Origin of Low-Energy Quadrupole Collectivity in Vibrational Nuclei

The coupling of the giant quadrupole resonance to valence-space configurations is shown to be the origin of the formation of low-lying quadrupole-collective structures in vibrational nuclei with symmetric and mixed-symmetric character with respect to the proton-neutron degree of freedom. For the first time experimental evidence for this picture is obtained from electron- and proton scattering experiments on the nucleus 92Zr that are sensitive to the relative phase of valence-space amplitudes by quantum interference.