Electronic states of doped spin ladders (Sr,Ca)(14)Cu24O41

We examine the electronic states of Sr14-xCaxCu24O41 by the ionic and cluster model approach. It is found that self-doped bores are likely to stay on the chain at x = 0 and Ca substitution drives the holes to move to the ladder. This feature is caused by changes of the positions of (Sr,Ca) layers, which results in enhancement of the electrostatic potentials in the chain. The optical conductivity is calculated with application of the exact diagonalization method to small Cu-O clusters. It is shown that the excitations below 2 eV are governed by the ladder, giving rise to the spectral weight transfer from the charge-transfer excitation at around 2 eV to low energy Drude excitation, while the contribution from the chain mainly emerges in a higher energy region showing large spectral weight at around 3 eV. These findings provide a consistent explanation for the results of a recent experiment.