocalized nature of Si:Fe as suggested on the one hand by its reduced hyperfine field (relative to the free atom) and extended spin density and by the occurrence of two closely spaced, stable charge states (within 0.4 eV) and on the other hand by the atomically localized picture (suggested, for example, by the stability of a high-spin, ground-state configuration) is resolved. We find a large reduction in the hyperfine field and contact spin density due to the covalent hybridization between the impurity 3d orbitals and the tails of the delocalized ${\mathrm{sp}}^{3}$ hybrid orbitals of the surrounding silicon atoms. Using the calculated results, we discuss (i) the underlying mechanism for the stability and plurality of charged states, (ii) the covalent reduction in the hyperfine field, (iii) the remarkable constancy of the impurity M\"ossbauer isomer shift for different charged states, (iv) comparison with the multiple charged states in ionic crystals, and (v) some related speculation about the mechanism of (${\mathrm{Fe}}^{2+}$/${\mathrm{Fe}}^{3+}$) oxidation-reduction ionizations in heme proteins and electron-transporting biological systems.

Towards a Kohn-Sham potential via the optimized effective-potential method

The optimized effective-potential (OEP) method is applied to a self-interaction-corrected local-spin-density (SIC-LSD) energy functional. The local potential which results has the useful properties of being both self-interaction free and orbital independent, and it can thus be regarded as a good approximation to the exact Kohn-Sham potential. A number of atomic systems are examined in the exchange-only and in the Ceperley-Alder exchange-correlation approximations. The resulting total energies are very close to those obtained by previous applications of the LSD-SIC functional. The resultant potentials are structurally similar to those derived by applying the OEP method to a Hartree-Fock Hamiltonian: showing much improved behavior over the local-spin-density approximation in both large- and small-$r$ regions as well as the characteristic intershell cusplike structure. The eigenvalues have less formal significance than the more standard approaches, especially those for unoccupied orbitals which seem to have no significance whatsoever. Nonetheless, the highest occupied eigenvalue agrees closely with the conventional LSD-SIC value. However, for the deeper levels, each eigenvalue lies higher than the comparable eigenvalue of the conventional SIC, although lower than the eigenvalue of the LSD potential---the deeper the level, the larger the difference. This property follows from the nonvariational character of the eigenvalues, and it is shown that one can obtain realistic excitation spectra from this formalism by utilizing the appropriate variational quantity. The results obtained illustrate some of the less understood issues in the application of the Kohn-Sham procedure within density-functional theory. Based on observations made here using atomic calculations, we can offer a plausible explanation of the underestimation by LSD calculations of the experimental band gaps in insulators.