Electronic structure of the spin chains in (Ca,Sr)14Cu24O41

The incommensurate composite systems (Ca,Sr)14Cu24O41 are based on three largely independent subsystems, Cu2O3 ladders, CuO2 chains, and electron donor ions. We focus on the electronic properties of the chain subsystem. The crystal structure of the Ca and Sr-rich compound differ in a symmetric or asymmetric alignment of the CuO2 chains. Substitution of Sr by Ca leads to a transfer of holes from the chains to the ladders. The spin order on the chains likewise depends strongly on the doping and ranges from dimers to antiferromagnetic order. To clarify the electronic structure, we perform calculations based on density functional theory. We present systematic investigations of the local density of states at the chain copper sites and the band structure. The hybridization between the chains, ladders and electron donor ions is found to be negligible. The band structure of coupled corner-shared CuO chains resembles the band structure of the composite system. The chains are well described in terms of two characteristic electronic bands at the Fermi energy. (orig.)