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Soft x-ray excited angle-resolved photoemission spectroscopy (ARPES) is performed for the valence bands of quasi-one-dimensional V 6 O 13 and SrCuO 2 in order to reveal behavior of the strongly correlated V 3d and Cu 3d states. The resonance enhancement of the V 3d state for the V 2p core excitation and the high photoionization cross section of the Cu 3d states compared with the O 2p states are fully utilized in addition to the high resolutions in energy and momentum facilitated by recent instrument developments. Clear differences from the results of low photon energy ARPES have been observed for both materials by virtue of the high 3d sensitivity as well as high bulk sensitivity. Coexistence of a quasiparticle peak with an incoherent peak is observed in the metallic phase of V 6 O 13 , whereas the quasiparticle peak collapses in the insulator phase, in which two incoherent peaks are observed. In SrCuO 2 , the dispersive behavior of the spectra is well understood on the basis of the one-dimensional half-filled Hubbard model with U / t = 7.5 and U = 3.0 eV (U: Coulomb repulsive energy and t: transfer energy) and substantial coupling between the spin and charge excitations is suggested.

We report on the Cu 1s resonant inelastic X-ray scattering (RIXS) of Cu-O one-dimensional (1D) strongly correlated insulator systems with contrasting atomic arrangements, namely edge-sharing CuGeO 3 and corner-sharing Sr 2 CuO 3 . Owing to good statistics of the high-resolution RIXS data, so far unresolved fine structures are revealed. Detailed photon-energy and momentum dependence of the RIXS spectra in comparison with theoretical calculations has clarified the natures of the low-energy charge excitations and hybridization of the electronic states.PACS Numbers: 78.70.Ck, 71.27.+a, 75.10.Pq Resonant inelastic X-ray scattering (RIXS) is a powerful tool to probe the momentum dependence of low-energy excitations in solids. 1,2 This technique is intriguing to clarify bulk electronic states of strongly correlated insulators, which are under a keen general interest in decades. 3,4,5 For metallic systems, highresolution angle-resolved photoemission (ARPES) is promising to detect their occupied states. Nowadays both surface-sensitive low-energy 6 and bulk-sensitive high-energy 7,8 ARPES measurements are feasible. Compared to ARPES, RIXS is really bulk sensitive and applicable to insulators with high resistivity, where the electron correlation is even stronger. 1,2,5,9 However, high energy resolution RIXS is rather demanding because of their poor count rate. For this purpose, high photon flux in a small spot size and a highly efficient analyzer crystal are required.Here we report photon-energy (hν) and momentum (Δk) dependence of the Cu 1s RIXS with good statistics for two contrasting Cu-O 1D insulating systems CuGeO 3 10 and Sr 2 CuO 3 2 with dominantly divalent Cu. As shown in Fig. 1c, CuGeO 3 has a single chain with the edge-sharing CuO 2 plane configuration with the Cu-O-Cu angle (θ) of 99˚, where the Cu-Cu chain axis is taken as the x-axis and the CuO 2 plane corresponds to the x-y plane. The 3d xy orbital is unoccupied because it has the highest energy among the whole d orbitals according to an extended d-p model calculation. 11,12 The transfer energy between the neighboring Cu 3d sites via O 2p sites is thought to be very small because of the orthogonality of the Cu 3d xy orbitals on the neighboring sites coupled to the O 2p orbitals in the edge-sharing CuGeO 3 . On the other hand, Sr 2 CuO 3 has a single Cu-O chain with the corner-sharing configuration as shown in Fig. 2b. The transfer energy is thought to be large in this case, in which the Cu 3d hole is thought to be in the 3d x 2 -y 2 state. Therefore very different behavior of charge dynamics is expected in these systems.RIXS experiment was performed at BL19LXU of SPring-8 with a 27m long X-ray linear undulator. By use of two channel cut crystal monochromators, the resolution of the incident hν was better than 300 meV. A horizontal focusing was better than 100 µm on the sample. The instrument with 1m Rowland circle was used for the measurement. The total resolution of 400 meV (full width at half maximum) was achieved by using a spherically bent Si(553) ana...

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Bulk-sensitive hard x-ray photoemission spectroscopy (HAXPES) reveals for as-grown epitaxial films of half-metallic ferromagnetic CrO 2 (100) a pronounced screening feature in the Cr 2p 3/2 core level and an asymmetry in the O 1s core level. This gives evidence of a finite, metal-type Fermi edge, which is surprisingly not observed in HAXPES. A spectral weight shift in HAXPES away from the Fermi energy is attributed to single-ion recoil effects due to high energy photoelectrons. In conjunction with inverse PES the intrinsic correlated Mott-Hubbard-type electronic structure is unravelled, yielding an averaged Coulomb correlation energy U av  3.2 eV.PACS numbers: 71.27.+a, 75.47.Lx, Transition metal oxides are strongly correlated electron systems, which exhibit a wealth and future potential of phenomena most challenging to modern solid state physics 1, 2. In the theoretical description of the electronic structure of transition metal oxides a seminal progress is owed to dynamic mean-field theory (DMFT) 3,4. This method has called upon intrinsic, bulk-sensitive photoemission spectroscopy (PES) 5. The reason is obvious because electronic states of the clean surface differ from those in the bulk due to the increase in U/t, where U is the on-site electron Coulomb repulsion energy and t is the electron hopping energy between lattice sites. Experimentally a breakthrough toward determining the intrinsic bulk electronic structure occurred due to the development of hard x-ray PES (HAXPES) with a probing depth of 5 -10 nm 6 -9. In this context a very overdue, controversial and provoking case is the half-metallic ferromagnet CrO 2 10,11, which exhibits a metastable surface, transforming into the stable antiferromagnetic insulator Cr 2 O 3 10. For CrO 2 a discrepancy exists between the correlated Fermi liquid-type metallic behavior 10,13 and the very small intensity of the sputter-cleaned surface in ultraviolet PES (UPES) near the Fermi energy E F 12. The latter was conjectured to be due to surface relaxation of CrO 2 (001) 14. The metallicity of CrO 2 (100) was even questioned based on UPES measurements 15.On the contrary, an enhanced spectral weight near E F due to an orbital Kondo effect has been predicted using DMFT 16. However, the theoretical description of electronic and (magneto-)optical data of CrO 2 has raised doubts about the relevance of strong Hubbard-type correlations 17-20. This controversy and the above surface calamity stress the indispensable need to employ HAXPES in comparison to soft x-ray PES (SXPES) 8,21,22. Despite the interest in CrO 2 for spintronics applications 23,24, because of its high spin polarization 12,17,25-28, the intrinsic correlated electronic structure still remains to be unravelled 29.Here we present the first bulk-sensitive investigation of valence band states and core levels of CrO 2 by means of HAXPES using photon energies of h  8 keV. The I. METHODS A. HAXPES measurementsThe HAXPES experiments were performed at the BL19LXU beam line of SPring-8 using a...