The local electronic structure of tin phthalocyanine studied by resonant soft X-ray emission spectroscopies

The electronic structure of thin films of the organic semiconductor tin phthalocyanine (SnPc) has been investigated by resonant and non-resonant soft X-ray emission (RXES and XES) at the carbon and nitrogen K-edges with excitation energies determined from near edge X-ray absorption fine structure (NEXAFS) spectra. The resultant NEXAFS and RXES spectra measure the unoccupied and occupied C and N 2p projected density of states, respectively. In particular, RXES results in site-specific C 2p and N 2p local partial density of states (LPDOS) being measured. An angular dependence of C 2p and N 2p RXES spectra of SnPc was observed. The observed angular dependence, the measured LPDOS and their correspondence to the results of density functional theory calculations are discussed. Observed differences on the same site-specific features between resonant (non-ionising) and non-resonant (ionising) NXES spectra are attributed to symmetry selection and screening.     

X-ray magnetic circular dichroism at rare-earth L23 absorption edges in various compounds and alloys

Theoretical interpretations of X-ray magnetic circular dichroism (XMCD) at rare-earth (called R hereafter) L₂₃ absorption edges are reviewed using differing models, depending on the material under investigation. In the first chapter, we present an overview of recent developments for XMCD in XAS with a few general remarks, especially at R atom absorption edges. In Section 2, we first describe basic mechanism of XMCD at L₂₃ edges of R systems, and then we essentially discuss two examples of XMCD spectra in: (i) R₂Fe₁₄B metallic compounds, with the help of a cluster model, and (ii) RFe₂ Laves-phase compounds, using a tight-binding approximation for R 5d and Fe 3d conducting states. A good agreement between theory and experiment for R₂Fe₁₄B suggests that a cluster model provides a valuable method to quantitatively calculate XMCD spectra of R systems, even with quite complicated atomic arrangements. For RFe₂ systems the XMCD spectral shape, especially for the L₂ edge of heavy R elements, is more complicated than those of R₂Fe₁₄B systems, and this is explained by the competition of some different XMCD mechanisms. In Section 3, we focus on special series of Ce systems, related to XAS and XMCD studies at the Ce L₂₃ edges. Two clearly differing cases are interpreted: (i) A well localized 4f¹ system, i.e. CeRu₂Ge₂; (ii) A less localized 4f¹ system, i.e. CeFe₂, with a 3d partner. Then, from a more experimental point of view, we investigate the influence of substitution on the low temperature properties of CePd₃ compounds, i.e. Ce(_Pd1-xMnx)3$\mathrm{Ce}({\mathrm{Pd}}_{1-x}{\mathrm{Mn}}_x{)}_3$ alloys where x   is about 0.03, giving rise to (_CePd3)8Mn$({\mathrm{CePd}}_3{)}_8\mathrm{Mn}$ ordered structure. We give another example: Ce(_Pd1-xNix)3$\mathrm{Ce}({\mathrm{Pd}}_{1-x}{\mathrm{Ni}}_x{)}_3$ alloys with x taken up to about 0.25. Also the Ce L₂₃ XMCD signal measured in pure CePd₃ demonstrates that in the Ce based dense Kondo materials, only the 4f¹ channel yields a magnetic response.     

Refining Fermi surface topologies from ab initio calculations through momentum density spectroscopies

It is well known that the agreement between the Fermi surface topologies predicted by ab initio electronic structure calculations and experiment can often be brought into much better agreement through small rigid-band-like shifts. A new method for refining these calculations using experimental data containing Fermi surface information, based on a rigid-band-like fitting approach is presented. In this method, experimental data from different methods can be combined to refine and deliver a ‘tuned’ bandstructure, allowing an investigation of FS nesting properties, a quantitative comparison between experiment and calculation, and highlighting the origin of inconsistencies. Results of the application of this method to positron annihilation experiments in vanadium are presented, showing significant improvement over the ab initio calculation. In order to demonstrate the versatility of this fitting method, it has been applied to a combination of positron annihilation measurements and magnetic Compton scattering experiments in ferromagnetic nickel.     

Contribution ofd-like states to magnesium,aluminium and phosphorusL-emission bands of MgO,Al2O3 and AlPO4

We have measured the MgL-, AlL- and PL-emission bands of MgO, -Al₂O₃ and AlPO₄, respectively. For MgO and Al₂O₃ the electronic structure and the X-ray emission bands have been calculated. In spite of different crystal structure and chemical composition of these compounds the cationL-emission bands are very similar. We have found that for the interpretation of theL-emission bands of these compounds the cation 3d-like electrons are crucial.     

Spin-resolved NEXAFS from resonant X-ray scattering (RXS): Application to MnO

Resonantly excited metal K core line spectra of NiO, MnO, CuO and other compounds have been investigated at the beamlines X21 (NSLS/BNL), BW1 and W1.1 (HASYLAB/DESY). A novel technique for quantitative resolution of NEXAFS spectra into spin-up and spin-down components has been developed. Since the method employs spin conservation and local spin references, it needs no circularly polarized radiation and no sample magnetization for taking both the RXS and NEXAFS spectra. Hence antiferromagnetic and paramagnetic materials can be investigated as well.By utilizing linear dichroism with angular-dependent measurements on single-crystal samples, additional resolution of NEXAFS spectra is possible with respect to the orbital symmetry. Application of the method to paramagnetic MnO, for the first time, provides new and unambiguous experimental results confirming modern (LSDA+U) calculations. Received: 29 May 2001 / Accepted: 4 July 2001 / Published online: 5 October 2001     

The electronic structure of KMnO4 investigated by resonant photoemission

The valence band of potassium permanganate (KMnO₄) was investigated by means of resonant photoemission spectroscopy (ResPES) at the Mn2p, Mn3p and O1s edge. These data confirm the previous conclusions of a strong deviation from a purely ionic charge distribution in this compound. The ResPES data are in agreement with previous results about the character of the individual valence band states. A simple cluster model is used to explain qualitatively the different structures seen in the valence band spectra and their dependence on the photon energy.     

Electronic structure of HgGa2S4

The electronic structure and chemical bonding in HgGa₂S₄ crystals grown by vapor transport method are investigated with X-ray photoemission spectroscopy. The valence band of HgGa₂S₄ is found to be formed by splitted S 3p and Hg 6s states at binding energies BE=3-7 eV and the components at BE=7-11 eV generated by the hybridization of S 3s and Ga 4s states with a strong contribution from the Hg 5d states. At higher binding energies the emission lines related to the Hg 4f, Ga 3p, S 2p, S 2s, Hg 4d, Ga LMM, Ga 3p and S LMM states are analyzed in the photoemission spectrum. The measured core level binding energies are compared with those of HgS, GaS, AgGaS₂ and SrGa₂S₄ compounds. The valence band spectrum proves to be independent on the technological conditions of crystal growth. In contrast to the valence band spectrum, the distribution of electron states in the bandgap of HgGa₂S₄ crystals is found to be strongly dependent upon the technological conditions of crystal growth as demonstrated by the photoluminescence analysis.     

Total electron yield XANES of zinc-blende MnTe

The electronic structure and chemical bond of zinc-blende (zb) MnTe have been studied by using total-electron-yield (TEY) X-ray absorption near-edge structure (XANES) spectroscopy. Close resemblances of the shape of Mn K-edge XANES in zb-MnTe and in Zn_1−xMn_xTe [A. Titov, X. Biquard, D. Halley, S. Kuroda, E. Bellet-Amalric, H. Mariette, J. Cibert, A.E. Merad, G. Merad, M.B. Kanoun, E. Kulatov, Yu.A. Uspenskii, Phys. Rev. B 72 (2005) 115209] indicated predominant influence of the 1st coordination shell. In particular, identical single-peak pre-edge structure for both cases was mainly ascribed to the Mn 1s-3d/4p weakly allowed dipole transitions. The quantitative analysis of XANES in zb-MnTe concerned the observed chemical shift of Mn K-edge threshold energy and a magnitude of the relevant cation-anion charge transfer (or effective cation charge), q(Mn-Te) [calculated after M. Kitamura, H. Chen, J. Phys. Chem. Solids 52 (1991) 731]. It also provided a comparison with our earlier X-ray absorption studies of Zn_1−xMn_xB alloys (B = S, Se). The estimated charge transfer within the chemical bond of zb-MnTe enabled us to complete the q(Mn-B) versus chalcogen ligand (B = S, Se, Te) dependence and to interpret it in terms of p-d hybridization and a contribution of Mn 3d electrons to the overall charge transfer.     

XMCD at LII,III edges of rare-earths: electric quadrupolar and dipolar effects

X-ray Magnetic Circular Dichroism (XMCD) at L_II,III edges of rare earths in rare earth–3d transition metal compounds is still a subject of study. We present experimental evidence of electric quadrupolar transitions at L_III and L_II edges of Er in the ErFe₂ Laves phase. These experimental results are compared with atomic calculation taking into account crystal electric field effects on the Er site. Received: 30 May 2001 / Accepted: 4 July 2001 / Published online: 5 October 2001     

Pre-edge features in X-ray absorption structure of Mn in GaMnN,GaMnAs and GeMn

_Ga1-xMnxN${\mathrm{Ga}}_{1-x}{\mathrm{Mn}}_x\mathrm{N}$ samples with a wide range of concentrations, from x=0.003$x=0.003$ to 0.057, were grown by molecular beam epitaxy. X-ray diffraction and the simulation of the extended X-ray absorption fine structure (EXAFS) confirmed the wurtzite structure of the samples, without any secondary phase, and the location of Mn in the gallium sublattice of GaN. The valence state of Mn was studied using the X-ray absorption near-edge structure (XANES) at the K-edge of Mn. The shape of the measured XANES spectra does not depend on the Mn concentration: that implies the same valence state and local atomic structure around the Mn atom in all samples. A calculation of the electronic band structure of GaMnN, GaMnAs, GeMn and the XANES spectra of Mn in GaMnN was performed using the linearized augmented plane wave (LAPW) method. The calculated spectra fit well our experimental data. In particular, the comparison demonstrates that a peak in the pre-edge structure is directly related to the 3+$3+$ valence state of Mn. This was confirmed experimentally by measurements of the Mn K-edge in GaMn²⁺As${\mathrm{GaMn}}^{2+}\mathrm{As}$ and ZnMn²⁺Te${\mathrm{ZnMn}}^{2+}\mathrm{Te}$. An application of proposed interpretation for GeMn is discussed.     

Contribution of valence s+p type electrons to the high momentum components of the electron-positron momentum density in gold

It is generally believed that the high momentum components (HMCs) of the angular correlation of annihilation radiation or Doppler broadening spectra map the positron annihilation with predominately core electrons, containing only a small fraction coming from localised d+f-type valence electrons. In present work, we study how far the contribution of valence electrons to the HMCs of the electron-positron (e-p) momentum density can be neglected. Calculations are performed for gold within both the independent particles model (IPM) and including the e-p correlation effects non-locally, using the weighted density approximation (WDA). In particular, the HMCs due to valence s+p-type electrons are compared with their core and d+f-type valence counterparts.     

Polarization dependence of resonant inelastic X-ray scattering spectroscopy in correlated electron systems

Resonant inelastic X-ray scattering (RIXS) at the transition-metal K-edge is studied as a tool to detect the electronic structure in correlated electron systems. We, in particular, focus on the polarization dependence of RIXS intensity and symmetry of the electronic excitations. It is shown that by analyzing the polarization of the initial and scattered X-rays, the symmetry of the 4p orbitals are selected. Combined effects of the polarization of X-rays and the momentum transfer in the scattering are also studied.     

Site Preference and Alloying Effect of Excess Ni in Ni-Mn-Ga Shape Memory Alloys

The formation energies and electronic structures of Ni-rich Ni-Mn-Ga alloys have been investigated by firstprinciples calculations using the pseudopotential plane wave method based on density functional theory. The results show that the alloying Ni prefers to occupy the Mn site directly in Ni9Mn3Ga4 and to occupy the Mn site and drive the displaced Mn atom to the Ga site in NigMn4Ga3, which is in accordance with the experimental result. According to the lattice constants and the density of states analyses, these site preference behaviours are closely related to the smaller lattice distortion and the lower-energy electronic structure when the excess Ni occupies the Mn site. The effect of Ni alloying on martensitic transformation is discussed and the enhancement of martensitic transformation temperature by Ni alloying is estimated by the calculated formation energy difference between austenite and martensite phases.     

Band structure calculations for Heusler phase Co2YBi and half-Heusler phase CoYBi (Y=Mn, Cr)

We have studied the electron structure and magnetic properties of Heusler phase Co₂YBi and half-Heusler phase CoYBi (Y=Mn, Cr) by using the full-potential linearized-augmented plane-wave (FLAPW) method. Co₂MnBi and Co₂CrBi are predicted to be half-metallic magnetism with a total magnetic moment of 6 and 5 μ_B, respectively, well consistent with the Slater-Pauling rule. We also predict CoMnBi to be half-metallic magnetism with a slight compression. The gap origin for Co₂MnBi and Co₂CrBi is due to the 3d electron splitting of Mn (Cr) and Co atoms, and the gap width depends on Co electron splitting. The atom coordination surroundings have a great influence on the electron structure, and consequently the Y site in the X₂YZ structure has a more remarkable electron splitting than the X site due to the more symmetric surroundings. The investigation regarding the lattice constant dependence of magnetic moment shows that the Co magnetic moment exhibits an opposite behavior with the change of the lattice constant for Heusler and half-Heusler alloys, consequently leading to the different variation trends for total magnetic moment. The variation of total and atom magnetic moment versus lattice constant can be explained by the extent of 3d electron splitting and localization of Mn (Cr) and Co atoms for both the series of alloys.     

X-ray and UV photoelectron spectroscopy study of Na-halogen-graphite intercalation compounds. Comparison between donor- and acceptor-graphite compounds

New results obtained with XPS and UPS on ternary Na-halogen-graphite intercalation compounds (GICs) complement previous ones obtained on binary alkali- and ternary alkali-oxygen-GICs yet published, to give a general overview about what can be learned from photo-electron spectroscopy. In addition to surface stoichiometry, the charge transfer, the Fermi level energy shift and the band structure are estimated thanks to the observation in UP spectra of the unoccupied Π^∗ level peak.     

X-ray magnetic circular dichroism involving valence states

Accepted: 6 March 1997     

Magnetic circular dichroism in soft X-ray resonant inelastic scattering

We discuss the application of resonant inelastic X-ray scattering to the study of magnetic systems in the soft X-ray range. To this end we distinguish two broad areas. In the first the layout of the experiment is such that the absorption magnetic circular dichroism (MCD) is not zero. In the second the magnetisation is perpendicular to the incident helical beam so that the absorption MCD is zero. In the first area we summarise published results on Fe-Co alloys and we present new data on Mn impurities in Ni together with calculations. In the second area we summarise published results on Ni in Ni-ferrite with final 3s shell excitation and we present new results on Co-metal and Co in Co-ferrite measured with a new approach. This is based on the incident energy dependence of the integral of the Raman spectrum in inner shell excitation (integrated resonant Raman scattering). The potentialities and the limitations of the above methods are critically presented. Received: 23 May 2001 / Accepted: 4 July 2001 / Published online: 5 October 2001     

Effect of Fe substitution on the magnetic properties of half-Heusler alloy CoCrAl

The effect of Fe substitution for the vacant site in half-Heusler alloy CoCrAl is studied. A series of single phase CoFe_xCrAl (x=0.0, 0.25, 0.5, 0.75 and 1.0) alloys has been successfully synthesized. The lattice constant is found to increase almost linearly with increasing Fe content, indicating Fe atoms enter the lattice of CoCrAl instead of existing as a secondary phase. When Fe entering the vacant site, spin polarization occurs and the alloy turns from a semimetal in CoCrAl to a half-metallic ferromagnet (HMF) in CoFeCrAl. This is due to the reconstruction of the energy band with Fe substitution. The Curie temperature and saturation magnetic moments are enhanced and increase monotonically with increasing Fe content. The variation of the spin moment follows the Slater-Pauling curve and agrees with the theoretical calculation as well.     

Ab-initio investigation of electronic properties and magnetism of half-Heusler alloys XCrAl (X=Fe, Co, Ni) and NiCrZ (Z=Al, Ga, In)

The electronic structures and magnetism of the half-Heusler alloys XCrAl (X=Fe, Co, Ni) and NiCrZ (Z=Al, Ga, In) have been investigated to search for new candidate half-metallic materials. Here, we predict that NiCrAl, and NiCrGa and NiCrIn are possible half-metals with an energy gap in the minority spin and a completely spin polarization at the Fermi level. The energy gap can be attributed to the covalent hybridization between the d states of the Ni and Cr atoms, which leads to the formation of bonding and antibonding peaks with a gap in between them. Their total magnetic moments are 1μ_B per unit cell; agree with the Slater-Pauling rule. The partial moment of Cr is largest in NiCrZ alloys and moments of Ni and Al are in antiferromagnetic alignment with Cr. Meanwhile, it is also found that FeCrAl is a normal ferromagnetic metal with a magnetic moment of 0.25μ_B per unit cell and CoCrAl is a semi-metal and non-magnetic.     

High spin polarization in ordered Cr3Co with the DO3 structure: A first-principles study

The electronic structure of the highly ordered alloy Cr₃Co with the DO₃ structure has been studied by FLAPW calculations. It is found that the ferrimagnetic state is stable and that the equilibrium lattice constant of Cr₃Co equals 5.77 Å. A large peak in majority spin density of states (DOS) and an energy gap in minority spin DOS are observed at the Fermi level, which results in a high spin polarization of 90% in the ordered alloy Cr₃Co. The total magnetic moment of Cr₃Co is 3.12μ_B, which is close to the ideal value of 3μ_B derived from the Slater-Pauling curve. An antiparallel alignment between the moments on the Cr (A, C) sites and the Cr (B) sites is observed. Finally, the effect of lattice distortion on the electronic structure and on magnetic properties of Cr₃Co compound is studied. A spin polarization higher than 80% can be obtained between 5.55 and 5.90 Å. With increasing lattice constant, the magnetic moments on the (A, C) sites increase and the moments on the (B, D) sites decrease. They compensate each other and make the total magnetic moment change only slightly.