Isospin structure of pairing collective motion

The isospin degree of freedom of the pairing force is studied in the framework of the rotational ($\ensuremath{\Delta}\ensuremath{\ne}0$) and vibrational ($\ensuremath{\Delta}=0$) models. The magnitude $\ensuremath{\Delta}$ is the BCS gap parameter. The predictions of the two models concerning two-nucleon transfer reactions and energy levels are compared and contrasted. Some of the available experimental material around ${\mathrm{Ni}}^{56}$ is discussed in this framework. Further experiments are suggested which would help decide between the coupling schemes.