Magnetic dipole excitations in deformed nuclei

Abstract

The many models adopted to study the properties of the low-lying magnetic dipole excitations known as the scissors mode observed in most deformed nuclei are reviewed. Attention is focused first on the geometrical two-rotor model (TRM), whose predictions gave the motivation for seeking such a mode. The consistency of these predictions with the most meaningful collective properties of the mode is emphasized. More sophisticated descriptions carried out within different boson models are then reviewed. Their strict connection with the TRM is proved. An even closer link is shown to exist between the TRM and the schematic random-phase approximation (RPA). From the phenomenological and schematic models, confined to the description of the collective features of the M1 transitions, the analysis moves to the fully microscopic approaches, the only ones capable of accounting for the global properties of the mode as well as for the fragmentation of its M1 strength. Shell-model approaches, widely adopted for light and medium-light nuclei, are discussed. A more detailed analysis is devoted to the RPA, the most widely adopted microscopic scheme, especially in heavy nuclei. The path leading from the early incomplete and too approximate approaches yielding contradictory results to the most recent and refined studies converging to similar conclusions is sketched. The quasiparticle–phonon model (QPNM) as a way of improving the RPA description of the M1 spectrum by including the coupling to two-phonon RPA states is finally illustrated, and the related results are discussed. The study of the M1 spectra observed recently in deformed odd-mass nuclei carried out in a QPNM context completes the review.