Nuclear spin relaxation in hole-doped two-leg ladders

The nuclear spin-lattice relaxation rate (1/T-1) has been measured in the s ingle crystals of the hole-doped two-leg ladder compound Sr14-xCaxCu24O41 a nd in the undoped parent material La6Ca8Cu24O41. Comparison of 1/T-1 at the Cu and the two distinct oxygen sites revealed that the major spectral weig ht of low-frequency spin fluctuations is located near q similar to (pi, pi) for most of the temperature and doping ranges investigated. The remarkable difference in the temperature dependence of 1/T-1 for the two oxygen sites in the heavily doped x = 12 sample revealed a reduction of singlet correla tions between the two legs in place of growing antiferromagnetic correlatio ns along the leg direction with increasing temperature, Such behaviour is m ost likely caused by the dissociation of bound hole pairs.