Mechanism of Ground State Selection in the Frustrated Molecular Spin   Cluster V15
  We report an inelastic neutron scattering (INS) study under a magnetic field on the frustrated molecular spin cluster $V_{15}$. Several field-dependent transitions are observed and provide a comprehensive understanding of the low-energy quantum spin states. The energy gap $2 \Delta_{0}\approx 27(3)\mu$eV between the two lowest $S=1/2$ Kramers doublets is unambiguously attributed to a symmetry lowering of the cluster. The INS data are mapped onto an S=1/2 Antiferromagnetic Heisenberg triangle with scalene distortion. A quantitative description of the wavefunction mixing within the ground state is derived. 
G. Chaboussant, S.T. Ochsenbein, A. Sieber, H.-U. Guedel, H. Mutka, A.   Mueller and B. Barbara