Interaction between the chain and ladder subsystems in (Ca,Sr,La) ₁₄Cu₂₄O₄₁ compounds

We investigate the influence of structural modulations on the electronic properties of incommensurate (Ca,Sr,La)₁₄Cu₂₄O ₄₁ compounds by band structure calculations based on density functional theory and the local density approximation (LDA). Using a supercell approach with ten CuO₂ chain and seven Cu₂O₃ ladder segments, we take into account the major effects of the structural incommensurability. We find that the LDA electronic states show very little response to these modulations. The coupling between the electronic and structural degrees of freedom, hence, can be well described in terms of two independent subsystems. The incommensurate charge density waves (CDWs) in the chains and the ladders are formed independently of each other.