C. Gu

Photoelectron spectroscopy study of YBa2Cu3Ox with varied oxygen stoichiometry: Possible evidence for strong coupling superconductivity

Photoelectron spectroscopy measurements are reported for samples of YBa2Cu3Ox with oxygen content varied in the range 6.2<x<6.9. For the most oxygen rich samples, a prominent cutoff at the Fermi edge is observed. The electronic density of states at the Fermi level N(EF) falls as oxygen is depleted. The superconducting transition temperature TcαtN(EF) suggest ing that a strong coupling mechanism controls the superconductivity.

Evidence from photoemission of strongly hybridized states at the Fermi level of Bi2Sr2CaCu2O8

The nature of the states at the Fermi level of the high temperature superconductors is of crucial importance since it is these states which form Cooper pairs below T c which are responsible for the superconductivity. In particular it is important to discern whether these states are derived from the Cu 3d or O 2p orbitals. Recent resonant photoemission experiments claim that there is no Cu 3d character at the Fermi edge of the Bi 2 Sr 2 CaCu 2 O 8 materials. While we observe similar resonant effects in these materials, an investigation over a wide range of photon energies, hv = 15 to 100 eV, has led us to reevaluate these results and to conclude that there is comparable O 2p and Cu 3d character at the Fermi edge. Our findings give support to strongly hybridized one band or two band descriptions of these materials.

Evidence from photoemission of comparable oxygen-2p and copper-3d character in the states at the Fermi level of YBa2Cu3O6.9

Abstract Only the states at the Fermi level of superconductors from Cooper pairs responsible for superconductivity. An understanding of the nature of these states is therefore essential for a proper description of the superconductivity. Previous resonant photoemission experiments claim that there is only O 2p character in the states at the Fermi level of the Bi 2 CaSr 2 Cu 2 O 8 superconductors. We have extended these studies to the YBa 2 Cu 3 O 7- x material employing a wider range of photon energies and conclude that there is comparable O 2p and Cu 3d character in the density of states at the Fermi level in these materials. This supports strongly hybridized models of the high temperature superconductors and casts doubt on simplified one band models.

Photoemission and electronic structure studies of YBa2Cu3Ox

Abstract High-resolution angle-resolved photoelectron spectroscopy measurements are reported for YBa2Cu3Ox when the oxygen stoichiometry x is varied between 6.9 (92 K superconductor) and 6.2 (insulator). Fermi surfaces obtained from energy distribution curve measurements on a grid scanning the entire first Brillouin zone are reported for the x = 6.9 sample. Bands along the Γ-S symmetry line were measured for samples with reduced oxygen stoichiometry. For superconducting samples, these bands are essentially independent of x. As the material becomes insulating (x

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 ≈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 ≈ 7kTc).

High resolution photoemission studies of untwinned YBa2Cu3O6.9

Abstract In this contribution we will report high energy and angle resolution photoemission studies of high quality, untwinned crystals of x=6.9 YBCO. We will emphasize results related to the role of chain derived states in the region near the Fermi surface. Valence band states near 1 eV will be discussed since these features are sensitive indicators of sample quality. In spite of the high resolution and the high quality samples, the expected large super-conducting gap is not seen. Possible reasons will be discussed.

The Acute Spectral Structure of Single-Domain YBa2Cu3O6.9

Extraordinarily sharp spectral features at binding energies near leV have been observed in the photoemission spectra of untwinned, single crystal YBa2Cu3O6.9. This is the first observation of such distinctive electronic structure away from the near-Fermi Energy regime. It suggests that the entire valence band electronic structure, not merely the Fermiology, may provide insight into the nature of high temperature superconducting cuprates.

Normal State Electronic Structure and the Superconducting Energy Gap in HTSC’s as Determined from Photoemission Spectroscopy

Photoemission spectroscopy is utilized to determine the electronic structure of high-Tc materials. The observation of dispersive bands at EF suggests a Fermi surface similar to that obtained from a band calculation. These results, together with a BCS-like energy gap, are not inconsistent with the notion of a Fermi liquid consisting of hybridized p-d bands.