Electronic structure of spin-chain compounds: Common features

The incommensurate composite systems M ₁₄Cu ₂₄O ₄₁ (M = Ca, Sr, La) are based on two fundamental structural units: CuO ₂ chains and Cu ₂O ₃ ladders. We present electronic structure calculations within density functional theory in order to address the interrelations between chains and ladders. The calculations account for the details of the crystal structure by means of a unit cell comprising 10 chain and 7 ladder units. It turns out that chains and ladders can be treated independently, which allows us to introduce a model system based on a reduced unit cell. For the CuO ₂ chains, we find two characteristic bands at the Fermi energy. Tight binding fits yield nearest and next-nearest neighbour interactions of the same order of magnitude.