Nuclear Spin Relaxation in Hole-Doped Two-Leg Ladders

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The nuclear spin-lattice relaxation rate (1/T₁) has been measured in the single crystals of the hole-doped two-leg ladder compound Sr_14-xCa_xCu₂₄O₄₁ and in the undoped parent material La₆Ca₈Cu₂₄O₄₁. Comparison of 1/T₁ at the Cu and the two distinct oxygen sites revealed that the major spectral weight of low-frequency spin fluctuations is located near q -- (π, π) for most of the temperature and doping ranges investigated. The remarkable difference in the temperature dependence of 1/T₁ for the two oxygen sites in the heavily doped x=12 sample revealed a reduction of singlet correlations between the two legs in place of growing antiferromagnetic correlations along the leg direction with increasing temperature. Such behavior is most likely caused by the dissociation of bound hole pairs.