Raman-active phonons and their doping dependence in spin-ladder Sr14Cu24O41

Raman spectra of Sr14-xCaxCu24O41 have been investigated in a number of wel l-defined single-crystal and polycrystalline samples. From the polarization and doping dependence, and from a comparison with previous reports on isos tructural compounds, we identify the (4A(g) + 4B(1g) + 3B(2g) + B-3g) modes that are Raman allowed within the simple Fmmm structure, consisting of two Cu-O sub-lattices. With increasing Ca content, we observe the intensity de crease in the modes involving the atoms of the ladders. This effect, analog ous to the CuO2-plane phonons in the high-T-c cuprates, can be attributed t o the hole transfer from the chains to the ladders. (C) 2000 Elsevier Scien ce B.V. All rights reserved.