TOPOLOGICALLY QUENCHED TUNNEL SPLITTING IN A SPIN SYSTEM OBTAINED FROM QUANTUM-MECHANICAL PERTURBATION-THEORY

The difference DELTAE between the two lowest energy levels of a one-spin system with classically degenerate ground state is calculated as a function of an external magnetic field B. Using standard quantum-mechanical perturbation theory, it is shown that the tunnel splitting DELTAE vanishes for specific discrete values of the field B. Consequently, the quenching derived previously by a semi-classical evaluation of path integrals for large spin is seen to persist for arbitrary spin lengths. Further, the structure of the full quantum-mechanical energy spectrum is obtained.