  We investigate the topological quenching of spin tunneling in Mn_12-acetate
molecules with an applied magnetic field along the hard axis. The model
Hamiltonian describing this system has a fourth-order term due to the
tetragonal anisotropy. We treat this model using the discrete phase integral
formalism in which the Schrodinger equation corresponding to the Hamiltonian
becomes a nine-term recursion relation. We solve this recursion relation and
find that the tunnel splitting is quenched at certain values of the applied
field. We also present a qualitative treatment based on the instanton approach.

Mesoscale and Nanoscale Physics