Spin Dynamics of the Spin-Ladder Dimer-Chain MaterialSr14Cu24O41

We have performed inelastic neutron scattering on a single crystal sample of ${\mathrm{Sr}}_{14}{\mathrm{Cu}}_{24}{\mathrm{O}}_{41}$ to study the spin dynamics of the ${\mathrm{Cu}}_2{\mathrm{O}}_3$ spin ladder layers and ${\mathrm{CuO}}_2$ chains. Our data are best fitted with a dispersion which has a spin gap of $32.5\pm 0.1\mathrm{meV}$ and a maximum of $193.5\pm 2.4\mathrm{meV}$, consistent with a coupling along the ladders, $J_{\parallel }=130\mathrm{meV}$ and a rung coupling $J_{\perp }=72\mathrm{}\mathrm{meV}$. Excitations with an energy transfer of approximately 11.5 meV arise from a dimer chain with an antiferromagnetic intradimer coupling $J_1=11.2\mathrm{}\mathrm{meV}$ between next-nearest-neighbor Cu ions, and a ferromagnetic interdimer coupling $J_2=-1.1\mathrm{}\mathrm{meV}$. The dimer chains have a periodicity of five units.