Towards understanding magnetic dipole excitations in deformed nuclei: Phenomenology

Abstract

The pseudo SU(3) model, in an extended version in which proton and neutron degrees of freedom are incorporated explicitly, is used to study the nature of low-lying magnetic dipole excitations in even-even rare-earth nuclei. Through a linear mapping between invariants of the rotor group and SU(3), a microscopic interpretation of the ``scissors'' mode of the two-rotor model is realized. The model allows for, first, a classification of SU(3) shell-model configurations in terms of collective degrees of freedom and, second, an adjustment of the relative energy of distinct ${1}^{+}$ states. Triaxiality of the parent proton and neutron distributions is shown to add a ``twist'' degree of freedom to the usual ``scissors'' mode picture. This new mode should be observed as: (1) a low-lying $K=0$ excitation, and (2) a second, doubly degenerate, $K=1$ transition only slightly higher in energy than the usual scissors mode. Fragmentation of the $M1$ strength distribution can be reproduced within framework of the model by including representation mixing terms in the Hamiltonian.