Spin gap of the one-dimensional CuO₂ chains in Sr_14-yY_yCu₂₄O_41-δ

We have measured the magnetic susceptibility X of Sr_14-yY_yCu₂₄O_41-δ consisting of two-dimensional Cu₂O₃ planes, namely, so-called spin-1/2 ladders with two legs and also of one-dimensional CuO₂ chains. The susceptibility X_chain(T) due to the CuO₂ chains, obtained by subtracting the susceptibility X_ladder(T) due to the Cu₂O₃ planes from the experimental data of X(T), is described by the Curie-Weiss law at T > T_max ∼ 80 K, where X exhibits the maximum for y = 0. With increasing y, X_chain becomes large owing to the increase of Cu₂₊ spins introduced into the CuO₂ chains. Moreover, the antiferromagnetic interaction between Cu₂₊ spins in the CuO₂ chains becomes weak with the increase in y, namely, with the increase of Cu₂₊ spins introduced into the CuO₂ chains, leading to suppression of the spin gap of the CuO₂ chains. As the result, the broad peak of X, observed around 80 K for y = 0, is found to disappear for y ≥ 1.0.