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 17νQ 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 17νQ 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 2pσ orbitals.