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The possible occurrence of multi-phase behavior in the high T sub c superconductor YBa sub 2 Cu sub 3 O sub x
We report here the results obtained for powder diffraction studies on four YBa{sub 2}Cu{sub 3}O{sub x} samples in which a model with two orthorhombic phases was used to obtain an improved agreement. The implications of this structural model on the systematic variation of {Tc} with oxygen content will be discussed. 28 refs., 3 figs., 2 tabs
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Normal state electronic structure and the superconducting energy gap in HTSC's as determined from photoemission spectroscopy
Photoemission spectroscopy has been utilized to determine the electronic structure of high-T{sub c} materials. The observation of dispersive bands at E{sub F} suggests a Fermi surface similar to that obtained from a band calculation. The results apparently are not inconsistent with the notion of a correlated Fermi liquid consisting of hybridized p-d bands. However, it is becoming more and more difficult to distinguish between Fermi liquid behavior in the new high-T{sub c} superconductors and behavior expected on the basis of the novel new non-Fermi liquid theories. The differences are now predicted to be on an energy scale smaller than our experimental resolution. We point out that, while deviations from simple band theory certainly do exist in the form of core and valence band satellites, band narrowing, and rapid photoemission peak broadening away from E{sub F}, there are sufficient agreements with the overall DOS that it should be considered a good starting point for the electronic structure. For example, the calculated Fermi surface for both the 123 and 2212 structures is reasonably well reproduced experimentally and the bands at E{sub F} consist of p-d hybridized orbitals just as predicted by local density functional theory. Our spectra clearly show that a BCS-like DOS is obtained at the Fermi energy as a gap opens up below T{sub c}. This is just one more indication that the old conventional models should be considered more seriously. 18 refs., 6 figs