Electrical and magnetic anomalies in the spin-ladder cuprate Sr_14-xA_xCu₂₄O₄₁(A=Ca, Y, La): Possibility of hole pairing in the ladder

We have re-investigated the electrical resistivity ρ and the magnetic susceptibility x of the spin-ladder cuprate Sr_14-xA_xCu₂₄O₄₁ (A=Ca, Y, La), consisting of two-dimensional Cu₂O₃ ladder planes and one-dimensional CuO₂ chains. The dlnρ/d(1/T) vs. T plot exhibits a broad peak around 200 K for x = 0. The peak temperature T_ρ goes down with increasing hole-concentration p in the ladder for x(Ca)> 0, while it goes up with decreasing p in the ladder for x(Y, La)> 0. The observed T_ρ is regarded as a temperature below which holes in the ladder are localized. On the other hand, the x^-1 vs. T plot exhibits inflection indicating deviation from the Curie-Weiss law around 200 K for x = 0. The inflection temperature T_x also goes down with increasing p in the ladder, and goes up with decreasing p in the ladder. The observed T_x is regarded as a temperature above which Cu²⁺ spins in the ladder notably contribute to the susceptibility. Values of T_x and its x-dependence have been found to be almost the same as those of T_ρ. This coincidence strongly suggests that holes in the ladder are paired and localized at low temperatures below T_x (and T_ρ) where almost all Cu²⁺ spins in the ladder complete to form spin-singlet pairs.