X-ray magnetic circular dichroism in d and f ferromagnetic materials: recent theoretical progress. Part I

The curreThe current status of theoretical understanding of the x-ray magnetic circular dichroism (XMCD) of 4 f and 5 f compounds is reviewed. Energy band theory based upon the local spin-density approximation (LSDA) describes the XMCD spectra of transition metal compounds with high accuracy. However, the LSDA does not suffice for lanthanide compounds which have a correlated 4 f shell. A satisfactory description of the XMCD spectra could be obtained by using a generalization of the LSDA, in which explicitly f electron Coulomb correlation are taken into account (LSDA + U approach). As examples of this group we consider GdN compound. We also consider uranium 5 f compounds. In those compounds where the 5 f electrons are rather delocalized, the LSDA describes the XMCD spectra reasonably well. As example of this group we consider UFe₂. Particular differences occur for the uranium compounds where the 5 f electrons are neither delocalized nor localized, but more or less semilocalized. Typical examples are UXAl (X = Co, Rh, and Pt), and UX (X = S, Se, Te). The semilocalized 5 f ’s are, however, not inert, but their interaction with conduction electrons plays an important role. We also consider the electronic structure and XMCD spectra of heavy-fermion compounds UPt₃, URu₂Si₂, UPd₂Al3₃, UNi₂Al₃, and UBe₁₃ where the degree of the 5 f localization is increased in comparison with other uranium compounds. The electronic structure and XMCD spectra of UGe₂ which possesses simultaneously ferromagnetism and superconductivity also presented. Recently achieved improvements for describing 5 f compounds are discussed

X-ray magnetic circular dichroism in Co₂FeGa: First-principles calculations

The electronic structure and x-ray magnetic circular dichroism (XMCD) spectra of the Heusler alloy Co₂FeGa were investigated theoretically from first principles, using the fully relativistic Dirac linear MT-orbital (LMTO) band structure method. Densities of valence states, orbital and spin magnetic moments are analyzed and discussed. The origin of the XMCD spectra in the Co₂FeGa compound is examined. The calculated results are compared with available experimental data

X-ray magnetic circular dichroism in d and f ferromagnetic materials: recent theoretical progress. Part I

The current status of theoretical understanding of the x-ray magnetic circular dichroism (XMCD) of 3d compounds is reviewed. Energy band theory based upon the local spin-density approximation (LSDA) describes the XMCD spectra of transition metal compounds reasonably well. Examples which we examine in detail are XPt₃ compounds (with X = V, Cr, Mn, Fe, Co, and Ni) in the AuCu₃ structure, Heusler compounds Co₂MnGe, Co₂NbSn, and compounds with noncollinear magnetic structure IrMnAl and Mn₃ZnC. Recently achieved improvements for describing the electronic and magnetic structures of 3d compounds are discussed

Electronic structure and magneto-optical Kerr effect in UCuAs₂

The optical and magneto-optical (MO) spectra of the ternary compound UCuAs₂ 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 the so-called LSDA+U approach. Better agreement between the theoretically calculated and the experimentally measured MO Kerr spectra is found with the LSDA+U approximation. The origin of the Kerr rotation in the compound is examined

Electronic structure and x-ray magnetic circular dichroism in uranium monochalcogenides

The electronic structure and x-ray magnetic circular dichroism (XMCD) spectra of US, USe, and UTe are investigated theoretically from first principles, using the fully relativistic Dirac LMTO 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 that in the presence of spin–orbit coupling the occupation matrix of localized electrons becomes non-diagonal in spin indexes. The origin of the XMCD spectra in the compounds is examined

Electronic structure and magneto-optical Kerr effect in the compound UCuP₂

The optical and magneto-optical (MO) spectra of the fernary compound UCuP₂ are investigated from first principles, using the fully relativistic Dirac linear-muffin-tin-orbital band structure method and density-functional theory in the local spin-density approximation. Within a band-like description of the 5f electrons, good agreement with the measured MO spectra is obtained. The origin of the Kerr rotation in the compound is examined

Electronic structure and x-ray magnetic circular dichroism in (Ge,Mn)Te diluted magnetic semiconductors

The electronic structure of the (Ge,Mn)Te diluted magnetic semiconductors was investigated theoretically from first principles, using the fully relativistic Dirac linear muffin-tin orbital (LMTO) band structure method. The electronic structure is obtained with the local spin-density approximation (LSDA) as well as the LSDA+U method. The x-ray magnetic circular dichroism (XMCD) spectra of (Ge,Mn)Te DMSs at the Mn L2,3 edges are investigated theoretically from first principles. The origin of the XMCD spectra in the compound is examined. The calculated results are compared with available experimental data

Weak-coupling superconductivity in a strongly correlated iron pnictide

Iron-based superconductors have been found to exhibit an intimate interplay of orbital, spin, and lattice degrees of freedom, dramatically affecting their low-energy electronic properties, including superconductivity. Albeit the precise pairing mechanism remains unidentified, several candidate interactions have been suggested to mediate the superconducting pairing, both in the orbital and in the spin channel. Here, we employ optical spectroscopy (OS), angle-resolved photoemission spectroscopy (ARPES), ab initio band-structure, and Eliashberg calculations to show that nearly optimally doped NaFe0.978Co0.022As exhibits some of the strongest orbitally selective electronic correlations in the family of iron pnictides. Unexpectedly, we find that the mass enhancement of itinerant charge carriers in the strongly correlated band is dramatically reduced near the Γ point and attribute this effect to orbital mixing induced by pronounced spin-orbit coupling. Embracing the true band structure allows us to describe all low-energy electronic properties obtained in our experiments with remarkable consistency and demonstrate that superconductivity in this material is rather weak and mediated by spin fluctuations

Approximative treatment of 5f-systems with partial localization due to intra-atomic correlations

Increasing experimental and theoretical evidence points towards a dual nature of the 5$f$ electrons in actinide-based strongly correlated metallic compounds, with some 5$f$ electrons being localized and others delocalized. In a recent paper (PRB xxx, 2004), we suggested the interplay of intra-atomic correlations as described by Hund's rules and a weakly anisotropic hopping (hybridization) as a possible mechanism. The purpose of the present work is to provide a first step towards a microscopic description of partial localization in solids by analyzing how well various approximation schemes perform when applied to small clusters. It is found that many aspects of partial localization are described appropriately both by a variational wavefunction of Gutzwiller type and by a treatment which keeps only those interactions which are present in LDA+U calculations. In contrast, the energies and phase diagram calculated within the Hartree Fock approximation show little resemblence with the exact results. Enhancement of hopping anisotropy by Hund's rule correlations are found in all approximations.Comment: 9 pages, 9 figure