Quenched spin tunneling and diabolical points in magnetic molecules. II. Asymmetric configurations - art. no. 094414

The perfect quenching of spin tunneling first predicted for a model with bi axial symmetry, and recently observed in the magnetic molecule Fe-8, is fur ther studied using the discrete phase integral or WKB (Wentzel-Kramers-Bril louin) method. The analysis of the previous paper is extended to the case w here the magnetic field has both hard and easy components, so that the Hami ltonian has no obvious symmetry. Herring's formula is now inapplicable, so the problem is solved by finding the wave function and using connection for mulas at every turning point. A general formula for the energy surface in t he vicinity of the diabolo is obtained in this way. This formula gives the tunneling amplitude between two wells unrelated by symmetry in terms of a s mall number of action integrals, and appears to be generally valid, even fo r problems where the recursion contains more than five terms. Explicit resu lts are obtained for the diabolical points in the model for Fe8 that closel y parallel the experimental observations, The leading semiclassical results for the diabolical points are found to agree precisely with exact results.