Unusual spin effect in alkali vapor induced by two orthogonal multiple harmonics of magnetic field

Abstract

In this paper, we describe the unusual low-frequency magnetic resonances in alkali vapor with oriented atomic spins regarding the framework of density matrix formalism. The feature of the resonance is the absence of a constant component in the external magnetic field. To explain steep increase of the spin orientation at certain frequencies, we define special closed atomic spin trajectories governed by periodic magnetic perturbation. Any closed trajectory is characterized by the frequency of spin motion. The resonance effect was numerically verified in the paper. For instance, these trajectories can be observed in an alkali vapor via optical excitation. Surprisingly, the width of the resonance line is found to be narrower, as one may expect