Direct Observation of Exchange Splittings in ${\mathrm{Cs}}_3$${\mathrm{Tb}}_2$${\mathrm{Br}}_9$ by Neutron Spectroscopy

The inelastic-neutron-scattering technique was used to measure the magnetic excitation spectrum of ${\mathrm{Tb}}^{3+}$ dimers in ${\mathrm{Cs}}_3$ ${\mathrm{Tb}}_2$ ${\mathrm{Br}}_9$. Between 2 and 3 meV four inelastic transitions are observed which exhibit a different behavior of intensity as a function of momentum transfer. This allows an unambiguous identification of the four types of dimer excitations and, in turn, a direct determination of both the nature and the magnitude of the exchange interaction: It is isotropic with $\mathcal{J}=-0.0049\mathrm{}$ meV.