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

doi:10.1016/0921-4534(89)90330-4

Physica C: Superconductivity

Volume 158, Issues 1–2, 1 April 1989, Pages 276-280

https://doi.org/10.1016/0921-4534(89)90330-4 Get rights and content

Abstract

Photoelectron spectroscopy measurements are reported for samples of YBa₂Cu₃O_x 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(E_F) falls as oxygen is depleted. The superconducting transition temperature $T_{c} α t ̊ N(E_{F})$ suggest ing that a strong coupling mechanism controls the superconductivity.

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X-ray photoelectron spectroscopy (XPS or ESCA) has been employed to investigate the compositional behavior of YBa₂Cu₃O_x (6.3<x<6.9) or 1-2-3 superconducting systems (in both bulk-ceramic and thin-film form) at room temperature. Several questions are addressed that relate to the oxygen stability of these materials and the corresponding attributes of the resulting Fermi edges. First, it is demonstrated that subjecting these materials to ultra-high vacuum conditions extracts oxygen from these systems (i.e., reduces x). We then show that this degradation may be substantially mitigated by the by-product layer that forms on the surface of all air-exposed 1-2-3 systems. As a result we have generated the first view of the above T_c (normal conductive) finite Fermi density for 1-2-3. We then show that this density may be removed by the destructive force of a direct focus X-ray system. All of these features may be cycled back and forth merely bt alternating exposures of the 1-2-3 systems to air and vacuum.
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Studies of the valence band electronic structure of the high T_c superconductor YBa₂Cu₃O_7-δ (YBCO), obtained from photoelectron spectroscopy measurements, are reviewed. We include a brief discussion of angle integrated measurements acquired with X-ray photoemission and with ultraviolet photoemission. However, the major effort is devoted to a discussion of high resolution angle resolved photoemission spectroscopy (ARPES) studies on samples cleaved in high vacuum at cryogenic temperatures. These samples show a clear (metallic) Fermi edge cutoff, and band dispersion that is generally consistent with predictions of band theory. Mostly, electron states lying close to E_F (within about 1 eV) are examined. The highly two-dimensional nature of the YBCO structure allows for reasonably straightforward measurement of the Fermi surface; consequently the Fermi surface has been measured in some detail. Evidence, from ARPES measurements, for the existence of a van Hove singularity near the Fermi level, that might play a role in high T_c superconductivity, is considered. Systematic studies are presented that show changes in the band structure and Fermi surface as the oxygen stoichiometry is reduced; with oxygen depletion, YBCO changes from a high T_c superconductor to a magnetically ordered insulator. ARPES studies of YBCO crystals with Co and Zn atoms substituted on Cu sites are also included. When ARPES measurements are performed at room temperature, the samples show rapid degradation; consequently, room temperature measurements are not extensively reviewed here. Despite extensive studies, some issues still remain largely unresolved or controversial. The ARPES technique is surface sensitive and the cleavage plane has not been clearly identified; consequently concerns associated with measurement of the cleaved surface are addressed. Efforts, generally unsuccessful, to observe a superconducting gap, are discussed.
Alkaline earth core level photoemission spectroscopy of high temperature superconductors
1994, Journal of Electron Spectroscopy and Related Phenomena
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Intrinsic photoemission signals, surface preparation, and surface stability of high temperature superconductors
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This XPS surface study shows that it is difficult to “clean” the a-b surfaces of large tetragonal and orthorhombic YBCO single crystals by either chemical etching or argon-ion-bombardment. For both treatments a carbon contamination peak was detected and found to arise from random patches on the large surfaces. However, both chemical etching and low angle ion-bombardment resulted in minimum disruption to the surface atom termination. For the chemically etched surfaces of the YBCO crystals the O 1s emission spectra have been analysed and the origin of the component peak discussed. The pairs of peaks centred at 527.5, 528.2 eV and 527.5, 528.8 eV are considered to be intrinsic to the o- and t-structures. For 45° incidence ion-bombarded surfaces of o- and t-YBCO crystals, both show the O 1s spectrum and the Cu(L₃VV)-Auger transition to be identical and consequently the surface structures formed are considered to be identical. In fact, these surfaces appear to have Cu₂BaO₂ formed in the surface region. Valence-band and low energy core level emissions are reported for the chemically etched surfaces of both types of crystals. In the case of the tetragonal crystals the valence-band spectrum is more intense and a significant difference is observed in the shape of the band at ~3 eV compared to that obtained from orthorhombic crystals. After ion-bombardment both spectra were very similar, the valence-bands having a maximum at 3.7 eV, increased intensity relative to the Ba 5p but were almost devoid of fine structure.

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

Abstract

Physica C

Phys. Lett. A

J. Magn. Magn. Mat.

Physica C

Physica C

Phys. Lett. A

Science

Phys. Rev. B

Phys. Rev. B

Photoelectron spectroscopy study of YBa₂Cu₃O_x with varied oxygen stoichiometry: Possible evidence for strong coupling superconductivity