Theoretical aspects of the new collective modes in nuclei

The interpretation of the low-energy orbital magnetic-dipole excitations of nonspherical nuclei, commonly called scissors modes, is examined. Different models are reviewed, and with the help of the fluid dynamical model and microscopic results from different groups, the relations between the models and their limitations are discussed. Contradictions between semiclassical models, algebraic models and microscopic calculations are shown to be only apparent. The emerging picture is that of a semiclassical vorticity mode which can only exist due to quantum effects but nevertheless, in the quantum limit, is spread over a number of states, each of them at most slightly collective. The mode is an example for the vorticity of nuclear currents which in this case has a direct measurable effect, namely the M1 strength itself. The importance of electric-quadrupole strengths to assess the nature of the modes is emphasized.