Anomalous Properties of Quadrupole Collective States in 136^{136}Te and beyond

The ground and low-lying states of neutron-rich exotic Te and Sn isotopes are studied in terms of the nuclear shell model by the same Hamiltonian used for the spherical-deformed shape phase transition of Ba isotopes, without any adjustment. An anomalously small value is obtained for $B(E2;0^+_1\to 2^+_1)$ in $^{136}$Te, consistently with a recent experiment. The levels of $^{136}$Te up to yrast $12^+$ are shown to be in agreement with observed ones. It is pointed out that $^{136}$Te can be an exceptionally suitable case for studying mixed-symmetry 1$^+$, 2$^+$ and 3$^+$ states, and predictions are made for energies, M1 and E2 properties. Systematic trends of structure of heavier and more exotic Sn and Te isotopes beyond $^{136}$Te are studied by Monte Carlo Shell Model, presenting an unusual and very slow evolution of collectivity/deformation.Comment: 8 pages, 7 figures, accepted for publication in Phys. Rev.

Onset of intruder ground state in exotic Na isotopes and evolution of the N=20 shell gap

The onset of intruder ground states in Na isotopes is investigated by comparing experimental data and shell-model calculations. This onset is one of the consequences of the disappearance of the N=20 magic structure, and the Na isotopes are shown to play a special role in clarifying the change of this magic structure. Both the electromagnetic moments and the energy levels clearly indicate an onset of ground state intruder configurations at neutron number N=19 already, which arises only with a narrow N=20 shell gap in Na isotopes resulting from the spin-isospin dependence of the nucleon-nucleon interaction (as compared to a wider gap in stable nuclei like 40Ca). It is shown why the previous report based on the mass led to a wrong conclusion.Comment: 9 pages, 6 figures, to be published in Phys. Rev.

T=1 T=1 Pairing Along the N=Z N=Z Line

Pairing energies for the addition of two neutrons on even-even nuclei with $N=Z$ are studied. The $Z$ dependence is attributed to the number and type of orbitals that are occupied in the valence shell-model space. Properties in the region from $Z=60-100$ depend on the location of the $0g_{9/2}$ orbital.Comment: 3 pages and 2 figure

Novel features of nuclear forces and shell evolution in exotic nuclei

Novel simple properties of the monopole component of the effective nucleon-nucleon interaction are presented, leading to the so-called monopole-based universal interaction. Shell structures are shown to change as functions of $N$ and $Z$ consistently with experiments. Some key cases of this shell evolution are discussed, clarifying the effects of central and tensor forces. The validity of the present tensor force is examined in terms of the low-momentum interaction V$_{low k}$ and the Q$_{box}$ formalism.Comment: 4 pages, 4 figure

Novel Extrapolation Method in the Monte Carlo Shell Model

We propose an extrapolation method utilizing energy variance in the Monte Carlo shell model in order to estimate the energy eigenvalue and observables accurately. We derive a formula for the energy variance with deformed Slater determinants, which enables us to calculate the energy variance efficiently. The feasibility of the method is demonstrated for the full $pf$-shell calculation of $^{56}$Ni, and the applicability of the method to a system beyond current limit of exact diagonalization is shown for the $pf$+$g_{9/2}$-shell calculation of $^{64}$Ge.Comment: 4 pages, 4figure

Nuclear Shell Model by the Quantum Monte Carlo Diagonalization Method

The feasibility of shell-model calculations is radically extended by the Quantum Monte Carlo Diagonalization method with various essential improvements. The major improvements are made in the sampling for the generation of shell-model basis vectors, and in the restoration of symmetries such as angular momentum and isospin. Consequently the level structure of low-lying states can be studied with realistic interactions. After testing this method on $^{24}$Mg, we present first results for energy levels and $E2$ properties of $^{64}$Ge, indicating its large and $\gamma$-soft deformation.Comment: 12 pages, RevTex, 2 figures, to be published in Physical Review Letter