Coexistence of S = 1/2 antiferromagnetic chains and dimers on hole-doped CuO₂ chains in Ca_1-xCuO₂

Quasi-one-dimensional cupric oxide Ca_1-xCuO_2+δ, comprising 25-50% hole-doped edge-sharing CuO₂ chains, is studied by uniform magnetic susceptibility and specific heat measurements on a series of polycrystalline samples with controlled metal and oxygen contents. Because the Cu-O-Cu bonds are nearly orthogonal, holes are almost localized, and only spin degrees of freedom survive at low temperature. The results reveal that antiferromagnetic chains made of 50% spins per formula unit always exist, independent of spin density, and the remainder of spins mostly form dimers of variable density. A two-sublattice model is proposed by considering that the nearest-neighbor couplings are negligibly small, due to both geometrical frustration and the special Cu-O-Cu bond angle of ~95°. Thus next-nearest-neighbor interactions dominate, and give rise to a charge-ordered state on one sublattice, which behaves as a Heisenberg antiferromagnetic chain. The rest of the spins tend to form dimers on the other sublattice with low spin density. Long-range antiferromagnetic ordering appears to occur at 12 K.