17O NMR study of the local charge state in the hole-doped Cu2O3 two-leg spin-ladder A14Cu24O41 (A14=La6Ca8, Sr14, Sr11Ca3, Sr6Ca8)

NMR measurements of the ¹⁷O nuclear quadrupole interactions ¹⁷vQ show that the charge environment of the S= 1/2 Cu2O3 two-leg spin-ladder layer of hole-doped A14Cu24O41 changes dramatically above a certain temperature T*. This temperature T* decreases with additional doping and is correlated with the magnetic crossover from the spin gap regime to the paramagnetic regime. We demonstrate that these changes in ¹⁷vQ are consistent with an increase in the effective hole concentration in the Cu2O3 two-leg ladder above T*. The effective hole concentration increases primarily in the oxygen 2p σ orbitals.