The influence of the intruder level on nuclear deformation is studied within the framework of the nucleon-pair shell model truncated to an SD-pair subspace. The results suggest that the intruder level has a tendency to soften the deformation and plays an important role in determining the onset of rotational behavior.

We study transitional patterns from the vibrational, U(5), to the rotational, SU(3), limit of the interacting boson model with a schematic Hamiltonian. The transitional behavior of low-lying energy levels, isomer shifts, E2 transition rates, and some other related quantities across the entire U(5)↔SU(3) transitional region are studied in detail. The analysis shows that nuclei in the critical region are soft.

We study the role of the strange quark mass in color-flavor locked matter. An effective interaction between kaons and particle-hole excitations near the Fermi surface is important and should be included in the effective Lagrangian. The new picture is based on the reasonable assumption of quark-hadron continuity. For a nonzero strange-quark mass, there is the possibility that both antikaon and kaon condensates may occur at different densities. A novel phase diagram of high-density QCD is given, which is different from that given in previous literature. The implications of the configuration are discussed briefly.

The nucleon-pair shell model truncated to the S-D subspace is applied to nucleus 134Ba for examining the effects on collectivity due to long-range n-p quadrupole forces. It is found that symmetric and mixedsymmetric states, consequently M1 transitions, are primarily controlled by the n-p quadrupole interaction. In comparison with surface-d interaction, when the quadrupole n-p coupling is weak, the model can produce approximately the vibrational limit, and when the quadrupole n-p coupling is strong, the O(6) limit can be realized.

Magnetic transition strengths in 134Ba are discussed within the framework of the nucleon-pair shell model (NPSM) truncated to the SD subspace. The S and D pairs are determined, respectively, by a variational method and a proton-neutron TDA approximation. The M1 and M3 transition strengths are found to be consistent with results from the proton-neutron interacting boson model calculations. The results confirm that the collective magnetic properties of 134Ba are primarily due to the orbital motion of nucleons.

The nucleon-pair shell model, truncated to an SD subspace, is applied to 128-134Xe and 130-136Ba. The spectra, eigenstates, and electromagnetic transition strengths of these nuclei, which are known to display O(6) symmetry, are described in terms of a three-parameter Hamiltonian. Agreement with experiment is good, in particular, mixed-symmetry configurations and the upward shift and fragmentation of the M1 strength with increasing neutron numbers emerges naturally. The results show that the SD subspace approximation improves with the number of pairs.

In the nucleon-pair shell model truncated to the S-D subspace with single-particle level splitting fully taken into account, the effects on the collectivity due to the pair structure are studied for 134Ba. It is found that the yrast states are not sensitive, but the others, especially the 02+ and 22+ states, are very sensitive to the D-pair structure. Among the lbur pair structures studied in the paper, only the SD pairs taken from the 01+, 21+ states of two-valence-nucleon systems with surface delta interaction can describe the spectra, and the B(E2) and B(MI) values of 134Ba. @ 1998 Published by Elsevier Science B.V.