Ga Sawatzky

Electronic structure of some 3D transition-metal pyrites

Bremsstrahlung Isochromat spectra of FeS2, NiS2 NiS1.2Se0.8 and NiSe2 are reported. These are the first direct experimental evidence for a sharp antibonding p-like state above the Fermi level. A comparison is made with experimental results in the literature. For FeS2 band structure calculations are also presented; these are in good agreement with experiment. The authors show that it is necessary in this calculation to include empty spheres when using a muffin-tin approximation. This is because the pyrite crystal structure is far from close packed. Furthermore, they briefly discuss the electrical and magnetic properties of the compounds.

EFFECTIVE TRANSFER FOR SINGLETS FORMED BY HOLE DOPING IN THE HIGH-TC SUPERCONDUCTORS

Abstract By comparing the exact results of an extended multiband Hubbard Hamiltonian for Cu 2 O 7 and Cu 2 O 8 clusters to those obtained from an effective single band Hamiltonian, we show that the low energy scale physics is very well described by a t - t ′- J model which describes the motion of singlets in an antiferromagnetic background of spins. We obtain values for t , t ′ and J for both hole and electron doping and show that these are different. We also study the dispersion relations and density of states within the quasi particle approximation both of which show rather interesting characteristics which could be relevant for high T c superconductors.

MOSSBAUER STUDY AND BAND-STRUCTURE OF FESI2

A study of the Mossbauer effect as a function of temperature has been carried out for the two crystallographic phases of FeSi2. The spectra show a quadrupole splitting for all Fe positions which slightly decreases with increasing temperature. The isomer-shift values are characteristic for covalent Fe compounds. For each structure a band model is proposed, explaining the differences in electrical and magnetic properties.

SYMMETRY OF THE OXYGEN HOLE STATES IN BI2SR2CACU2O8 INVESTIGATED BY XAS

Abstract We have observed strong polarization dependence in the X-ray absorption near the oxygen K edge in a single crystal of Bi 2 Sr 2 CaCu 2 O 8 ( T c =85 K). The results show that O-derived holes near the Fermi-level have p x, y (perpendicular to the c -axis) symmetry. Some consequences for models of superconductivity are discussed. The concentration of holes is estimated to be about equal to that in YBa 2 Cu 3 O 7 .

Core hole satellites in Ni and Au Ni

Abstract The 2p core hole satellite structure in Ni and 5% Ni in Au are compared and discussed in terms of recent theoretical models. The large differences found for these two materials are explained in terms of the large differences in the Ni 3d band, and virtual bound state widths. The satellite structure for Au Ni is found to agree with the multiplet splitting of the free atom in a 2p53d9 configuration.

IDENTIFICATION OF THE RELATIVE POPULATION OF SPIN ORBIT SPLIT STATES IN THE GROUND-STATE OF A SOLID

The authors introduce a method to investigate experimentally the relative populations of spin-orbit-split states in the ground state of a material. The method uses multiplet structure in X-ray absorption spectra and selection rules limit the subset of terms that can be reached in a given final-state configuration. This subset is dependent on the precise details of the ground state. A test of this method on Ce materials is presented. The results are consistent with an increase in the ratio of 4f72/1 to 4f52/1 contributions in the ground state with increasing hybridization between the 4f levels and the conduction states.

Photoelectron spectra of 2H-TaS2 and SnxTaS2

Photoelectron spectra of Sn1/3TaS2 and SnTaS2 are reported and compared with spectra of 2H-TaS2 and SnS. It is possible to trace the charge transfer from Sn to Ta from the observed increase of the occupation of the lowest Ta 5d band and from the observed decrease of the positive charge on Ta. The charge transfer is the same in Sn1/3TaS2 as in SnTaS2. The data indicate the presence of two types of Sn atoms in SnTaS2. The Ta 4f peaks show an asymmetric broadening; and an anomalous intensity ratio of the 4f7 and 4f5/2 peaks. These effects are ascribed to shake-up effects involving excitations of 5d electrons. The average relaxation energy of a Ta 4f hole is found to be 1.1 eV.

Super transfer in doped LaFeO3

The effects of doping the orthoferrite LaFeO3 with Ga, Ni, Cr, Sc and Al on the hyperfine fields at the57Fe nuclei are investigated and discussed. The dependence of the magnetic hyperfine field at 4.2 K on the kind of nearest neighbour can be described in terms of a “supertransfer” model based upon the LCAO method.

Unoccupied band critical point energies of noble metals determined with bremsstrahlung isochromat spectroscopy

Abstract We show that Bremsstrahlung Isochromat Spectroscopy (BIS) is a powerful and simple tool to determine unoccupied band critical point energies of solids. We determine some critical point energies of the noble metals and compare these to band structure calculations. We also determine the energy dependent lifetime broadening providing information about the imaginary part of the self energy corrections.

EXTENDED FRENKEL EXCITONS IN ONE-DIMENSIONAL NARROW-BAND SYSTEMS

Abstract A model is developed for excitons in a linear chain of dimers of radical ions in the region intermediate between the nearly free-electron limit and the localized-electron limit. When only on site electron—hole interactions are taken into account, an extended exciton state results, which in the limit of zero bandwidth is the normal Frenkel exciton solution. For the case of RbTCNQ, it is shown that from a comparison of the experimentally found dipolar splitting parameters and the calculated ones, it may be concluded that the on site electron—hole interaction and the one-electron bandwidth are of the same order of magnitude for this salt.