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 $\mathrm{Cu}(\text{quinoxaline}){\mathrm{Br}}_{2}$. Magnetic neutron scattering is identified above an energy gap of $1.9\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$. 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 comparisons 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 ${J}_{\ensuremath{\Vert}}=2.0\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ and rung exchange ${J}_{\ensuremath{\perp}}=3.3\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$.