17O NMR Study of the Local Charge State in the Hole Doped Cu2O3 Two-Leg   Spin-Ladder A14Cu24O41 (A=La6Ca8, Sr14, Sr11Ca3, Sr6Ca8)
  NMR measurements of the ^{17}O nuclear quadrupole interactions, ^{17}\nu_{Q}, show that the charge environment of the S=1/2 Cu_{2}O_{3} two-leg spin-ladder layer of hole-doped A_{14}Cu_{24}O_{41} 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 ^{17}\nu_{Q} are consistent with an increase in the effective hole concentration in the Cu_{2}O_{3} two-leg ladder above T*. The effective hole concentration increases primarily in the oxygen 2p\sigma orbitals. 
K. R. Thurber, K. M. Shen, A. W. Hunt, T. Imai, F. C. Chou