Average and local structures of strongly overdoped YBa2Cu3O7+x

Recent reports show that some cuprates, such as Cu0.75Mo0.25Sr2YCu2O7+x and Ba2CuO3+x, exhibit high-Tc superconductivity in the 80-90 K range in the strongly overdoped regime, for doping levels p~0.4 hole/Cu or higher, well beyond the superconducting dome, where a conventional Fermi liquid behaviour is rather expected. This surprising result, achieved by means of high-pressure oxygenation, raises the question of whether the holes provided by the extra oxygens, x, contribute to the superfluid density or rather form a phase-separated metallic phase. In order to address this point, we propose to investigate the average and local structures of a series of YBa2Cu3O7+x samples with different oxygen concentrations up to x=0.4 or larger, by means of neutron powder diffraction and pair distribution function analysis. This is required to determine the local order of the extra oxygens, which shall enable us to establish a correlation between x, electronic structure, and normal-state and superconducting properties. In order to carry out a systematic study on 3-4 samples with different x, we should request 4 days of beamtime at POLARIS, which we consider the most appropriate instrument for our purpose considering its higher count rate in the high-Q range relevant to obtain a high-resolution PDF and the limited amount of sample available.