Metal-insulator transition and spin gap in the spin-1/2 ladder system Sr_14-xA_xCu₂₄O₄₁ (A = Ba and Ca)

We have investigated the transport and magnetic properties of the spin-1/2 ladder system with two legs Sr_14-xA_xCu₂₄O₄₁ (A = Ba and Ca). For x = 0, both electrical resistivity and thermoelectric power exhibit semiconductive behavior. It becomes more insulating with the increase in x(Ba), while it becomes more conductive with the increase in x(Ca). A metal-insulator transition is found at x(Ca) = 6.0-8.4 from the thermoelectric power measurements. The magnetic susceptibility exhibits a broad peak around 80 K for x = 0. Subtracting the Curie component at low temperatures, the remainder of the susceptibility χ_s(T) decreases toward zero with decreasing temperature below ∼ 80 K, which implies the presence of a spin gap. The temperature at which the broad peak shows the maximum changes little through the partial substitution of Ba and Ca for Sr. However, the value of χ_s(T) decreases with the increase in x(Ba), while it tends to increase with the increase in x(Ca). These properties are discussed in terms of the redistribution of holes between the two different sites of Cu. It appears that the observed spin gap behavior is due to Cu²⁺ spins in the CuO₂ chain rather than in the Cu₂O₃ plane.