Parameter-free description of orbital magnetic dipole strength

The low-lying orbital magnetic dipole strength in even-even nuclei is discussed using a sum-rule approach. It is shown that both the mean excitation energies as well as the summed excitation strengths from the experiment can be described well in heavy nuclei if the moments of inertia and the g factors are substituted by the parameters of the ground-state rotational bands. The influence of the high-lying scissors mode is taken into account explicitly, leading to a successful description of the low-lying mode in heavy deformed nuclei with no free parameters. A quantitative estimate of the gross features of the high-lying mode is deduced. The application of the sum-rule approach to medium-mass nuclei is presented, and the discrepancies with the experimental data are discussed.