Inelastic neutron scattering study of a quantum spin trimer

We present an inelastic neutron scattering (INS) study of the excitation spectrum of a quantum S=1/2 equilateral Heisenberg trimer, Cu{sub 3}(O{sub 2}C{sub 16}H{sub 23}){sub 61.2}C{sub 6}H{sub 12}. The magnetic properties of the system can be described by an ensemble of independent equilateral triangles of S=1/2 Cu{sup 2+} ions. With antiferromagnetic Heisenberg coupling, the ground state of each trimer is a degenerate pair of S=1/2 doublets, with a quartet S=3/2 excited state. Previous bulk measurements led to an estimate for the excitation energy of 28 meV. Here, we report INS measurements that can provide a direct measurement of magnetic excitation energies. These measurements are challenging since inter- and intramolecular vibrational modes associated with the organic ligands are at frequencies similar to the magnetic excitations. Measurements on a nonmagnetic compound with the same ligands as well as the temperature dependence of the neutron scattering cross section are used to identify the vibrational modes. This leads to an identification of the magnetic excitation energy as being approximately 37 meV at T=10 K, with a gradual softening with increasing temperature.