We have performed high energy resolution angle-resolved photoemission studies of the normal state band structure of oxygen overdoped Bi 2 Sr 2 Ca 1 Cu 2 O 8+x . We find that there is an extended saddle point singularity in the density of states along Γ −M − Z direction. The data also indicate that there is an asymmetry in the Fermi surface for both the Γ −M − Z and perpendicular directions.

High-resolution angle-resolved photoemission measurements were performed on single crystals of Bi 2 Sr 2 Ca 1 Cu 2 O 8+ δ with different oxygen stoichiometries. The data establish that the gap anisotropy (ratio of the gap along Γ-M to the gap along Γ-X) can be reversibly changed from ∼20:1 (optimal or underdoped) to ∼2:1 (overdoped). Differences in sample doping explain the conflicting reports on gap anisotropy in the literature. Possible effects of this change in gap anisotropy on the symmetry of the order parameter are discussed. There remains some ambiguity as to the relation between the order parameter and doping.

Angle-resolved photoemission spectra taken with high energy resolution on the one-dimensional cornpound (TaSe&)zI confirm the suppression of the spectral intensity at the Fermi edge for one-dimensional systems and lend support to correlation effects as its most probable cause.The photoemission spectrum of nearly-onedimensional materials like (TaSez)2I is a very interesting problem in condensed-matter physics. Several experiments have suggested a very low spectral intensity near the Fermi energy EF. ' Dardel et al. examined this striking result with very high energy resolution, concluding that the effect is most likely not related to a low quasiparticle density of states near EF, but rather to a significant difference between the excitation spectrum and the density of states.Several mechanisms were proposed to explain the suppression of spectral intensity near EF, but no final evidence in favor of one of them could be extracted from angle-integrated photoemission data. %e did perform high-energy-resolution, angle-resolved studies of (Tase4)2I. The results strongly favor one of the hypotheses formulated in Ref. 2: the suppression of the photoemission intensity near EF is due to correlation effects that drive an infrared catastrophe, as suggested by Schulz in the Luttinger model framework.The samples studied in our experiments were similar to those of Ref. 2; the most significant differences in the procedure were the use in our study of monochromatized synchrotron radiation, and a high angular resolution of +0. 5'. The energy resolution in the different runs of our experiments ranged from 25 to 50 meV. Typical experimental results are shown in Figs. 1 and 2. In Fig. 1 we see a series of high energy and angular resolution spectra, taken at a constant photon energy of 22 eV, and at different azimuthal angles. The spectra exhibit two important features: first, a dispersive peak that does not cross the Fermi energy, but reaches its maximum energy, approximately 0.5 eV below EF, at a k vector corresponding to the border of the first Brillouin zone (ki =0.245 A ' in the direction of the crysta1).Second, we also see that the photoemission spectral intensity is very weak near EF, as was observed in the angle-integrated spectra of Ref. 2. Note that the intensity is weak, but not zero. This is clear from Fig. 2, which shows the spectral region near EF. For all directions in Fig. 2, we do see detectable signal up to the Fermi level, but never an indication of a band crossing a Fermi edge. Several photon energies were used in the experiments, in the range 15 -45 eV. The results of these tests indicate that the suppression of the near-edge spectral signal is not a trivial final-state effect. Similarly, experiments performed with different photon polarizations rule out polarization effects as a possible cause of the low intensity near 46 13 624