R. J. Kelley

Temperature Dependence of the Superconducting Gap Anisotropy in Bi2Sr2CaCu2O8+x

Detailed data on the momentum-resolved temperature dependence of the superconducting gap of Bi2Sr2CaCu2O8+x are presented, complemented by similar data on the intensity of the photoemission superconducting condensate spectral area. The gap anisotropy between the Γ-Mand Γ-X directions increases markedly with increasing temperature, contrary to what happens for conventional anisotropic-gap superconductors, such as lead. Specifically, the size of the superconducting gap along the Γ-X direction decreases to values indistinguishable from zero at temperatures for which the gap retains virtually full value along the Γ-M direction. These data rule out the simplest type of d-wave order parameter.

Observation of a van Hove singularity in Bi2Sr2CaCu2O8+x with angle-resolved photoemission.

Keywords: HIGH-TC SUPERCONDUCTORS ; DENSITY-OF-STATES ; FERMI-SURFACE ; ELECTRONIC-STRUCTURE ; TEMPERATURE Note: Univ wisconsin,ctr appl superconduct,madison,wi 53706. ecole polytech fed lausanne,inst phys appl,ch-1015 lausanne,switzerland. Ma, j, univ wisconsin,dept phys,1150 univ ave,madison,wi 53706.ISI Document Delivery No.: QH079 Reference LSE-ARTICLE-1995-019View record in Web of Science Record created on 2006-10-03, modified on 2017-05-12

Electromagnetic coupling character of [001] twist boundaries in sintered Bi2Sr2CaCu2O8+x bicrystals

Abstract The electromagnetic characteristics of [001] twist grain boundaries in Bi 2 Sr 2 CaCu 2 O 8+ x (BSCCO-2212) have been deduced from measurements of the resistive transitions, voltage-current ( V - I ) characteristics, and field-dependent transport critical current densities ( J ct ( H )) of single- and bicrystalline samples. The bicrystals were prepared by solid-state sintering of (001) faces of freshly-cleaved, bulk-scale single crystals placed at a pre-determined misorientation angle, θ. A low-angle 2° bicrystal was strongly coupled with V - I and J ct ( H ) characteristics that were indistinguishable from those of the single crystals, indicating that the sintering process itself does not produce weak coupling. 30° and 36° [001] bicrystals were weakly coupled. 23° and 88° twist bicrystals were strongly coupled, even though their boundary T c values were reduced. This behavior contrasts starkly with that of [001] tilt boundaries in YBa 2 Cu 3 O 7 − δ , where a reduced boundary T c is always associated with weak coupling.

Advances in single-crystal Bi2Sr2CaCu2O8+δ superconductors

Abstract Susceptibility, resistivity and X-ray diffraction measurements have been performed on single crystals of Bi2Sr2CaCu2O8+δ. Very sharp (0.5 K width) superconducting transition temperatures as low as 55 K have been reached in the overdoped part of the phase diagram. Anneals in low oxygen pressures have allowed us to access the entire undordoped region of the phase diagram. The samples have been reversibly changed from a high-temperature superconductor to an insulator.

Doping-induced change of superconducting gap anisotropy in Bi2Sr2Ca1Cu2O8+δ

High-resolution angle-resolved photoemission measurements were performed on single crystals of Bi2Sr2Ca1Cu2O8+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.

Superfluid and normal fluid density in high-Tc superconductors

The electronic properties of the cuprate superconductors have been studied by measuring the reflectance over the frequency range from the far-infrared to the near-ultraviolet(roughly, 10 meV-5 eV). There is an interesting behavior in both the normal state and the superconducting state. In the normal state, there is the well-known non-Drude distribution of the doping-induced spectral weight. In the superconducting state, the spectral weight of the superconducting condensate correlates with Tc in a variety of materials. Remarkably, in optimally doped superconductors, only about 20% of the doping-induced carriers joins the superfluid; the rest of the spectral weight remains at finite frequencies.

Pb0.4bi1.6sr2cacu2o8+X and Oxygen Stoichiometry - Structure, Resistivity, Fermi-Surface Topology, and Normal-State Properties

Keywords: ANGLE-RESOLVED-PHOTOEMISSION ; CU-O SUPERCONDUCTORS ; ELECTRONIC-STRUCTURE ; MODULATED STRUCTURE ; CONVERGENT-BEAM ; BAND-STRUCTURE ; BI2SR2CACU2O8 ; PB ; YBA2CU3O7-DELTA ; YBA2CU3O6.9 Note: Univ wisconsin,ctr appl superconduct,madison,wi 53706. ecole polytech fed lausanne,inst phys appl,ch-1015 lausanne,switzerland. Ma, ja, univ wisconsin,dept phys,1150 univ ave,madison,wi 53706.ISI Document Delivery No.: QT246 Reference LSE-ARTICLE-1995-022View record in Web of Science Record created on 2006-10-03, modified on 2017-05-12

INTERLAYER COUPLING AND THE METAL-INSULATOR TRANSITION IN PR-SUBSTITUTED BI2SR2CACU2O8+Y

Substitution of rare-earth ions for Ca in Bi2Sr2CaCu2O8+y is known to cause a metal-insulator transition. Using resonant photoemission we study how this chemical substitution affects the electronic structure of the material. For the partial Cu-density of states at E_F and in the region of the valence band we observe no significant difference between a pure superconducting sample and an insulating sample with 60% Pr for Ca. This suggests that the states responsible for superconductivity are predomi- nately O-states. The partial Pr-4f density of states was extracted utilizing the Super- Koster-Kronig Pr 4d-4f resonance. It consists of a single peak at 1.36eV binding energy. The peak shows a strongly assymetric Doniach-Sunjic line- shape indicating the presence of a continuum of electronic states with sharp cut off at E_F even in this insulating sample. This finding excludes a bandgap in the insulating sample and supports the existance of a mobility gap caused by spatial localization of the carriers. The presence of such carriers at the Pr-site, between the CuO_2 planes shows that the electronic structure is not purely 2-dimensional but that there is a finite interlayer coupling. The resonance enhancement of the photoemission cross section, at the Pr-4d threshold, was studied for the Pr-4f and for Cu-states. Both the Pr-4f and the Cu-states show a Fano-like resonance. This resonance of Cu-states with Pr-states is another indication of coupling between the the Pr-states and those in the CuO_2 plane. Because of the statistical distribution of the Pr-ions this coupling leads to a non-periodic potential for the states in the CuO_2 plane which can lead to localization and thus to the observed metal-insulator transition.

The symmetry of the order parameter in highly overdoped Bi2Sr2CaCu2O8+x

We report results of an angle-resolved high-resolution photoemission study of strongly overdoped Bi2Sr2CaCu2O8+x single crystals with T-c similar to 60 K. We find a nonzero superconducting (SC) gap along all three high symmetry directions in the Brillouin zone, in contrast with a d-wave scenario of high temperature superconductivity. Our data indicate that both the maximum gap value and the gap anisotropy decrease with overdoping

Theab-plane optical conductivity of high-T c superconductors

There is anisotropy in theab-plane optical properties of the high-temperature superconductors, both in the normal state and in the superconducting state. In both states, two components appear in the optical conductivity: a free carrier part and a “midinfrared” component. BelowTc, the free carriers form the superconducting condensate. In YBa2Cu3O7−δ, the anisotropy of the penetration depth shows that the chains contribute strongly to this superfluid. In Bi2Sr2CaCu2O8, where chains are absent, there is stillab plane anisotropy. BelowTc a finite absorption parallelb remains at frequencies as small as 20 meV. This anisotropy could be due to anisotropy either of the superconducting gap or the midinfrared component.