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.


[1] A. Damascelli et al., Rev. Mod. Phys. 75, 473 (2003).
[2] D.C. Peets et al., cond-mat/0211028.