The nucleus as a condensate of monopole and quadrupole pairing vibrations

It has been shown that the aligned wave function, and the wave function obtained by restricting pairs of particles to be coupled to angular momentum zero and two, as assumed by the quadrupole phonon model (QPM) and by the interacting boson model (IBM) are, for strongly deformed systems, rather different. They become similar in the vibrational limit and display different degrees of similarity for intermediate (anharmonic) situations. To what extent this difference reveals itself in the predicted properties of the low-energy nuclear spectrum is an open question. In an attempt to clarify this point we have calculated the spectrum and the electromagnetic and two-nucleon transfer probabilities for some strongly anharmonic and transitional nuclei, in the framework of the nuclear field theory (NFT) version of the pair aligned model. These calculations, which are microscopic, depend on the strength of the pairing and particle-hole interactions. We find that for standard values of these parameters, the moment of inertia of both the β- and the γ-bands are too small. While the main pattern of the phase transition observed in the Sm isotopes is displayed by the model, major deviations are observed concerning the properties of the β-vibrations, and in connection with the two-nucleon transfer strength associated with the 2+ member of the ground-state rotational band. (orig.)