Particle-hole excitations in the interacting boson model (IV). The U(5)-SU(3) coupling

In the extended interacting boson model (EIBM) both particle- and hole-like bosons are incorporated to encompass multi-particle-multi-hole excitations at and near to closed shells. We apply the group theoretical concepts of t he EIBM to the particular case of two coexisting systems in the same nucleu s exhibiting a U(5) (for the regular configurations) and an SU(3) symmetry (for the intruder configurations). Besides the description of "global" symm etry aspects in terms of I-spin, also the very specific local mixing effect s characteristic for the U(5)-SU(3) symmetry coupling are studied. The mode l is applied to the Po isotopes and a comparison with a more realistic calc ulation is made. (C) 1999 Elsevier Science B.V. All rights reserved.