R.S. List

Superconducting Gap in Bi-Sr-Ca-Cu-O by High-Resolution Angle-Resolved Photoelectron Spectroscopy

Detailed studies indicate a superconducting gap in the high-temperature superconductor Bi2Sr2CaCu2O8. Photoemission measurements with high energy and angle resolution isolate the behavior of a single band as it crosses the Fermi level in both the normal and superconducting states, giving support to the Fermi liquid picture. The magnitude of the gap is 24 millielectron volts.

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.

High resolution-angle resolved photoemission studies of high temperature superconductors

Abstract Recent photoemission studies of Y 123 and Bi 2212 performed with high energy and angular resolution have provided detailed information on the nature of the states near the Fermi level. Measurements of the superconducting gap, band dispersion, and the density of states near the Fermi level in the normal state all support a Fermi liquid description of these materials.

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.

Photoemission measurement of Fermi surface, band dispersion and quasiparticle lifetime in YBa2Cu3O6.9

Abstract High resolution (20 meV) angle-resolved photoemission measurements on single-crystals of YBa 2 Cu 3 O 6.9 were analyzed to obtain band dispersion and quasiparticle lifetimes. The mass enhancement is 1.2 for a Cu-O plane band mass and 1.0 for the pdπ chain band. Fitting measured energy distribution curves to a model spectrum that includes self-energy corrections, we find that the imaginary part of the self-energy varies quadratically in the immediate vicinity of E F , and becomes linear above ~20 meV.

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.

The fermi surfaces and band dispersion of YBa2Cu3O6.9 as seen by angle-resolved photoemission

Abstract We have carried out angle-resolved photoemission Spectroscopy on single crystals of YBa 2 Cu 3 O 6.9 . The crystals were cleaved in situ, under ultra high vacuum while the sample stage was cooled to below 10°K. By observing the dispersion of the electron energy bands as they cross the Fermi energy, we have mapped the Fermi surfaces. There is reasonable agreement between the experimental results and the predictions of band structure calculations using the local density approximation, as well as with positron annihilation experiments.

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 temperature superconductors: Correlated Fermi liquids

Abstract Most early descriptions of the high temperature superconductors started with localized, Hubbard-like pictures both for the undoped parent materials and the superconductors [1–3]. This was due in part to the apparent failure of band structure calculations in predicting the band gaps in these materials evident in early experimental results [4–6]. More recent photoemission experiments, however, on higher quality superconducting materials are reported which indicate that these materials are not only metals with a large density of states at the Fermi edge, but also exhibit electronic structure in very good agreement with band structure calculations. The effects of correlations are evident in bands near the Fermi level and in the Cu satellites, but overall the evidence suggests the validity of a band-like, Fermi liquid approach to these materials, and that correlations do not dominate the physics of these superconductors.