Neutron-scattering and susceptibility study of spin chains and spin ladders in ${\left({\mathrm{Sr}}_{0.8}{\mathrm{Ca}}_{0.2}\right)}_{14}$${\mathrm{Cu}}_{24}$${\mathrm{O}}_{41}$

We have performed inelastic-neutron-scattering and dc susceptibility measurements on a polycrystalline sample of ${\left({\mathrm{Sr}}_{0.8}{\mathrm{Ca}}_{0.2}\right)}_{14}$ ${\mathrm{Cu}}_{24}$ ${\mathrm{O}}_{41}$, which has a complex crystal structure comprising alternating layers containing isolated Cu${\mathrm{O}}_2$ chains and ${\mathrm{Cu}}_2$ ${\mathrm{O}}_3$ 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 Cu${\mathrm{O}}_2$ chains. We see some evidence in our susceptibility and neutron-scattering measurements to suggest that portions of the Cu${\mathrm{O}}_2$ chains are dimerized, or broken up into larger finite chains by nonmagnetic Cu${\mathrm{O}}_2$ units.