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.
Categories: cond-mat.str-el cond-mat.supr-con