Topological quenching of spin tunneling in Fe8-tacn molecules

A Berry phase in the path integral for spin gives rise to an interference effect in the tunneling of a biaxially symmetric spin Hamiltonian. As the magnetic field applied along a hard axis is varied, the tunnel splitting between the two lowest energy states oscillates, with field values where it is completely quenched. This Hamiltonian is realized in Fe-8-tacn molecules, where these oscillations have just now been seen and provide, in fact, strong evidence for tunneling below 0.36 K in the first place. (C) 2000 Elsevier Science B.V. All rights reserved.