Antiferromagnetic Ordering in the Spin Singlet State of the Ladder/Chain Material: Sr_2.5Ca_11.5Cu₂₄O₄₁

We report on specific heat and neutron scattering measurements performed for Sri_14-xCa_xCu₂₄O₄₁ single crystals. The specific heat data of Sr_2.5Ca_11.5Cu₂₄O₄₁ showed a sharp phase transition at T_N≈2.1K, indicating the possible onset of a long-range order. Specific heat measurements under magnetic fields also revealed that this phase transition is not due to charge ordering or structural phase transition but to magnetic ordering. In the neutron scattering experiments, we observed several Bragg peaks, corresponding to the magnetic ordering observed in the specific heat measurements. Furthermore, we confirmed from polarized neutron scattering measurements that these Bragg peaks are undoubtedly magnetic in origin. Considering both the nuclear magnetic resonance and neutron scattering data, a possible magnetic structure on the chain and ladder sites has been proposed. In this system, the singlet state and antiferromagnetic order induced by appropriate hole-doping coexist at ambient pressure and the dissolved hole pairs move under high pressure, leading to the occurrence of superconductivity.