Antiferromagnetism and spin gap state in the two-leg ladder system

We report here the magnetic ground state in the hole-doped two-leg ladder system Sr_2.5Ca_11.5Cu₂₄O₄₁ at ambient pressure. We clearly observed the singlet ground state, of which values are almost unchanged for the different Ca concentration (approximately 32 meV). We also observed the antiferromagnetic long-range order below T_N, of which values are between approximately 0.6 K(x = 9) and approximately 3.6 K(x = 1.36). A possible magnetic structure of this system, was also discussed. These facts can be interpreted in line with scenario that the singlet state and antiferromagnetic order coexist at ambient pressure and these localized hole pairs begin to move under pressure, resulting in the occurrence of superconductivity.