Quantum tunnelling of magnetization in an alternating magnetic field: localization and chaos

Abstract

Tunnelling of mesoscopic quantum spins, i.e. magnetization, in a time-periodic external field is studied analytically. Three independent mechanisms of localization or blocking of the magnetization are isolated, namely the crossing of Floquet eigenvalues, special frequencies, and the breakdown of a symmetry which would allow tunnelling between degenerate minima. Symmetry breakdown is discussed in detail in the adiabatic region where the external field changes slowly. For high frequencies, we take advantage of the averaging method and are able to confirm that 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 averaging method.