Two-level interacting boson models beyond the mean field

10 pages, 1 table, 12 figures.-- PACS nrs.: 21.60.Fw; 73.43.Nq.-- ArXiv pre-print available at: http://arxiv.org/abs/nucl-th/0609001v1
The phase diagram of two-level boson Hamiltonians, including the interacting boson model (IBM), is studied beyond the standard mean field approximation using the Holstein-Primakoff mapping. The limitations of the usual intrinsic state (mean field) formalism concerning finite-size effects are pointed out. The analytic results are compared to numerics obtained from exact diagonalizations. Excitation energies and occupation numbers are studied in different model space regions (Casten triangle for IBM) and especially at the critical points.
This work has been partially supported by the Spanish Ministerio de Educación y Ciencia and by the European regional development fund (FEDER) under project numbers BFM2003-05316-C02-02, FIS2005-01105, and FPA2003-05958.