Spin dynamics of an isotropic singlet-ground-state antiferromagnet with alternating strong and weak interactions: An inelastic-neutron-scattering study of the dimer compound ${\mathrm{Cs}}_3$${\mathrm{Cr}}_2$${\mathrm{Br}}_9$

Powder and single-crystal samples of the dimer compound ${\mathrm{Cs}}_3$ ${\mathrm{Cr}}_2$ ${\mathrm{Br}}_9$ have been studied by inelastic neutron scattering. At 1.6 K, the singlet-to-triplet dimer excitation exhibits pronounced energy dispersion as a result of weak interdimer interactions. The results are interpreted within the random-phase approximation with use of a Heisenberg Hamiltonian and considering only nearest-neighbor interactions. The agreement between theory and experiment is excellent. Three exchange parameters have been obtained at 1.6 K: $J=-1.03\mathrm{}$ meV (intradimer), $J_p=-0.054\mathrm{}$ meV (interdimer, intrasublattice), and $J_c=-0.039\mathrm{}$ meV (interdimer, intersublattice). The excitation shows soft-mode behavior. However, an ordering does not occur above 1.6 K.