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Abstract:
The nature of the metallic phase in the high- transition- temperature (
high-T(c)) copper oxide superconductors, and its evolution with carrier
concentration, has been a long-standing mystery(1). A central question
is how coherent electronic states, or quasiparticles, emerge from the
antiferromagnetic insulator with doping. Recent quantum oscillation
experiments on lightly doped copper oxides have shown evidence for small
pockets of Fermi surface(2-5), the formation of which has been
associated with the opening of the pseudogap - ananisotropic gap in the
normal state excitation spectrum of unknown origin(1). As the doping is
increased, experiments suggest that the full Fermi surface is
restored(6,7), although the doping level at which the pseudogap closes
and the nature of the electronic ground state beyond this point have yet
to be determined. Here we report the observation of quantum oscillations
in the over-doped superconductor Tl(2)Ba(2)CuO(6+delta) that show the
existence of a large Fermi surface of well- defined quasiparticles
covering two-thirds of the Brillouin zone. These measurements confirm
that, in overdoped superconducting copper oxides, coherence is
established at all Fermi wavevectors, even near the zone boundary where
the pseudogap is maximal and electronic interactions are strongest; they
also firmly establish the applicability of a generalized Fermi-liquid
picture on the overdoped side of the superconducting phase diagram.
