Neutron-scattering and susceptibility study of spin chains and spin ladders in (Sr0.8Ca0.2)(14)Cu24O41

We have performed inelastic-neutron-scattering and dc susceptibility measurements on a polycrystalline sample of (Sr0.8Ca0.2) 14Cu24O41, which has a complex crystal structure comprising alternating layers containing isolated CuO2 chains and Cu2O3 two-legged ladders. The observed excitation spectrum is consistent with a spin gap for the ladder of 35 meV. At lower energy transfers we have observed a broad, nondispersive, feature centered on 10 meV that corresponds to singlet-triplet excitations in the CuO2 chains. We see some evidence in our susceptibility and neutron-scattering measurements to suggest that portions of the CuO2 chains are dimerized, or broken up into larger finite chains by nonmagnetic CuO2 units.