High-resolution angle-resolved photoemission study of electronic structure near the Fermi level in strongly correlated f-electron materials

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We have succeeded in performing an energy resolution better than 5 meV by using a new micro-wave excited discharge lamp. With this photoemission spectrometer, we have studied several f- or d-electron correlated materials and obtained the following new findings.1. We have succeeded in directly observing the change in the Fermi surface topology accompanied with the magnetic phase transition in CeSb. We found that the driving force in the transition is the anisotropic p-f mixing.2. We found for the first time a temperature dependent small pseudogap at the Fermi level in CeRhAs and CeRhSb. We have ascribed the pseudogap to the Kondo insulator gap formed through the f-d hybridization.3. We have mapped out the "band structure" of the heavy-fermion superconductors UPtィイD23ィエD2 and URuィイD22ィエD2SiィイD22ィエD2. By comparing with the band calculations, we found that the main body of the valence-band structure consisting of p and d orbitals shows a good agreement between the experiment and the calculation while the bands near EィイD2FィエD2 originating in the f orbital are remarkable narrower in the experiment, probably due to the strong electron correlation.4. We have mapped out the "band structure" of USb and compared with CeSb. We found that USb is a metal with Fermi surfaces consisting of U6d and U5f electrons in contrast with semimetallic CeSb. We have ascribed this to the difference in the energy position of 4f and 5f levels.5. We found a pseudogap in Bi2212 high-TィイD2cィエD2 superconductors, which opens at EィイD2FィエD2 above the superconducting transition temperature (TィイD2cィエD2) for various hole concentrations from under to optimal doping. We found two different pseudogaps with different energy scale and temperature dependence ; one is a precursor of the superconducting gap and the other originates in the antiferromagnetic correlation.6. We have determined the precise Fermi surface of Bi2212 and confirmed its hole-like topology centered at X point.