Abstract
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.
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