Quantum Mechanical Transitions in a Dissipative Environment

We have constructed an algorithm to treat quantum mechanical dynamics, such as the nonadiabatic transitions in a dissipative environment, by making use of the formula of reduced density matrix, i.e., Quantum Master Equation (QME). We applied the method to investigate dynamics of magnetization of nanoscale magnets, such as Mn12 and Fe8, in a time dependent field. Due to the discreteness of the energy levels in small systems the resonating tunneling phenomena play an important role. At relatively high temperatures the excited levels contribute to the transition and the transition rate depends significantly on the temperature. On the other hand, at very low temperatures the magnetization process does not depend on the temperature. Even at such a low temperature, the effect of environments is still relevant. We demonstrate such a process with the method of QME. We also analyze temperature dependent phenomena of hysteresis of the nanoscale magnets.