Structure electronique des oxydes de cobalt CoO et Co3O4

The total density of occupied states in the valence band of CoO and Co₃O₄ is determined by XPS and UPS. From variations of excitation probability of the bands, the 4 e V wide O2_p band is shown to be located around 5 eV for both oxides, while structures obtained from photoionisation of the localized 3d band spread over 10 eV range below the Fermi level overlapping with O2_p band. The 3d peaks located at binding energy <3 eV correspond to the calculated energy of the d_n ?1 manifold final state in the octahedral and tetrahedral crystal field of CoO and Co₃O₄. The 3d levels at higher binding energy are shown to occur from configuration interaction in both final and initial states. These last peaks are higher in intensity for CoO relative to Co₃O₄. A superior limit for the width of the 3d initial band in a one electron energy diagram is given to be <3 eV. This value associated to the Coulomb correlation energy measured equal to ~3 eV. This value associated to the Coulomb correlation energy measured equal to ~3 eV from shake-up and Auger energy confirms the Mott insulator nature of CoO.     

Electronic properties of anatase TiO2 doped by lanthanides: A DFT+U study

The effects of mono-doping of 4f lanthanides with and without oxygen vacancy defect on the electronic structures of anatase TiO₂ have been studied by first-principles calculations with DFT+U (DFT with Hubbard U correction) to treat the strong correlation of Ti 3d electrons and lanthanides 4f electrons. Our results revealed that dopant Ce is easy to incorporate into the TiO₂ host by substituting Ti due to its lower substitutional energy (∼−2.0 eV), but the band gap of the system almost keeps intact after doping. The Ce 4f states are located at the bottom of conduction band, which mainly originates from Ti 3d states. The magnetic moment of doped Ce disappears due to electron transfer from Ce to the nearest O atoms. For Pr and Gd doping, their substitutional energies are similar and close to zero, indicating that both of them may also incorporate into the TiO₂ host. For Pr doping, some 4f spin-down states are located next to the bottom of the conduction band and narrow the band gap of the doping system. However, for Gd doping, the 4f states are located in deep valence band and there is no intermediate band in the band gap. The magnetic moment of dopant Gd is close to the value of isolated Gd atom (∼7 μB), indicating no overlapping between Gd 4f with other orbitals. For Eu, it is hard to incorporate into the TiO₂ host due to its very higher substitutional energy. The results also indicated that oxygen vacancy defect may enhance the adsorption of the visible light in Ln-doped TiO₂ system.     

Electronic structure of PbI2 from photoelectron spectra and the exciton problem

The valence band density of states for PbI₂ is determined from X-ray and u.v. induced photoelectron spectra. It is shown that the band derived from Pb 6s states is at 8 eV binding energy and not at the top of the valence bands as suggested by band structure and charge density calculations. A rigid shift in the predominantly iodine 5p derived bands to lower binding energy brings the band structure calculations into essential agreement with experiment. Pb 5d core level binding energies determined here are used to derive core level exciton energies of 0.7 eV from published reflectivity spectra.     

UV-photoemission study of barium,europium and ytterbium

Measurements are reported of the photoemission spectra and absolute yield of thin films of Eu, Ba and Yb for the photon energy range 2 to 21 eV. Transitions from the occupied 4f-states in Eu and Yb have been observed and the binding energies deduced. The excitation probability of the 4f electrons is found to be very low and an explanation based on their atomic-like nature is given. Transitions involving valence band states are compared with predictions based on published energy band schemes and the density of states for the unoccupied 5d band in Yb is deduced. The effect of chemisorbed oxygen on Ba and Eu surfaces is reported.     

The influence of Ni impurities on the magnetic properties and ESR in Gd(Al1−xNix)2 compounds for x ⩽ 0.2

X-ray, magnetic and ESR studies in Gd(Al_1?xNi_x)₂ compounds were performed. The obtained results, analysed by means of the theoretical models proposed by P.A. Lindgård and E. Zipper show that nickel is nonmagnetic in the studied compounds and its presence changes the conduction band structure and decreases the density of states on the Fermi level.     

YNi4Cu: XPS measurements and electronic structure calculation

The valence band and the core levels of the YNi₄Cu compound are studied using the X-ray photoemission spectroscopy. The valence band is compared with the theoretical calculation by the spin-polarized Tight Binding Linear Muffin Tin Orbital method. The dominance of the Ni 3d and Cu 3d states down to 5 eV below the Fermi level is found. The modification of the bands’ widths and positions can be well explained by the d–d repulsion model. The Ni 2p and Y 3d peaks resemble the results for pure metals.     

Electronic structure of copper halides CuI and CuCl: A comparative X-Ray photoelectron and absorption spectroscopy study

The energy distributions of the occupied and unoccupied electronic states for copper halides CuCl and CuI have been investigated using X-ray photoemission and absorption spectroscopy with a highenergy resolution on the equipment of the Russian-German beamline for outlet and monochromatization of synchrotron radiation from the electron storage ring BESSY II. A quasi-molecular analysis of the obtained experimental spectra has revealed that there is a fundamental similarity of the energy structures of the valence band and the conduction band of copper halides CuX (X = Cl, I) due to the identical atomic structure of the studied compounds. The differences in the positions of individual energy subbands in the valence band and the conduction band of CuX and in their intensities in the spectra are associated with different degrees of hybridization of the Cu 3d, 4s and X(n + 1)s, np valence states, as well as with different sizes of structural units (CuCl₄ and CuI₄ quasi-molecules) of the studied crystals.     

A Mechanism of the influence of Gd(III) and other Ln(III) ions on sensitized luminescence of Eu(III) and Tb(III) ions in aqueous and ethanol solutions: The role of hydroxyl bridges

We studied sensitization of Eu(III) and Tb(III) ions by molecules of 1,10-phenanthroline and 2,2-bipyridil in D₂O and d ₆-ethanol and the influence of Nd(III), Pr(III), Sm(III), Gd(III), and Ho(III) ions on the luminescence intensity I _lum and lifetime τ_lum of Eu(III) and Tb(III) in solutions. The stability constants of complexes of Eu(III) and Gd(III) with 2,2′-bipyridil are measured by spectrophotometric and luminescence methods. It is shown that luminescence of Eu(III) is quenched by Gd(III) ions at the ion concentration equal to 10^?2–10^?1 M, which is caused by competing between these ions for a sensitizer. At the concentration of Ln(III) ions equal to 10^?6?10^?3 M, the sensitized luminescence of Eu(III) and Tb(III) was quenched and τ_lum decreased in the presence of Nd(III) ions, whereas in the presence of Gd(III) the luminescence intensity increased. It is proved that a bridge that connects the two ions upon energy transfer is formed by hydroxyl groups. The intensity of luminescence of Eu(III) and Tb(III) in aqueous solutions and its lifetime decreased in the presence of hydroxyl groups, while upon addition of Gd(III) to these solutions these quantities were restored. We also found that the addition of Gd(III) to deoxygenated ethanol solutions of 2,2′-bipyridil and Eu(III) slows down photochemical and thermal reactions between bipyridil and Eu(III), resulting in the increase in the luminescence intensity of Eu(III).     

Electronic structure of Gd-doped MgO

The electronic structure of Gd-doped MgO is investigated using the LSDA+U (local spin density approximation with U-correction) method and compared with the MgO structure. The total density of states obtained accounting for the correlation effects in the 4f shell of gadolinium is found to be formed by the oxygen 2p states at the valence band and the 4f gadolinium occupied states, while the conduction band is represented by a mixture of empty electronic states. Magnetic properties of the calculated Gd-doped MgO are found to be formed solely by the Gd-4f-magnetic moment of about 7μ_B, in good agreement with recent experimental results suggesting a ferromagnetic coupling of the local magnetic moments induced by Gd.     

Cross sections and γ-rays from multiple neutron capture in151Eu

Primary capture gamma rays following multiple neutron capture in¹⁵¹Eu have been measured as a function of the irradiation time. Neutron capture cross sections of the¹⁵²Eu 3^? groundstate and of the¹⁵²Eu 0^? isomer (T _1/2=9.3h) were determined. These cross sections are relevant for the interpretation of inelastic scattering of neutrons at isomeric states with energy gain (neutron acceleration). The level scheme of¹⁵³Eu has been extended. Neutron binding energies of¹⁵²Eu,¹⁵³Eu,¹⁵³Gd and¹⁵⁴Gd are given.     

Valence band structure of single crystal titanium carbide,studied by soft X-ray and ultraviolet photoelectron spectroscopy

Valence states of single crystal titatium carbide (TiC_x, X?0.88) have been studied with photon energies ranging from far ultraviolet (u.v.) to soft X-ray. The valence band consists of two peaks located at 3 and 10 eV below the Fermi level. This is in good agreement with recent APW band structure calculations that predict a strong hybridization of the Ti 3d and C 2p bands and a C 2s band at lower energy.     

Room temperature ferromagnetism in Cu–Gd co-doped GaN nanowires: A first-principles study

The electronic structure and room temperature ferromagnetism of wurtzite Cu–Gd co-doped GaN nanowires have been investigated by means of the first-principles calculations within the density functional theory, including the on-site Coulomb energy U. The magnetic coupling between Gd atoms in the Gd-doped GaN nanowire is paramagnetic instead of ferromagnetic (FM) as in the bulk structure. After replacing Ga with Cu atom we find a stable FM coupling between Gd magnetic moments in this p-type system. p–d coupling between Cu-3d and N-2p states pushes N-2p states up to Fermi level due to the existence of hole states introduced by Cu dopants. While the p–d coupling between host N-2p and Gd-5d states near Fermi level stabilizes a FM phase of Gd magnetic moments. Furthermore, we get a FM state above room temperature by increasing the holes concentration.     

Comparative study of the core level photoemission of the ZrB2 and ZrB12

X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS) were used to investigate the binding energies and valence band for ZrB₂ and ZrB₁₂. The Zr 3d and B 1s core levels were identified. The Zr 3d core level shows a spin–orbit split 3d_5/2 and 3d_3/2 while that for B 1s core level exhibited a single symmetric peak, these being typical of zirconium and boride signals. Comparing the Zr 3d and B 1s core levels with metallic Zr, B₂O₃ and ZrO₂ reference materials only a negative chemical shift for Zr 3d associated to ZrB₂ was observed, which suggests that the charge transfer model based on the concept of electronegativity was not applicable to explain the superconductivity in the ZrB₁₂ sample. The measured valence band using UPS is consistent with the band-structure calculations indicating a higher density of states (DOS) at E_F for ZrB₁₂ respect to ZrB₂. Finally, we found that the weak mixed B-p and Zr-d states for ZrB₁₂ is crucial for the superconductivity due to the state population increased the DOS at the E_F.     

Thermoreflectance of Mo from 0.5 to 35 eV

The thermoderivative spectrum, ΔR/R, is presented for Mo from 0.5 to 35 eV. Much of the observed structure is interpreted in terms of transitions involving states at the Fermi level. Spin—orbit splitting of the Δ₅ band is observed and is estimated to be 0.24 eV. Structures at 10.3 and 24.2 eV are identified as thermally modulated volume plasmons. A large feature at 11.5 eV corresponds to transitions from the occupied d bands to high lying bands. Structure is observed at 33.7 eV which can be related to modulation of core state transitions.     

Revised model for 5d electrons in the heavy rare earth metals

A revised version of a recently published model for 5d electrons in the ferromagnetic state of the heavy rare earth metals is described. The model involves the broadening of local 5d states into overlapping bands with individual widthsW. In the new approach it is assumed that the local 5d wave functions lie at some point between those for atomic 4f _n 5d 6s ² configurations and those calculated for such configurations subject to the restriction that the 4f shell is kept with its moment rigidly fixed in some given direction. The admixture of non-aligned 4f states as in the atom lowers the local energy, but it also lowers the 5d bandwidth due to misfit of the 4f states which occur with and without the presence of a 5d electron. This second effect raises the energy of the low lying states in the band. The best local states are determined by minimising the total electronic energy of the system, using approximations which are most suitable for 4f shells with large excitation energies. Bandwidths are found by fitting the observed saturation magnetic moments in Gd and Tm, and satisfyW?1 eV.     

Ultraviolet luminescence of Li6Gd(BO3)3: Ce crystals under selective excitation in the region of 4d → 4f transitions

The subnanosecond time-resolved ultraviolet luminescence of Li₆Gd(BO₃)₃: Ce crystals under selective excitation by ultrasoft X-rays in the region of the 4d??4f core transitions at temperatures of 7 and 293 K has been investigated for the first time. The performed investigation has revealed the following features: an intense fast component of the luminescence decay kinetics in the subnanosecond range due to the high local density of electronic excitations and the processes of Auger relaxation of the core hole; the modulation of the luminescence excitation spectrum by the ??giant resonance?? absorption band of the 4d-4f photoionization in the energy range 135?C160 eV; and a new broad luminescence band at an energy of 4.44 eV due to the direct radiative recombination between the genetically related electron in the states of the conduction band bottom and hole in the 4f ground state of the Ce³⁺ ion.     

Valence band states of semi-magnetic semiconductors: Cd1-xMnxTe

Valence band electron states of Cd_1-xMn_xTe mixed crystals were determined over the composition range 0?x?0.7 by ultra-violet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS). A peak at 3.5 eV binding energy (BE) whose magnitude increases with the manganese mole fraction x was identified as originating from the Mn 3d⁵ level. A previously reported structure at 6.5 eV BE was also observed for x>0.4; it is, however, believed to be a satellite of the 3.5 eV peak originating from a shake-up process.     

Electronic structures of MRhO2, MRh2O4, RhMO4 and Rh2MO6 on the basis of X-ray spectroscopy and ESCA data

X-ray O Kα, Rh Mγ and a series of M Lα emission spectra, ESCA spectra of the valence and inner levels, and O K and Rh M_III quantum-yield spectra for X-ray photoemission of the rhodium double oxides MRhO₂ (M = Li, Na, K), MRh₂ O₄ (M = Be, Mg, Ca, Sr, Ba, Co, Ni, Cu, Zn, Cd, Pb), RhMO₄ (M = V, Nb, Ta) and Rh₂MO₆ (M = Mo, W) have been measured and the dependence of electronic structure on the metal M analysed. For all compounds the inner part of the valence band corresponds to O 2p_σ + O 2p_π + Rh 4d states, while the outer part corresponds to Rh 4d. The valence band is separated from the conduction band by a narrow gap of width less than 1 eV. The first empty band, near the bottom of the conduction band, is formed by Rh 4d states, followed by a band due to vacant O 2p states.     

Valence and core state modifications in Pt3Ti

XPS data for the valence band, the Pt 4? states, and the Ti 2p states are presented for the intermetallic Pt₃Ti. Relative to the Pt valence band, the Pt₃Ti band shows a decrease in the density of states just below the Fermi level and a shift of the centroid to higher (binding) energy. Ti 2p and Pt 4? binding energies showed relatively large shifts with respect to the pure metals. These changes in the valence band density of states and core level binding energies are interpreted as arising from hybridization of the d-orbitals in both metals to form strong intermetallic bonds.     

Zur Zustandsdichte der Übergangsmetalle

X-ray isochromats of the bcc transition metals Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, W have been measured with high resolution and under comparable conditions. These curves show, that the rigid-band-model is valid for these metals within one row of the periodic table. The width of the conduction band grows from the 3d- to the 5d-row. These results of the isochromat measurements are compared with conclusions about the density of states at the Fermi level, calculated from values of the electronic specific heat, magnetic susceptibility, and superconducting transition temperature. A comparison with theoretical band calculation shows, that a lowered value of the exchange term in the potential would bring into better agreement the widths of the conduction band, taken from calculation and from isochromat measurements.