SU(2) instantons with boundary jumps and spin tunneling in magnetic molecules

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Coherent state path integrals are shown in general to contain instantons with jumps at the boundaries, i.e., with boundary points lying outside classical parameter or phase space. As an example, the magnetic molecule Fe₈ is studied using a realistic Hamiltonian, and instanons with jumps are shown to dominate beyond a certain external magnetic field. An approximate formula is found for the fields where ground state tunneling is quenched in this molecule.