QUANTUM TUNNELING OF MAGNETIZATION IN AN ALTERNATING MAGNETIC-FIELD -LOCALIZATION AND CHAOS

Tunnelling of mesoscopic quantum spins, i.e. magnetization, in a time- periodic external field is studied analytically. Three independent mec hanisms of localization or blacking of the magnetization are isolated, namely the crossing of Floquet eigenvalues, special frequencies, and the breakdown of a symmetry which would allow tunnelling between degen erate minima. Symmetry breakdown is discussed in detail in the adiabat ic region where the external held changes slowly. For high frequencies , we take advantage of the averaging method and are able to confirm th at AC hampering is a rather general property. We also numerically show that spin hampering persists in the classically chaotic region, even outside the domain of applicability of the avenging method.