Neutron scattering from a coordination polymer quantum paramagnet

Inelastic neutron scattering measurements are reported for a powder sample of the spin-1/2 quantum paramagnet $\mathrm{C}\mathrm{u}(\mathrm{Q}\mathrm{u}\mathrm{i}\mathrm{n}\mathrm{o}\mathrm{x}\mathrm{a}\mathrm{l}\mathrm{i}\mathrm{n}\mathrm{e})\mathrm{B}{\mathrm{r}}_2$. Magnetic neutron scattering is identified above an energy gap of 1.9 meV. Analysis of the sharp spectral maximum at the onset indicates that the material is magnetically quasi-one-dimensional. Consideration of the wave vector dependence of the scattering and polymeric structure further identifies the material as a two-legged spin-1/2 ladder. Detailed comparison of the data to various models of magnetism in this material based on the single mode approximation and the continuous unitary transformation are presented. The latter theory provides an excellent account of the data with leg exchange $J_{\parallel }=2.0$ meV and rung exchange $J_{\perp }=3.3$ meV.