AVERAGING OF AN ALTERNATING MAGNETIC-FIELD AND SPIN QUANTUM TUNNELING

Spin quantum tunneling in an alternating magnetic field of frequency o mega is studied. Without the field, tunneling occurs at a frequency om ega(0). Here we focus on the high-frequency domain omega much greater than omega(0), exploit the averaging method, and develop a theory cove ring most of the high-frequency axis. A simple proof of the averaging method has been included as well. We show that there is an intermediat e regime where the alternating field can dominate and, thus, enhance t unneling. A second case of enhancement is provided by resonance tunnel ing via a nearly degenerate pair with higher energy eigenvalues than t he ones considered. In general, however, no enhancement is to be expec ted. We also propose an experiment to probe spin quantum tunneling thr ough resonance in a circularly polarized magnetic field. This setup ex ploits a spin both as a dynamic variable and asa generator of rotation s, thus distinguishing it uniquely from a particle.