Quantum Tunneling in Half-Integer Spin Systems

Motivated by the experimental observations of resonant tunnelings in thesystems with half-integer spin, such as V$_{15}$ and Mn$_4$, we study themechanism of adiabatic change of the magnetization in systems with thetime-reversal symmetry. Within the time-reversal symmetric models, effects ofseveral types of perturbations are investigated. Although tunneling between theground states is suppressed in a simple Kramers doublet, we show that thenonadiabatic transition governed by the Landau-Zener-St\"uckelberg mechanismoccurs in many cases due to the additional degeneracy of the ground state. Wealso found more general cases where LZS mechanism can not be applied directlyeven the system shows a kind of adiabatic change of the magnetization