Coexistence ofS=1/2Antiferromagnetic Chains and Dimers on Hole-Doped CuO2Chains in Ca1-xCuO2

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Abstract
Quasi-one-dimensional cupric oxide Ca 1- x CuO 2+δ , comprising 25–50% hole-doped edge-sharing CuO 2 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.