Kondo semiconductor YbB 12 and the Lu-substituted Yb 1−x Lu x B 12 ͑x =1/ 8͒ alloy have been studied by the bulk-sensitive hard x-ray ͑h ϳ 8 keV͒ photoelectron spectroscopy at temperatures from 200 down to 20 K. To check the Kondo lattice effects for the bulk electronic states, the temperature dependence of the bulk Yb valence and 4f peak positions in both systems is analyzed with use of the single-impurity Anderson model ͑SIAM͒ by considering the crystalline electric field effects. For x = 0, the temperature dependence of the bulk electronic states cannot be interpreted within the SIAM, whereas those for x =1/ 8 could be understood by the SIAM predictions due to the collapse of the lattice coherence by the Lu substitution. These results indicate that the Kondo lattice effects are important for pure YbB 12 and play essential roles for forming a narrow gap at low temperatures.

The need for not only bulk sensitive but also extremely high resolution photoelectron spectroscopy for studying detailed electronic structures of strongly correlated electron systems is growing rapidly. Moreover, easy access to such a capability in one’s own laboratory is desirable. Demonstrated here is the performance of a microwave excited rare gas (Xe, Kr, and Ar) lamp combined with ionic crystal filters (sapphire, CaF2, and LiF), which can supply three strong lines near the photon energy of hnyu hν=8.4, 10.0, and 11.6 eV, with the hν resolution of better than 600 μeV for photoelectron spectroscopy. Its performance is demonstrated on some materials by means of both angle-integrated and angle-resolved measurements.

Dependence of the spectral functions near the Fermi level on temperature and rare-earth atom doping was studied in detail for strongly correlated alloys Sm 1−x Eu x B 6 and Yb 1−x Lu x B 12 by photoelectron spectroscopy

We have performed high-resolution hard-x-ray photoemission spectroscopy for the metal-insulator transition (MIT) system (V 1−x Cr x ) 2 O 3 in the paramagnetic metal, paramagnetic insulator, and antiferromagnetic insulator phases. The quality of the spectra enables us to conclude that the on-site Coulomb energy U does not change through the MIT, which eliminates all but one theoretical MIT scenario in this paradigm material.

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...