Agenda

Résumé : Recent reports have given growing evidence that bulk superconductivity unexpectedly persists in the strongly overdoped region of the phase diagram of cuprates at hole concentrations as high as p = 0.5 hole/Cu, well beyond the superconducting dome widely accepted as a universal property of cuprates. Such an extended superconducting region has been hitherto reported in four systems, YBa2Cu3O7+x (YBCO) [1-2], Cu1-yMoySr2YCu2O7+x [3-5], BaCuO4-y [6] and La2-xCaCuO4 [7]. Notably, in YBCO and Cu1-yMoySr2YCu2O7+x, where overdoping is achieved by means of high-pressure oxygenation methods, various groups have found that Tc does not change appreciably with x up to 0.4 [1,28], which is explained by a x-independent superfluid density measured by muon-spin-relaxation spectroscopy [5].
In order to reconcile the above results with the scenario of superconducting dome, an electronic phase separation between superconducting and normal carriers in strongly overdoped cuprates has been invoked, as suggested by muon-spin-relaxation spectroscopy [9,5], specific heat [10] and optical conductivity data [11]. This picture is consistent with the proposal of two-band model where both Cu dx2-y2 and dz2 [11,12] bands are partially occupied owing to a record short apical oxygen distance [3-5].
Here we present and discuss very recent results obtained on high-quality single-phase YBCO powder samples by means of neutron diffraction, magnetization and specific heat measurements that rule out the existence of normal carriers in the strongly overdoped region [3]. This prompts us to re-examine the phenomenology of strongly overdoped cuprates in the light of a multi-orbital scenario, which would require a challenging study of the structural and electronic properties on single crystals.

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