S=1/2 antiferromagnetic chains and dimers on hole-doped edge-sharing CuO₂ chains

Holes doped on edge-sharing CuO₂ chains are almost localized because of nearly orthogonal Cu-O-Cu bonds. Consequently, only spin degrees of freedom survive at low temperatures, which sometimes presents unusual quantum magnetism. Quasi-one-dimensional cupric oxides Ca_1-xCuO_2+δ and Sr₁₄Cu₂₄O₄₁, both containing 25-50% hole-doped edge-sharing CuO₂ chains, are studied by uniform magnetic susceptibility and specific heat measurements on a series of polycrystalline samples with controlled metal and oxygen contents. Most spins are always dimerized in Sr₁₄Cu₂₄O₄₁, while, in contrast, antiferromagnetic chains made of nearly 50% spins exist with the remainder forming dimers of variable density in Ca_1-xCuO_2+δ. 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°.