Charge and spin dynamics in spin-ladder Sr14Cu24O41 investigated by Raman scattering

We report on Raman scattering study of changes in the phonon spectrum and low-energy electronic excitations for Sr14Cu24O41. From the polarization and Ca-doping dependence, and from a comparison with previous reports on isostructural compounds, we identify the Raman-allowed modes within the simple structure, consisting of two orthorhombic CuO subcells. At low temperatures, the spectra exhibit a number of normally forbidden phonons as well as additional low-energy excitations. For T < similar to 200 K, we observe a large loss of low-energy spectral weight that linked to changes in the linewidth of the 246-cm(-1) phonon. These effects, analogous to the pseudogap effect in the high-T-c cuprates, can be correlated with recently reported resistivity experiments that show the pair formations in the ladders for T < similar to 200 K.