By introducing the concepts of quasi-spin pairing and quasi-spin seniority, the Lipkin model is extended to a variable number of particles. The properties of quasi-spin pairing are seen to be quite similar to those of ordinary pairing. The quasi-spin seniority allows one to obtain a simple classification of excited multiplets. A "pairing plus monopole" model is studied in connection with the Hartree-Fock theory.
The most general Hartree-Fock orbitals of the so-called lattice-density-wave character are introduced and its properties studied. Application to the nuclear matter problem is made by employing a density dependent interaction of the Skyrme type. ~.-clustering effects are predicted at low densities.
Abstract.The relationship between ground-state correlations and collectiveness is investigated for the case of low-lying nonrotational states in the rare earth nuclei. Both octupole and quadrupole modes of excitation are studied and the quasiparticle virtual populations associated with each of them are discussed. The relative importance of particleparticle and particle-hole interaction matrix elements is also analyzed in connection with the shape of the correlation patterns. The fundamental role of the Nilsson + BCS scheme is emphasized and the consistency of the quasiparticle random-phase approximation is established.
The competition between pairing and multipole fields is studied within the framework of a two-level model, employing different theoretical approaches. The importance of a selfconsistent tretment is clearly revealed. Special attention is payed to the microscopic mechanisms that take place when the system undergoes various phase transitions.
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