We have studied the electronic structure and charge ordering (Verwey) transition of magnetite (Fe3O4) by soft X-ray photoemission. Due to the enhanced probing depth and the use of different surface preparations we are able to distinguish surface and volume effects in the spectra. The pseudogap behavior of the intrinsic spectra and its temperature dependence give evidence for the existence of strongly bound small polarons consistent with both dc and optical conductivity. Together with other recent structural and theoretical results our findings support a picture in which the Verwey transition contains elements of a cooperative Jahn-Teller effect, stabilized by local Coulomb interaction.

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We have studied the low-energy electronic structure of a Kondo insulator YbB 12 by high-resolution photoemission spectroscopy. A "Kondo peak" is observed ϳ25 meV below the Fermi level, which agrees well with the Kondo temperature, whereas the gap at the Fermi level is found much smaller, indicating that the magnetic properties at higher temperatures (կ75 K) are indeed determined by the Kondo effect in spite of the gap formation at lower temperatures. A renormalized band picture is presented to describe the coexistence of the Kondo peak and the transport gap as well as the highly asymmetric line shape of the Kondo peak.

We report the Fermi surfaces of the superconductor Sr2RuO4 and the non-superconductor Sr1.8Ca0.2RuO4 probed by bulk-sensitive high-energy angle-resolved photoemission. 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. 71.18.+y, 74.70.Pq Clarification of Fermi surfaces (FSs) is fundamental to understand the physical properties of functional materials such as superconducting transition metal oxides, heavy fermion systems, and organic conductors. Quantum oscillation measurements by virtue of the de Haasvan Alphen or Shubnikov-de Haas effect are known as useful techniques to detect bulk FSs. However, their electron-or hole-like character and their shape cannot be experimentally revealed by these measurements alone. In addition, these techniques require low temperatures and almost defect-free single crystals, so that they are not easily applicable to doped or partially substituted systems such as the high-temperature superconductors La 2−x Sr x CuO 4 , Bi 2 Sr 2 CaCu 2 O 8+δ , or the here reported Sr 1.8 Ca 0.2 RuO 4 . The number of measurements by using quantum oscillations on oxides is actually very few. On the other hand, low-energy angle-resolved photoemission (ARPES) is known as a tool for probing FSs as well as quasi-particle dispersions of correlated electron systems. 1 However, it is still unclear whether so far reported low-energy ARPES (hν < ∼ 120 eV) results fully reflect bulk electronic structures because of its high surfacesensitivity. Since high-energy photoemission (hν ≥ 500 eV) has an advantage in probing bulk states, 2,3 highenergy ARPES with high angular resolution can be a complementary and promising technique for the bulk Fermiology of solids besides the quantum oscillations measurements.It is known that Sr 2 RuO 4 shows "triplet" superconductivity 4,5 , which disappears with a very small amount of Ca-substitution. 6 Combination of the quantum oscillation measurements and band-structure calculations suggests one hole-like FS sheet centered at (π, π) (α sheet) and two electron-like FS sheets centered at (0,0) (β and γ sheets) in Sr 2 RuO 4 . 7,8,9 On the other hand, so far re-ported results of low-energy ARPES for Sr 2 RuO 4 are controversial although ARPES has an advantage in determining the character of FSs. Yokoya et al. have first concluded two hole-like and one electron-like FSs. 10 However, the following ARPES studies 1,11,12 have suggested that the earlier finding originates from surface states, and that the bulk FSs are qualitatively similar to the result of the band-structure calculation. It has also been reported that a lattice distortion takes place at the surface, 13 giving FSs different from the bulk. Thus the characters and shapes of the two-dimensional bulk FS...

Material specific electronic band structure of the electron-doped high-Tc cuprate Nd1.85Ce0.15CuO4 (NCCO) is calculated within the pseudo gap regime, using the recently developed generalized LDA+DMFT+Σ k scheme. LDA/DFT (density functional theory within local density approximation) provides model parameters (hopping integral values, local Coulomb interaction strength) for the one-band Hubbard model, which is solved by DMFT (dynamical mean-field theory). To take into account pseudogap fluctuations LDA+DMFT is supplied with "external" k-dependent self-energy Σ k , which describes interaction of correlated conducting electrons with non-local Heisenberg-like antiferromagnetic (AFM) spin fluctuations responsible for pseudo gap formation. Within this LDA+DMFT+Σ k approach we demonstrate the formation of pronounced "hot-spots" on the Fermi surface (FS) map in NCCO, opposite to our recent calculations for Bi2Sr2CaCu2O 8−δ (Bi2212), which have produced rather extended region of FS "destruction". There are several physical reasons for this fact: (i) the "hot-spots" in NCCO are located closer to Brillouin zone center; (ii) correlation length of AFM fluctuations ξ is larger for NCCO; (iii) pseudogap potential ∆ is stronger, than in Bi2212. Comparison of our theoretical data with recent bulk sensitive high-energy angle-resolved photoemission (ARPES) data for NCCO provides good semiquantitative agreement. Based on that comparison alternative explanation of the van-Hove singularity at -0.3 eV is proposed. Optical conductivity both for Bi2212 and NCCO is also calculated within LDA+DMFT+Σ k and compared with experimental results, demonstrating satisfactory agreement.