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Angle-resolved photoemission measurements have been performed on Sr 14 Cu 24 O 41 which consists of twoleg Cu 2 O 3 ladders and edge-sharing CuO 2 chains. The data are consistent with the existence of a dispersive band near the Fermi level which has the periodicity of the ladder sublattice. The band shows a minimum binding energy at kaϭ(2nϩ1)/2, suggesting that a Mott-Hubbard gap opens on the ladder. The experimental result shows a qualitative agreement with the t-J ladder calculation. It was also found that replacement of Sr with Ca causes a transfer of holes from the chain to the ladder. ͓S0163-1829͑97͒05137-0͔

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We have performed a high-resolution angle-resolved photoemission spectroscopy ͑ARPES͒ on antiferromagnetic USb to study the electronic structure near the Fermi level. We found that USb has a metallic band structure with the fully occupied Sb 5p bands in contrast to semimetallic CeSb that has the partially filled Sb 5p bands. This suggests that the magnetic phase transition of USb is not understood within the framework of the p-f mixing model. This difference in the electronic structure between USb and CeSb is ascribed to the energy position of the respective bare f level with respect to the Sb 5p band. The observed fully occupied Sb 5p bands in USb is consistent with the band calculation based on the itinerant U 5 f model, but different from that of the localized model. On the other hand, we found two dispersionless bands just below E F in ARPES spectra of USb, which are well described in terms of the 5 f 2 -final-state multiplet structure calculated based on the localized 5 f model. These experimental results suggest the dual ͑itinerant and localized͒ character of 5 f electrons that characterizes anomalous properties of USb.

High-resolution photoemission spectroscopy ͑HRPES͒ was performed on V 2Ϫy O 3 (yϭ0.00, 0.03, and 0.04͒, which has been considered as the prototype of the Mott-Hubbard model for the metal-insulator transition ͑MIT͒. The density of states at the Fermi level (E F ) observed by HRPES is one order of magnitude smaller than that of the band calculation based on the local-density approximation. We observed no distinguishable sharp structure at E F for hole-doped V 1.96 O 3 at 15 K as well as negligible temperature-dependence of the HRPES spectrum of V 2 O 3 between 200 and 300 K. The analysis of the V 3d spectrum with a local self-energy gives a too large quasiparticle effective mass compared to the specific-heat measurement, which implies the importance of spatial fluctuations near the MIT. All these facts strongly question the applicability of the dynamical mean-field theory of the Hubbard model to the V 2Ϫy O 3 system.

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