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We report bulk-sensitive hard X-ray (hν = 5.95 keV) core-level photoemission spectroscopy (PES) of single crystal V1.98Cr0.02O3 and the high-Tc cuprate Bi2Sr2CaCu2O 8+δ (Bi2212). V1.98Cr0.02O3 exhibits low binding energy "satellites" to the V 2p "main lines" in the metallic phase, which are suppressed in the antiferromagnetic insulator phase. In contrast, the Cu 2p spectra of Bi2212 do not show temperature dependent features, but a comparison with soft X-ray PES indicates a large increase in the 2p 5 3d 9 "satellites" or 3d 9 weight in the bulk. Cluster model calculations, including full multiplet structure and a screening channel derived from the coherent band at the Fermi energy, give very satisfactory agreement with experiments.PACS numbers: 71.30.+h, 74.72.Hs, 78.20.Bh, Core-level photoemission spectroscopy (PES) has played a very important role in our understanding of the electronic structure of correlated transition metal (TM) and rare-earth compounds.[1] The appearance of strong satellite structure accompanying the main peaks in correlated systems is well known and systematic variations in the position and intensities of these satellites provide us important clues to their electronic structure.[2, 3, 4] The inter-atomic configuration-interaction approach, using a cluster model or Anderson impurity model , gives a quantitative interpretation for satellite intensities and positions, leading to an accurate description of the ground state and excitation spectrum. [2,3,4] In this approach, the physics of TM compounds can be described in terms of a few parameters, namely, the d-d Coulomb repulsion energy U , the charge-transfer energy ∆, the ligand p-TM d hybridization energy V , and the core-hole-d electron Coulomb attraction energy U dc . Zaanen, Sawatzky and Allen[5] proposed a classification scheme for TM compounds which soon evolved into a paradigm. In this scheme, the band gaps of late TM compounds are socalled charge-transfer (CT) type with U > ∆. NiO and CuO are typical examples of CT insulators while the high-T c cuprates are CT insulators driven metallic by doping. In contrast, the early TM compounds, with U < ∆ are Mott-Hubbard (MH) systems. V 2 O 3 , with its alloys, plays the role of a classic MH system displaying a correlation induced metal-insulator transition. [6,7,8] While the old picture of the MH metal-insulator transition involved a complete collapse or a coalescence of the lower and upper MH bands into a single band in the metal phase, photoemission studies showed the formation of a well-defined coherent band at the Fermi level in the presence of remnant MH bands for a series of correlated oxides [9] and very recently, also for V 2 O 3 .[10] The experimental results are in excellent agreement with calculations using dynamic mean-field theory (DMFT). [10,11] In spite of these successes of PES, the surface sensitivity of PES has often led to controversies regarding surface versus bulk electronic structure, and hence, hard Xray (HX)-PES is very important and promising. [12,13] With the develop...

The silicon clathrate compound Ba 8 Si 46 shows superconductivity below the critical temperature ͑T c ͒ of 8 K, and the T c decreases monotonically with doping Ag. In order to reveal effects of Ag doping on the electronic states, we have applied soft x-ray photoemission spectroscopy to Ag-doped silicon clathrate compounds Ba 8 Ag x Si 46−x ͑x =0,1,3,6͒. The valence band photoemission spectra show that a Ba 5d-derived state at the Fermi level ͑E F ͒, which is prominently observed in Ba 8 Si 46 , decreases with increasing Ag content. The reduction in the peak intensity at E F with increasing Ag content is therefore in accord with the decrease of T c in Ba 8 Ag x Si 46−x. Band structure calculation using local-density approximation reproduces the observed valence band spectra of x = 0 and 6. The Si 2p and Ba 4d core-level photoemission spectra demonstrate that the valence electron of Si is attracted to the Ag site in x = 1 and the 5d electron of Ba inside the Si 24 cage is further donated to Ag in x ജ 3. Hence, Ag doping leads to the reduction of the peak at E F .

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We study the mixed valence transition (T v ∼80 K) in EuNi2(Si0.2Ge0.8)2 using Eu 3d-4f X-ray absorption spectroscopy (XAS) and resonant photoemission spectroscopy (RESPES). The Eu 2+and Eu 3+ main peaks show a giant resonance and the spectral features match very well with atomic multiplet calculations. The spectra show dramatic temperature (T )-dependent changes over large energies (∼10 eV) in RESPES and XAS. The observed non-integral mean valencies of ∼2.35 ± 0.03 (T = 120 K) and ∼2. 7 In contrast, a static or inhomogeneous MV state is one in which electron configurations are different at different sites, representing one specific electron configuration at a site. While the MV in f -electron systems are often homogeneous, there do exist exceptions. 1,8Many f -electron systems exhibit a MV transition induced by temperature (T ), magnetic field and/or pressure.These include the α-γ transition in Ce metal, 1 the pressure-induced transitions in SmS, 6 and TmTe, 9 the T -dependent transitions in YbInCu 4 , 10 Tm-monochalcogenides, 11 as well as Eubased intermetallics, EuPd 2 Si 2 , 12 Eu(Pd 1−x Au x ) 2 Si 2 , 13and EuNi 2 (Si 1−x Ge x ) 2 . 14 Among T -induced transitions, the Eu systems exhibit the largest change in valency, ∆v ∼ 0.3-0.5. 12,13,14 And of these, EuNi 2 (Si 1−x Ge x ) 2 has been extensively studied to show T -, 14,15 magnetic field-, 15 and pressure-16 induced valence transitions. By tuning composition [x in EuNi 2 (Si 1−x Ge x ) 2 ], the transition is observed to be first-order like for compositions close to x = 0.8, with a hysterisis as a function of T , pressure and magnetic field. 15,16,17 The MV transition in EuNi 2 (Si 0.2 Ge 0.8 ) 2 has thus been investigated across the critical T (T v ) of ∼80 K by magnetic susceptibility, high-energy bulk-sensitive Eu L-edge X-ray absorption spectroscopy (XAS), and X-ray diffraction to show that the transition is accompanied by a Kondo-like volume collapse across T v . 14,15,16,17 XAS and resonant photoemission spectroscopy (RE-SPES) are important techniques for studying the electronic structure (ES) of f -electron systems. 18,19,20 In XAS applied to a solid, a core electron of a particular site or element is excited to an empty state, and hence, it probes site-specific angular momentum projected unoccupied states of a solid.21 RESPES is a complementary technique which probes the resonantly enhanced partial occupied density of states (DOS) of a solid.20 These techniques provide important insights into the physical properties of strongly correlated materials, including MV, Kondo effect, heavy fermion behavior, etc. However, recent studies using ultraviolet photoemission spectroscopy (PES) of MV systems revealed modifications of the surface ES compared to the bulk. 22,23,24 While signatures of T -dependent MV are observed, the mean valence estimated from these measurements are incompatible with bulk thermodynamic studies. Significantly, the important role of hard X-ray (HX: hν ∼ 3 to 8 keV) PES in general, 25,26,27,28,29,30,31 as well as soft X-ray (SX: hν ∼ ...