Pressure dependence of Tc and charge transfer in YBa2Cu3Ox (6.35 <= x <= 7) single crystals.

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

Pressure dependence of Tc and charge transfer in YBa2Cu3Ox (6.35 <= x <= 7) single crystals.

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

Vortex dynamics differences due to twin-boundary pinning anisotropy in YBa2Cu3Ox at low temperatures for H parallel to ab planes

We measured the magnetization M of a twin-aligned single crystal of YBa2Cu3Ox (YBaCuO), with T-c=91 K, as a function of temperature T and magnetic field H, with H applied along the ab planes. Isothermal M-vs-H and M-vs-time curves were obtained with H applied parallel (parallel to) and perpendicular (perpendicular to) to the twin boundary (TB) direction. M-vs-H curves exhibited two minima below 38 K, which resembled similar curves that have been obtained in YBaCuO for Hparallel toc axis. Above 12 K, the field positions of the minima for Hparallel toTB and Hperpendicular toTB were quite similar. Below 12 K, the position of the second minimum H-min occurred at a higher field value with Hparallel toTB. Below 6 K, only one minimum appeared for both field directions. At low temperatures, these minima in the M-vs-H curves produced maxima in the critical current. It was determined that vortex lines were expelled more easily for Hparallel toTB than for Hperpendicular toTB and, therefore, below a certain field value, that J(c)(Hperpendicular toTB) was larger than J(c)(Hparallel toTB). At T<12 K with Hparallel toTB, the relaxation rate for flux lines leaving the crystal was found to be different from that for flux entering the crystal. We also observed flux jumps at low temperatures, with their sizes depending on the orientation of magnetic field with respect to the TB's.71

Photoemission from single crystals of EuBa2Cu3O7-x cleaved below 20 K: Temperature-dependent oxygen loss.

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

Photoemission from single crystals of EuBa2Cu3O7-x cleaved below 20 K: Temperature-dependent oxygen loss.

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

Large, dispersive photoelectron Fermi edge and the electronic structure of YBa2Cu3O6.9 single crystals measured at 20 K.

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

A Fermi Liquid Electronic Structure and the Nature of the Carriers in High-Tc Cuprates: A Photoemission Study

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