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Angle-resolved photoemission spectroscopy studies of Tl₂Ba₂CuO₆+δ and YBa₂Cu₃O₇-δ : analysis of recent results and the construction of a new system Mottershead, Jeffrey Daniel


Recent angle-resolved photoemission spectroscopy (ARPES) results, from experiments performed at the Swiss Light Source, Stanford Synchrotron Radiation Lightsource, and Advanced Light Source synchrotrons, on the high-temperature superconductors Tl₂Ba₂CuO₆+δ (Tl2201) and YBa₂Cu₃O (YBCO₆.₅) are presented. An overdoped Tl2201 sample with a TC of 30K was found to have a Fermi surface, consisting of a large hole pocket centred at (Pi ,Pi ), which is approaching a topological transition. A superconducting gap consistent with a dχ₂−y₂ order parameter was detected. In contrast with the underdoped HTSCs, where the quasiparticle (QP) linewidth at the top of the band is maximal in the antinodal direction and minimal in the nodal direction, overdoped Tl2201 was revealed to have a reverse nodal-antinodal anisotropy, with sharp QP peaks in the antinodal region and broader peaks in the nodal region. The Tl2201 results establish Tl2201 as a valuable material for exploring the overdoped side of the phase diagram with ARPES. Synchrotron experiments also yielded the first successful ARPES results on underdoped YBa₂Cu₃O₇−δ (YBCO). Surface-sensitive techniques were previously unsuccessful in studying YBCO because its cleaved surfaces are polar, resulting in an overdoped surface regardless of the doping level of the bulk. By doping the cleaved surfaces with potassium, the surface was progressively hole underdoped from as-cleaved continuously to the doping of the bulk, and subsequent ARPES experiments performed revealed a transition from a holelike Fermi surface on the as-cleaved surface to disconnected Fermi arcs when the surface doping matched the bulk. In parallel with the synchrotron-based research, an in-house ARPES system was constructed at the University of British Columbia (UBC). Unlike conventional ARPES systems, the ARPES setup at UBC incorporates a molecular beam epitaxy (MBE) system, which allows novel materials to be grown, characterized, and transferred to the ARPES chamber in vacuo. Novel design techniques to improve the accuracy of ARPES measurements are presented. The in-house ARPES system also serves as a prototype for an ARPES–MBE endstation being constructed at the Canadian Light Source (CLS). Design studies of some potential improvements to the in-house system, to be implemented at the CLS, are also presented.

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