G. Chaboussant, S. T. Ochsenbein, A. Sieber, R. Bircher, H.-U. Güdel, B. Barbara, A. Müller and H. Mutka
Magnetic field reveals nature of quantum spin state in the molecular spin cluster V₁₅
ILL Annual Report 2003, 18-19 (2004)                 

Abstract: The molecular spin cluster V₁₅ is a polyoxovanadate with 15 spins-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 called "butterfly" hysteresis and the ground state is composed of two non-degenerate S = 1/2 Kramers duoblets with a gap 2D₀ 30meV. Using the upgraded multi-chopper spectrometer IN5 at 45 mK and magnetic fields up to 1 Tesla, it is possible to completely elucidate the mechanism leading to this zero-filed split ground state. Combining magnetic fields and high-resolution inelastic neutron scattering provides a detailed insight into the nature of the S = 1/2 ground state: The magnetism of this "large" molecular cluster (15 spins) is mapped into a S = 1/2 Heisenberg antiferromagnetic triangle with quantum entanglement of the ground state doublets. Slight deviations from trigonal symmetry relieve part of the intrinsic frustration inherent to such systems.