Electron correlations are known to play an important role in determining the unusual physical properties of a variety of compounds Such properties include high-temperature superconductivity, heavy fermion behaviour and metal-to-insulator transitions High-resolution photoelectron spectroscopy (PES) provides a means of directly probing the electronic states (particularly those near the Fermi level) in these materials, but the short photoelectron mean free paths (< or = 5 A) associated with the low excitation energies conventionally used (< or = 120 eV) make this a surface-sensitive technique Now that high-resolution PES is possible at much higher energies, with mean free paths as long as 15 A (ref 6), it should become feasible to probe the bulk electronic states in these materials Here we demonstrate the power of this technique by applying it to the cerium compounds CeRu2Si2 and CeRu2 Previous PES studies of these compounds revealed very similar spectra for the Ce 4f electronic states, yet it is expected that such states should be different owing to their differing degrees of hybridization with other valence bands Our determination of the bulk Ce 4f electronic states of these compounds resolves these differences

Probing bulk states of correlated electron systems by high-resolution resonance photoemission

The description of realistic strongly correlated systems has recently advanced through the combination of density functional theory in the local density approximation (LDA) and dynamical mean field theory (DMFT). This LDA+DMFT method is able to treat both strongly correlated insulators and metals. Several interfaces between LDA and DMFT have been used, such as (N-th order) Linear Muffin Tin Orbitals or Maximally localized Wannier Functions. Such schemes are however either complex in use or additional simplifications are often performed (i.e., the atomic sphere approximation). We present an alternative implementation of LDA+DMFT, which keeps the precision of the Wannier implementation, but which is lighter. It relies on the projection of localized orbitals onto a restricted set of Kohn-Sham states to define the correlated subspace. The method is implemented within the Projector Augmented Wave (PAW) and within the Mixed Basis Pseudopotential (MBPP) frameworks. This opens the way to electronic structure calculations within LDA+DMFT for more complex structures with the precision of an all-electron method. We present an application to two correlated systems, namely SrVO3 and beta-NiS (a charge-transfer material), including ligand states in the basis-set. The results are compared to calculations done with Maximally Localized Wannier functions, and the physical features appearing in the orbitally resolved spectral functions are discussed.

Plane-wave based electronic structure calculations for correlated materials using dynamical mean-field theory and projected local orbitals

X-ray absorption near-edge structure (XANES) spectroscopy is the ideal non-destructive technique for characterizing and quantifying S species in compositionally complex natural materials such as silicate glasses and minerals, coals, asphalts and asphaltenes, kerogens and humic substances. Sulfur absorption edges represent the transition of S 1 s and 2 p core electrons to unoccupied antibonding orbitals at the bottom of the conduction band. Shifts in the position of the absorption-edge feature of S K - and L -edge XANES spectra constitute a chemical ruler for oxidation state of both inorganic and organic species of S; the S K edge shifts from 2469.5 eV for chalcopyrite (2− oxidation state) to 2482 eV for gypsum (6+). However, chemical state of S in Earth materials is most readily assigned by comparing the overall XANES profile with spectra for reference compounds. Sulfur XANES spectra are reviewed for pyrite, troilite, pyrrhotite and NiAs-type Co 0.923 S and Ni 0.923 S, niningerite (MgS), oldhamite (CaS), alabandite (MnS) and cubic FeS, and sphalerite and related phases, as well as for selected solid-solutions of the monosulfides. Sulfur XANES spectra for FeS, CoS, NiS, MgS, CaS, MnS and ZnS have been simulated by multiple scattering calculations. The S K -edge XANES of transition-metal-bearing monosulfides generally show anomalous absorption consistent with hybridization of the final S 3 p σ* antibonding states with empty 3 d orbitals on the metal atoms. Various applications of S K - and L -edge XANES fingerprinting are discussed, including speciation of inorganic S in basaltic glasses, lazurite and hauyne, identification of organic functional groups of S in coals, kerogens and humic substances extracted from subtropical soils and marine sediments, and the association of sulfated sugars with calcification of coral aragonite skeletons.

/pdf/xanes-spectroscopy-of-sulfur-in-earth-materials-3e7f15c3kf.pdf

Xanes spectroscopy of sulfur in earth materials

The wide variety of metal sulfide structures and their accommodation of atomic substitution, non-stoichiometry and metal-metal ( M - M ) and ligand-ligand interactions allows for diverse physical, chemical and electronic properties. The energy band structure of 3 d transition-metal sulfides, in particular, is strongly influenced by the covalence of metal-S bonds, which results in hybridization of S 3 p and metal 3 d bonding states and direct or indirect M - M bonding interactions in favorable cases. Differences in the phase relations of isostructural metal sulfides are often attributable to subtle changes in electronic states. The literature on metal sulfide phase relations relevant to the earth sciences is very extensive and could not possibly be summarized in a single chapter. Therefore, following Craig and Scott (1974), this chapter focuses on the base metal (Fe, Co, Ni, Cu, and Zn) sulfides, with the literature for other metal chalcogenides and pnictides and some sulfosalts summarized in a single table (Table 1⇓). The relevant phase relations of the platinum-group element (PGE) chalcogenides and pnictides have been comprehensively reviewed in Makovicky (2002). A section on the halite structure sulfides (niningerite, alabandite and oldhamite) is also included in this chapter. The material presented here relates closely to that discussed in other chapters, in particular the chapter on sulfide thermochemistry (Sack and Ebel 2006). The importance of understanding phase equilibria in the context of electronic and magnetic properties and, hence, electronic structure is also emphasized, thereby reinforcing material presented in several other chapters (Pearce et al. 2006; Vaughan and Rosso 2006).

View this table:

 Table 1. 
Summary of studies on sulfide phase relations relevant to Earth sciences.



Abbreviations used for mineral/phase names include: tr- troilite; po- pyrrhotite; hpo- hexagonal pyrrhotite; mpo- monoclinic pyrrhotite; py- pyrite; pn- pentlandite; hpn- high pentlandite; vs- vaesite; mss - (Fe,Ni) monosulfide solid …

Phase equilibria at high temperatures

Mechanism for SOFC anode degradation from hydrogen sulfide exposure

We have measured high-resolution photoemission spectra of NiS, which undergoes a nonmetal-metal transition at ${T}_{t}\ensuremath{\sim}260$ K. Below ${T}_{t}$, a small band gap opens; the band edge is essentially a step function broadened only by \ensuremath{\sim} 15 meV. This observation is difficult to reconcile with one-electron band theory, which predicts a much broader band edge, implying a dramatic influence of electron correlation on quasiparticle excitations and on the opening of a band gap in 3D antiferromagnetic insulators.

Opening of a correlation-induced band gap in NiS

In order to reveal the microscopic origin of the metal-nonmetal transition in NiS, we have studied the Ni 2p x-ray-absorption spectra of substituted systems Ni 1-x M x S (M=Fe,Co), whose transition temperature T t varies from ∼500 K (Ni 05 Fe 05 S) to below 4 K (Ni 094 Co 006 S) Spectra taken at room temperature have been analyzed using a configuration-interaction cluster model to deduce changes in the electronic structure parameters, namely, the S-3p-to-Ni-3d charge-transfer energy Δ and the Ni-3d-S-3p transfer integral T We find that Fe substitution does not increase the local Ni-S distance as would be expected from the lattice parameters but affects energy levels as is reflected in Δ

A. Misu

Papers

Opening of a correlation-induced band gap in NiS

Metal-nonmetal transition in NiS induced by Fe and Co substitution: X-ray-absorption spectroscopic study.