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Abstracts of the main activities
)Single-element materials in the same column in the periodic table often show mutually similar features as seen in alkaline metals. On the other hand, it is known for noble metals that gold has considerably different chemical stability from that of silver. The valence-band electronic configurations of these solids per atom have so far been recognized to be composed of fully occupied nd states (nd10) plus one conduction electron occupying an (n+1)sp state (n = 4 for silver and n = 5 for gold), but it has not been experimentally verified whether these configurations are correct or not.
We have newly developed a method to study linear polarization-dependent hard x-ray photoemission, by which the extraction of the s and p contributions as well as that of the d and f contributions to the bulk valence band and conduction electrons in solids is feasible. Contrary to the above common expectation, it is found that the 5d-orbital electrons contribute prominently to the conduction electrons in gold while the conduction electrons in silver are to some extent free-electron-like with negligible 4d contribution. The 4d electron correlation effects are found to be essential for the conduction electron character in silver.
Our findings might be related to a well-known fact that gold is more stable than silver in air. In addition, the presence of the intrinsic 5d holes could be an origin of recently found magnetism of the gold nanoparticles. This method will be powerful to reveal the bulk conduction-electron character in solids.
We have developed a new technique for probing bulk three-dimensional electronic structures and Fermi surfaces by using hν-dependent high-energy soft x-ray angle-resolve photoemission, which has been applied for a strongly correlated ferromagnet CeRu2Ge2 in the paramagnetic phase at a "high" temperature of 20 K. A clear difference of the Fermi surface topology from either band-structure calculation or de Haas-van Alphen results in the ferromagnetic phase is observed and interpreted by considering the difference of the 4f contribution to the Fermi surfaces in the paramegnetic phase.
Bulk-sensitive high-resolution photoemission was carried out on a prototype 3d1 metallic system Sr1-xCaxVO3. The experimental spectra are compared with parameter-free LDA+DMFT (local density approximation + dynamical mean-field theory) results. In contrast to earlier investigations the bulk spectral functions are found to be insensitive to x with a pinning of the spectrum at the Fermi energy. The good agreement between experiment and theory confirms the bulk-sensitivity of the high-energy photoemission spectra. Our study has demonstrated the importance of high-energy and high-resolution photoemission spectroscopy for revealing detailed bulk electronic states of transition metal oxides.
We report the bulk Fermi surfaces of the superconductor Sr2RuO4 and the non-superconductor Sr1.8Ca0.2RuO4 probed for the first time by bulk-sensitive high-energy angle-resolved photoemission at hν = 700 eV. It is found that there is one square-shaped hole-like, one square-shaped electron-like and one circle-shaped electron-like Fermi surface in both compounds. These results provide direct evidence for nesting instability giving rise to magnetic fluctuations. Our study clarifies that the electron correlation effects are changed with composition depending on the individual band. Our results support that disappearance of the triplet superconductivity with very small x is caused by the disorder intrinsic to the Ca-substitution.
Bulk Ce 4f and Ru 4d spectral functions of strongly valence-fluctuating CeRu2 are revealed by Ce 4d-4f resonance photoemission with unprecedented resolution (∼100 meV at hν ∼880 eV). The experimental spectra revealed that the Ce 4f states are itinerant and beyond the treatment of the single impurity Anderson model. Bulk Ru 4d states are found to be different from the surface states, implying that even the valence-band structure is modified by such a strong hybridization effect in the bulk. The controversy on the photoemission spectral shapes of Ce materials with very high Kondo temperature is now settled.
"Bulk-sensitive" Ce 3d-4f resonance photoemission (RPES) with an unprecedentedly high resolution has revealed a clear difference of bulk Ce 4f spectral weights of heavy fermion system CeRu2Si2 and CeRu2Ge2 in the region of the tail of the Kondo peak and its spin-orbit partner. The significant spectral difference in both materials is in strong contrast to the mutually similar "surface-sensitive" 4d-4f RPES. The obtained bulk-sensitive spectra are well reproduced by a single impurity Anderson model. Consideration of the crystalline electric field splitting is important for estimating a realistic Kondo temperature from the bulk 4f spectra.
Here we show the power of the technique "High-energy and high-resolution photoemission spectroscopy", which has become a reality very recently at BL25SU on SPring-8, by applying it to the Ce compounds CeRu2Si2 and CeRu2. Previous photoemission studies of these compounds by using low-energy hν (~120 eV) revealed mutually 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.
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We have performed a photoemission study of Yb4Bi3. The valence band spectra taken at hν = 125 eV have shown no multiplet structures associated with the trivalent Yb and revealed that the mean valence of the Yb ion is very close to 2.0. The divalent Yb 4f spectra have shown strong surface components besides weak but sharp bulk components. The surface 4f level shift is about 0.45 eV (300 K), 0.48 eV (150 K) and 0.54 (20 K). A clear energy shift of the bulk Yb 4f peaks toward EF with decreasing temperature has been observed, although Yb4Bi3 shows neither phase transition nor crossover. Such an unusual peak shift of the bulk component may originate from a change of the lattice constant with decreasing temperature. The observed narrowing of the bulk 4f peaks is thought to be due to a phonon broadening (about 7kBT).
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We have measured Ce 3d-4f resonant photoemission spectra of CeNiSn and CePdSn with high resolution of E/ΔE = 4000. Although the Ce valences are close to 3+ for both compounds, the Ce 3d-4f resonance spectra show stronger f1 final states near the Fermi level compared with Ce 4d-4f resonance spectra. Calculations based on the single impurity model give consistent spectral functions to the experimental results. The valence band states except for the Ce 4f states in the bulk are essentially the same as those in the surface and consistent with the result of a band-structure calculation.
Electronic structures of the charge-ordering manganite Nd1-xSrxMnO3 (x = 0.47, 0.5) have been studied by resonant and high-resolution photoemission. The Mn 2p-3d (Nd 3d-4f) resonant photoemission confirmed the Mn 3d (Nd 4f) states hybridized with the O 2p states. The high-resolution Mn 3p resonance spectra show a clear transition from the metal to the charge-ordered insulator for x=0.5 upon cooling while a Fermi cut-off is seen for x=0.47 at all temperatures, suggesting that the transition is induced by a subtle competition between the charge-ordering instability and the double-exchange interaction. Temperature and filling dependence of the spectra implies that the electronic states change not gradually but suddenly at the phase boundary.
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