Chapter 201 Angle-resolved photoemission studies of untwinned yttrium barium copper oxide

In summary, extensive studies of the dispersive quasiparticle states in optimally doped untwinned single crystals of Y123 using polarization-dependent angle-resolved photoelectron spectroscopy have illuminated a number of points regarding the electronic structure of this material. Based on analysis of the Ba 5p core level and inference from other surface-sensitive probes, the observed surface-to-bulk intensity ratio can be reproduced with a simple model incorporating a mixed surface termination of BaO and CuO₃ regions. Furthermore, with an electron attenuation length in this material of approximately 5, the bulk of the photoemission intensity arises from the surface and first subsurface CuO₂ planes, with significant attenuation of the signal from the bulk chains. Extensive arguments, based on photon polarization dependence, photon energy dependence, intensity variations between twinned, untwinned, and Y124 crystals, and oxygen, Co, and Pr doping results, favor attribution of the intense, narrow feature found near the Y point to a surface-related chain state. This feature, which appears at both X and Y in twinned crystals, is extrinsic to the bulk electronic states of Y123, and contaminates data in the crucial near-E_F region in crystals which are not exceptionally well detwinned. The measured band dispersions clearly reveal two plane-derived quasiparticle states, one with a binding energy of 0.22 eV at Γ, and the other with 0.53 eV, which are associated with the antibonding and bonding bilayer bands. The former disperses upward to form a van Hove singularity with a binding energy of 0.13 eV at X, while the latter is weakly dispersive along ΓX, and disperses rapidly upward along XS. While no measurable c-axis dispersion is seen in the antibonding band by changing the photon energy, the bonding band shifts upward by 0.11 eV when hv is changed from 28eV to 21eV. Observation of two distinct plane states and c-axis dispersion provides clear evidence for bilayer splitting in Y123, consistent with the simplest band-theoretical arguments. Quantitative Fermi surface determination based on a momentum-space DOS analysis reveals a strongly nested inner Fermi surface pocket centered on S attributed to the CuO₂ bonding pdσ bands. The bilayer splitting of the plane bands, which is clearly resolved in polarization-dependent energy-distribution curves appears as a more weakly resolved outer sheet in the k-space data. Vestiges of the principal chain sheet are also weakly apparent in some of the data, but are significantly suppressed, consistent with a strongly modified surface chain termination. The data show no sign of a "stick" Fermi surface centered at S, consistent with previous measurements but disagreeing with LMTO calculations. Evidence for antiferromagnetic shadow Fermi surfaces appears to be lacking in optimally doped Y123, supporting a structural origin for such features seen in Bi2212. Finally, the presence of leading-edge shifts with a k-dependence characteristic of pure d-wave order parameter untwinned Y123 has been observed. These shifts cannot be conclusively associated with the onset of the superconducting transition due to the surface instability of this material above T_c, but are consistent with a highly anisotropic superconducting gap of d-wave form. The lack of phase information makes it difficult to exclude the possibility of symmetries such as d + id or highly anisotropic s-wave mimicking the d-wave form.