Dipole resonances in light neutron-rich nuclei studied with time-dependent calculations of antisymmetrized molecular dynamics

In order to study isovector dipole response of neutron-rich nuclei, we have applied a time-dependent method of antisymmetrized molecular dynamics. The dipole resonances in Be, B and C isotopes have been investigated. In $^{10}$Be, $^{15}$B, $^{16}$C, collective modes of the vibration between a core and valence neutrons cause soft resonances at the excitation energy $E_x=10-15$ MeV below the giant dipole resonance(GDR). In $^{16}$C, we found that a remarkable peak at $E_x=14$ MeV corresponds to coherent motion of four valence neutrons against a $^{12}$C core, while the GDR arises from the core vibration in the $E_x >20$ MeV region. In $^{17}$B and $^{18}$C, the dipole strengths in the low energy region decline compared with those in $^{15}$B and $^{16}$C. We also discuss the energy weighted sum rule for the $E1$ transitions.Comment: 12 figures, submitted to Phys. Rev.

Negative parity states of 11^{11}B and 11^{11}C and the similarity with $^{12}C

The negative parity states of $^{11}$B and $^{11}$C were studied based on the calculations of antisymmetrized molecular dynamics(AMD). The calculations well reproduced the experimental strengths of Gamov-Teller(GT), $M1$ and monopole transitions. We, especially, focused on the $3/2^-_3$ and $5/2^-_2$ states, for which GT transition strengths were recently measured. The weak $M1$ and GT transitions for the $3/2^-_3$ in $^{11}$B and $^{11}$C are described by a well-developed cluster structure of $2\alpha$+$t$ and $2\alpha$+$^3$He, respectively, while the strong transitions for the $5/2^-_2$ is characterized by an intrinsic spin excitation with no cluster structure. It was found that the $3/2^-_3$ state is a dilute cluster state, and its features are similar to those of the $^{12}$C$(0^+_2)$ which is considered to be a gas state of three $\alpha$ clusters.Comment: 10 pages, 4 figures, submitterd to Physical Review

Cluster structure in stable and unstable nuclei

Cluster structure in stable and unstable nuclei has been studied. We report recent developments of theoretical studies on cluster aspect, which is essential for structure study of light unstable nuclei. We discuss negative parity bands in even-even Be and Ne isotopes and show the importance of cluster aspect. Three-body cluster structure and cluster crystallization are also introduced. It was found that the coexistence of cluster and mean-field aspect brings a variety of structures to unstable nuclei.Comment: 6 pages, 3 figures, submitted to Euro. Phys. J.

Application of the generalized two-center cluster model to 10Be

A generalized two-center cluster model (GTCM), including various partitions of the valence nucleons around two alpha-cores, is proposed for studies on the exotic cluster structures of Be isotopes. This model is applied to the 10Be = alpha + alpha + n + n system and the adiabatic energy surfaces for alpha-alpha distances are calculated. It is found that this model naturally describes the formation of the molecular orbitals as well as that of asymptotic cluster states dependeing on their relative distance. In the negative-parity state, a new type of the alpha + 6He cluster structure is also predicted.Comment: 5 pages, 3 figure

Structure of excited states of Be-11 studied with Antisymmetrized Molecular Dynamics

The structures of the ground and excited states of Be-11 were studied with a microscopic method of antisymmetrized molecular dynamics. The theoretical results reproduce the abnormal parity of the ground state and predict various kinds of excited states. We suggest a new negative-parity band with a well-developed clustering structure which reaches high-spin states. Focusing on a $2\alpha$ clustering structure, we investigated structure of the ground and excited states. We point out that molecular orbits play important roles for the intruder ground state and the low-lying $2\hbar \omega$ states. The features of the breaking of $\alpha$ clusters were also studied with the help of data for Gamow-Teller transitions.Comment: 24 pages, 7 figures, to be submitted to Phys.Rev.

Antisymmetrized molecular dynamics studies for exotic clustering phenomena in neutron-rich nuclei

We present a review of recent works on clustering phenomena in unstable nuclei studied by antisymmetrized molecular dynamics (AMD). The AMD studies in these decades have uncovered novel types of clustering phenomena brought about by the excess neutrons. Among them, this review focuses on the molecule-like structure of unstable nuclei. One of the earliest discussions on the clustering in unstable nuclei was made for neutron-rich Be and B isotopes. AMD calculations predicted that the ground state clustering is enhanced or reduced depending on the number of excess neutrons. Today, the experiments are confirming this prediction as the change of the proton radii. Behind this enhancement and reduction of the clustering, there are underlying shell effects called molecular- and atomic-orbits. These orbits form covalent and ionic bonding of the clusters analogous to the atomic molecules. It was found that this "molecular-orbit picture" reasonably explains the low-lying spectra of Be isotopes. The molecular-orbit picture is extended to other systems having parity asymmetric cluster cores and to the three cluster systems. O and Ne isotopes are the candidates of the former, while the $3\alpha$ linear chains in C isotopes are the latter. For both subjects, many intensive studies are now in progress. We also pay a special attention to the observables which are the fingerprint of the clustering. In particular, we focus on the monopole and dipole transitions which are recently regarded as good probe for the clustering. We discuss how they have and will reveal the exotic clustering.Comment: 96 pages, 44 figure

New effective nuclear forces with a finite-range three-body term and their application to AMD+GCM calculations

We propose new effective inter-nucleon forces with a finite-range three-body operator. The proposed forces are suitable for describing the nuclear structure properties over a wide mass number region, including the saturation point of nuclear matter. The forces are applied to microscopic calculations of $Z=N$ ($A\le 40$) nuclei and O isotopes with a method of antisymmetrized molecular dynamics. We present the characteristics of the forces and discuss the importance of the finite-range three-body term.Comment: 15 pages, 11 figures, submitted to Phys.Rev.

Superdeformation and clustering in 40^{40}Ca studied with Antisymmetrized Molecular Dynamics

Deformed states in $^{40}$Ca are investigated with a method of antisymmetrized molecular dynamics. Above the spherical ground state, rotational bands arise from a normal deformation and a superdeformation as well as an oblate deformation. The calculated energy spectra and $E2$ transition strengths in the superdeformed band reasonably agree to the experimental data of the superdeformed band starting from the $0^+_3$ state at 5.213 MeV. By the analysis of single-particle orbits, it is found that the superdeformed state has particle-hole nature of an $8p$-$8h$ configuration. One of new findings is parity asymmetric structure with $^{12}$C+$^{28}$Si-like clustering in the superdeformed band. We predict that $^{12}$C+$^{28}$Si molecular bands may be built above the superdeformed band due to the excitation of inter-cluster motion. They are considered to be higher nodal states of the superdeformed state. We also suggest negative-parity bands caused by the parity asymmetric deformation.Comment: 13 figures, submitted to Phys. Rev.

Structure of Excited States of 10Be studied with Antisymmetrized Molecular Dynamics

We study structure of excited states of 10Be with the method of variation after spin parity projection in the framework of antisymmetrized molecular dynamics. Present calculations describe many excited states and reproduce the experimental data of E2 and E1 transitions and the new data of the $\beta$ transition strength successfully. We make systematic discussions on the molecule-like structures of light unstable nuclei and the important role of the valence neutrons based on the results obtained with the framework which is free from such model assumptions as the existence of inert cores and clusters.Comment: 15 pages, RevTex, seven postscript figures (using epsf.sty

Deformations in N=14 isotones

Systematic analysis of deformations in neutron-rich N=14 isotones was done based on the method of antisymmetrized molecular dynamics. The property of the shape coexistence in $^{28}$Si, which is known to have the oblate ground state and the prolate excited states, was successfully described. The results suggest that the shape coexistence may occur also in neutron-rich N=14 nuclei as well as $^{28}$Si. It was found that the oblate neutron shapes are favored because of the spin-orbit force in most of N=14 isotones. $Q$ moments and $E2$ transition strengths in the neutron-rich nuclei were discussed in relation to the intrinsic deformations, and a possible difference between the proton and neutron deformations in $^{24}$Ne was proposed.Comment: 13 pages, 7 figures, sumitted to Phys.Rev.