We have investigated the transport and magnetic properties of the spin-ยฝ ladder system with two legs, Sr14_~AxCu2404~ (A = Ba and Ca). For x ~ 0, the electrical resistivity p exhibits semiconductive behavior. It becomes more insulating with the increase in x(Ba), while it becomes more conductive with the increase in x(Ca). The broad peak of the magnetic susceptibility around 80 K has been studied in detail. Subtracting the Curie component at low temperatures, the remainder of the susceptibility xs(T) decreases towards zero with decreasing temperature below ~ 80 K, which implies the presence of a spin gap. The value of xs(T) decreases with the increase in x(Ba), while it tends to increase with the increase in x(Ca). Moreover, for the samples with x = 0 annealed under various atmospheres, p tends to decrease and xs(T) increases with the decrease in oxygen content. These properties are discussed in terms of the redistribution of holes between the two different sites of Cu. It seems that the observed spin-gap behavior is due to Cu 2รท spins in the CuO2-chain rather than in the Cu203-plane.
Recently, special attention has been paid to another kind of cuprate with Cu203-planes instead of CuO2-planes, because it was theoretically predicted by Rice et al. [1] that SrCu203 synthesized under high pressure [2] can be regarded as a typical spin ladder system with two legs, that it should have a spin gap and would exhibit high-T~ superconductivity through hole doping. The presence of a spin gap in SrCu20 3 was confirmed by Azuma et al. [3], but hole doping in SrCu20 3 has not yet been achieved. The crystal structure of Sr14Cu24041 is composed of the stack of layers represented as .-.