Topological Quenching of Spin Tunneling in Mn₁2-acetate Molecules

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We investigate the topological quenching of spin tunneling in Mn₁2-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.