Low-energy spin excitations in the molecular magnetic cluster V15
  We report an Inelastic Neutron Scattering (INS) study of the fully deuterated molecular compound K6[V15As6O42].9(D2O) (V15). Due to geometrical frustration, the essential physics at low temperatures of the V15 cluster containing 15 coupled V4+ (S=1/2) is determined by three weakly coupled spin-1/2 on a triangle. The INS spectra at low-energy allow us to directly determine the effective exchange coupling J0 = 0.211 meV within the triangle and the gap 2 \Delta = 0.035(2)meV between the two spin-1/2 doublets of the ground state. Results are discussed in terms of deviations from trigonal symmetry and Dzyaloshinskii-Moriya (DM) interactions. 
G. Chaboussant, R. Basler, A. Sieber, S.T. Ochsenbein, A. Desmedt,   R.E. Lechner, M.T.F. Telling, P. Koegerler, A. Mueller ...