Department of Physics & Astronomy, University of British Columbia, 6224 Agricultural Road, Vancouver, B.C. V6T 1Z1, Canada
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In recent years, angle-resolved photoemission spectroscopy (ARPES) on
the
high-Tc copper oxides superconductors (such as, in particular,
La2-xSrxCuO4
and the Bi-based cuprates) has provided us with crucial insights into the
complex electronic structure of these materials. On the other hand,
despite
the intense effort, no conclusive agreement on the interpretation of some
of the most fundamental issues was reached yet. In this context,
important
breakthroughs could come from the study, by ARPES, of the Tl-based
cuprates
and in particular of the single CuO2 plane compound Tl2Ba2CuO6+δ
(Tl2201).
Owing to the well ordered crystal structure (with very flat CuO2 planes far
apart from each other) and to the very high Tc,max''K, Tl2201 would be in
principle an ideal system to study by ARPES; it could in fact provide one
of the best opportunities to investigate the intrinsic properties of the
doped CuO2 plane offering, at the same time, an alternative approach to
superconductivity (i.e., from the very overdoped side of the phase
diagram). Unfortunately, until now the study of Tl2201 by ARPES was
prevented by the lack of high-quality single crystals, due to the
difficulties of the growth process and in particular to the high toxicity
and volatility of thallium oxides. Very recently, however, these
difficulties were successfully dealt with and single crystals of
unprecedented quality were produced at UBC, which gave us the
opportunity
of obtaining an extensive set of angle-resolved photoemission data from
Tl2201 for three different doping levels (ranging from slightly overdoped
to the very overdoped regime). This study has provided us with very
detailed information on the normal state Fermi surface and quasiparticle
dispersion for Tl2201. In particular, the experimentally determined Fermi
surface for a very overdoped Tl2201 sample (Tc0K) consists of a single
large hole pocket centered at the (p,p) point, which at this doping level
appears to be approaching a topological transition from hole-like to
electron-like. Thus the picture that emerges from this investigation is
that the low-energy normal-state electronic structure of Tl2201 is
dominated by a single CuO band. While a superconducting gap
consistent with
the d-wave symmetry is detected, the quasiparticle evolution with
momentum
and binding energy exhibits a marked departure from what is observed in
under and optimally doped cuprates. The relevance of these findings to
electronic scattering, many-body, and quantum-critical phenomena in the
cuprates is discussed.
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