Probing mixing of quantum spin states in frustrated molecular magnets with neutron scattering

The molecular spin cluster V-15 is a polyoxovanadate with 15 spins s=1/2 but total spin ground,state S=1/2 due to strong antiferromagnetic (AFM) interactions. Below 1 K, the relaxation of the magnetisation exhibits a very peculiar behaviour in connection of the multi-spin character of the cluster and the ground state is composed of two non-degenerate S=1/2 Kramers doublets separated by a gap of about 0.3 K. The mechanism leading to such gap formation in the ground state has been elucidated using inelastic neutrons scattering (INS) under magnetic field. The observation and characterisation of several field-dependent INS transitions enable a comprehensive understanding of the low-energy quantum spin states. The energy gap between the two Kramers doublets is caused by slight deviations from the trigonal symmetry (symmetry lowering) of the cluster. The INS data are successfully mapped onto an S=1/2 AFM Heisenberg triangle with scalene distortion and a quantitative description of the wavefunction mixing (entanglement) within the ground state is derived. (C) 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.