In the past decade ARPES has emerged
as one of the leading tools in the study of the high-Tc cuprate superconductors
[1]. However, while
ARPES has provided us with crucial insights into the complex electronic
structure of these materials, no conclusive agreement has been reached
yet on the interpretation of some of the most fundamental results.
This is partly due to the fact that among all cuprates most of the
available ARPES data have been obtained on a limited set of systems,
such as La2-xSrxCuO4 (LSCO) and the Bi-based cuprates, the study of
which is complicated by several material issues such as intrinsic chemical
disorder, lattice distortions and/or charge instabilities. In this
context the Tl-based cuprates, and in particular the single CuO2 plane
compound Tl2Ba2CuO6+d (Tl2201), would be
ideal systems to study by ARPES and would probably provide the best
opportunity to investigate the intrinsic normal state and superconducting
properties of the `doped CuO2 plane`. In fact, Tl2201 has a well ordered
crystal structure with very flat CuO2 planes without any superstructure
modulations (contrary to the Bi-cuprates). In turn, this system has
a very high Tc=91K, as compared to the 34 K observed in Bi2201 and
LSCO, and can be studied by ARPES well into the superconducting regime.
Furthermore, Tl2201 can be synthesized over a wide doping range extending
from the optimally doped to the very overdoped regime, thus offering
an alternative approach to superconductivity from the overdoped side
of the phase diagram, where there are important hints of Fermi Liquid
behavior.
Unfortunately until now the study of
Tl2201 by ARPES has been 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. As a consequence,
previous attempts to probe the low energy electronic structure by ARPES
have been hampered by the non stoichiometry and high impurity concentration
of the available samples. Very recently, however, these difficulties
have been successfully dealt with and single crystals of unprecedented
quality were obtained at UBC in the Superconductivity group led by
D.A. Bonn and W.N. Hardy [2]. The exceptional quality of these crystals
is evidenced by the superconducting transitions nearly an order of
magnitude narrower than that reported before for this compound (Fig.1),
and by the fact that these are the first orthorhombic single crystals
ever obtained of Tl2201 (orthorhombicity is associated with lower cation
disorder). Investigating the electronic structure of these superior
crystals by high resolution ARPES experiments would provide us with
a unique opportunity to challenge the mystery of high-Tc superconductivity.