Metal-insulator transition and spin gap in the spin-ladder cuprate Sr_14-xA_xCu₂₄O₄₁ (A = Ca, Ba, Y, La)

We have investigated the transport and magnetic properties of Sr_14-xA_xCu₂₄O₄₁ (m = Ca, Ba, Y, La) consisting of two-dimensional Cu₂O₃ planes, namely, so-called spin-ladders with two legs and also of one-dimensional CuO₂ chains. Sr₁₄Cu₂₄O₄₁ exhibits a semiconductive behavior. For x(Ca) ≧ 8.4, metallic behavior is observed in the thermoelectric power measurements, but superconductivity does not appear. The susceptibility exhibits a broad peak around 80 K for x = 0. With incresing x, the Curie component at low temperatures increases and the broad peak disappears. It has been concluded that the broad peak is due to spin gap of the CuO₂ chains and that the Cu²⁺ spins in the Cu₂O₃ planes have already formed spin-singlet pairs below room temperature. A model on the spin configuration in the CuO₂ chains at low temperatures is proposed, depending on the value of x.