Distribution of electrons and holes in cuprate superconductors as determined from ¹⁷O and ⁶³Cu nuclear magnetic resonance

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The distribution of electrons and holes in the CuO₂ plane of the high-temperature superconducting cuprates is determined with nuclear magnetic resonance through the quadrupole splittings of ¹⁷O and ⁶³Cu. Based on new data for single crystals of electron-doped Pr₂₋ₓCeₓCuO₄(x=0, 0.05, 0.10, 0.15) as well as Nd₂₋ₓCeₓCuO₄ (x=0, 0.13) the changes in hole contents nd of Cu 3d(x²-y²) and nₚ of O 2pσ orbitals are determined and they account for the stoichiometrically doped charges, similar to hole-doped. It emerges that while nd+2nₚ=1 in all parent materials as expected, nd and nₚ vary substantially between different groups of materials. Doping holes increases predominantly nₚ, but also nd. To the contrary, doping electrons predominantly decreases nd and only slightly nₚ. However, nₚ for the electron doped systems is higher than that in hole doped La₁.₈₅Sr₀.₁₅CuO₄. Cuprates with the highest maximum T cs appear to have a comparably low nd while, at the same time, nₚ is very high. The rather high oxygen hole content of the Pr₂CuO₄ and Nd₂CuO₄ with the low nd seems to make them ideal candidates for hole doping to obtain the highest T c.