Towards understanding magnetic dipole excitations in deformed nuclei: Phenomenology
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 [Formula Presented] 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 [Formula Presented] excitation, and (2) a second, doubly degenerate, [Formula Presented] transition only slightly higher in energy than the usual scissors mode. Fragmentation of the [Formula Presented] strength distribution can be reproduced within framework of the model by including representation mixing terms in the Hamiltonian. © 1998 The American Physical Society.