The superconducting critical temperature Tc of YBa2Cu3Ox single crystals (6.35 7) has been measured resistively as a function of pressure P (0P 20 kbar). The initial rate dTc/dP exhibits three distinct regimes, a narrow plateau near 7 with dTc/dP 0.04 K/kbar, a plateau in the range 6.4 6.8, where dTc/dP 0.43 K/kbar, and a maximum value 0.8 K/kbar at 6.35. An analysis of the Tc(x,P) data using a phenomenological model yields a pronounced peak near x=6.8 in dnh(x)/dP, where nh(x) is the change in the hole density in the CuO2 planes relative to the value corresponding to the fully oxygenated sample. © 1992 The American Physical Society.

We have performed angle-integrated and angle-resolved photoemission measurements at 20 K on well-characterized single crystals of high Tc cuprates (both 1:2:3-type and 2:2:1:2-type) cleaved in situ, and find a relatively large, resolution limited Fermi edge which shows large amplitude variations with photon energy, indicative of band structure final state effects. The lineshape of the spectra of the 1:2:3 materials as a function of photon energy are well reproduced by band structure predictions, indicating a correct mix of 2p and 3d orbitals in the calculations, while the energy positions of the peaks agree with calculated bands to within approx;;0.5 eV. This may yet prove to reflect the effects of Coulomb correlation. We nevertheless conclude that a Fermi liquid approach to conductivity is appropriate. Angle-resolved data, while still incomplete, suggest agreement with the Fermi surface predicted by the LDA calculations. A BCS-like energy gap is observed in the 2:2:1:2 materials, whose magnitude is twice the weak coupling BCS value (i.e., 2Δ approx;; 7kTc). © 1990 IOP Publishing Ltd.

We have performed angle-integrated photoemission measurements at 20 K on well-oxygenated (Tc=92 K) single crystals of YBa2Cu3O6.9 cleaved n situ, and find a relatively large, resolution-limited Fermi edge which shows large amplitude variations with photon energy, indicative of band-structure final-state effects. Some dispersion is seen even in our angle-integrated measurements. Our best estimate of N(EF) per Cu atom is that it is about 20% that of Cu metal with about a 20-80 mix of Cu 3d and O 2p orbitals. Dispersive and final-state effects are seen throughout the valence bands. The line shapes of the spectra as a function of photon energy are very well reproduced by band-structure predictions, indicating a correct mix of 2p and 3d orbitals in the calculations, while the energy positions of the peak agree with calculated bands to within 0.5 eV. We conclude that a Fermi-liquid approach to conductivity is appropriate. © 1989 The American Physical Society.

The first low-temperature photoemission spectra from well-oxygenated and characterized, cleaved single crystals of the 1:2:3-type superconductors, specifically EuBa2Cu3O7-x, are presented. In contrast with polycrystalline or higher-temperature single-crystalline studies, a distinct and very stable density of states is found at the Fermi edge below 20 K. As the crystal is warmed even to 80 K the emission is rapidly lost from both the upper and lower portions of the Cu-O hybrids. At room temperature there is a loss of intensity at the Fermi edge with an accompanying change in the Ba 4d core-level line shape and the presence of a charging effect indicative of an insulating surface. These results imply that upon warming from 20 K there is very rapid oxygen loss from the cleaved surface with a resulting loss of superconducting behavior. This suggests that future photoemission experiments, especially those investigating detailed band structure, should be performed at low temperatures. © 1988 The American Physical Society.