Underlying Structure of Collective Bands and Self-Organization in Quantum Systems

Quasi-SU(3) Coupling Induced Oblate-Prolate Shape Phase Transition in the Casten Triangle

Shapes and shape evolution in the mass-130 region, including the Te, Xe, and Ba isotopes, have long been a focus of discussion in nuclear physics. This mass region consists of complex many-body systems that can behave in astonishingly simple and regular ways, as classified in the Casten symmetry triangle. By applying the shell model Hamiltonian proposed recently, we carry out calculations using the Hartree-Fock-Bogolyubov plus generator coordinate method, in the large model space containing the (1g_{9/2},1g_{7/2},2d_{5/2},2d_{3/2},3s_{1/2},1h_{11/2},2f_{7/2}) orbits. Based on good reproduction of the experimentally known energy levels, spectroscopic quadrupole moments, and E2 transition probabilities, we identify the quasi-SU(3) couplings across the N=50 and 82 shell gaps, which play a role in driving shape evolution and phase transition discussed in the extended Casten triangle. Specifically, we demonstrate that the quasi-SU(3) coupling mechanism in the proton partner orbits (1g_{9/2}, 2d_{5/2}) tends to drive the system to be more γ soft, and that in the neutron partner orbits (1h_{11/2}, 2f_{7/2}) are responsible for the oblate-to-prolate shape phase transition. With an emphasis on discussing spectroscopic quadrupole moments, our Letter uncovers hidden symmetries from the vast shell-model configurations and adds microscopical insights into the empirical symmetry triangle.

ΔI=2 Bifurcation as a Characteristic Feature of Scissors Rotational Bands

We report microscopic many-body calculations indicating that rotational bands based on nuclear scissors vibrations exhibit systematic splitting between neighboring spin states (ΔI=2 bifurcation) in which the magnitude of the moment of inertia oscillates between states having even and odd spins. We show that this unexpected result is caused by self-organization of the deformed proton and neutron bodies in the scissors motion, which is further amplified by the K^{π}=1^{+} two-quasiparticle configurations near the scissors states. We propose that the puzzling excited state found above the 1^{+} scissors state in ^{156}Gd [Phys. Rev. Lett. 118, 212502 (2017)PRLTAO0031-900710.1103/PhysRevLett.118.212502] is the first evidence of this effect, and predict that bifurcation may generally appear in all other scissors rotational bands of deformed nuclei, and possibly in other systems exhibiting collective scissors vibrations.

α-Clustering in atomic nuclei from first principles with statistical learning and the Hoyle state character

A long-standing crucial question with atomic nuclei is whether or not α clustering occurs there. An α particle (helium-4 nucleus) comprises two protons and two neutrons, and may be the building block of some nuclei. This is a very beautiful and fascinating idea, and is indeed plausible because the α particle is particularly stable with a large binding energy. However, direct experimental evidence has never been provided. Here, we show whether and how α(-like) objects emerge in atomic nuclei, by means of state-of-the-art quantum many-body simulations formulated from first principles, utilizing supercomputers including K/Fugaku. The obtained physical quantities exhibit agreement with experimental data. The appearance and variation of the α clustering are shown by utilizing density profiles for the nuclei beryllium-8, -10 and carbon-12. With additional insight by statistical learning, an unexpected crossover picture is presented for the Hoyle state, a critical gateway to the birth of life.

Alpha particles are considered the building blocks for some nuclei in alpha-clustering. Here the authors discuss quantum many-body simulations with nucleon-nucleon interaction to characterize the Hoyle state, the first excited 0+ state of the 12C nucleus, and find complexity in its alpha-clustering.

Emerging Concepts in Nuclear Structure Based on the Shell Model

Some emerging concepts of nuclear structure are overviewed. (i) Background: the many-body quantum structure of atomic nucleus, a complex system comprising protons and neutrons (called nucleons collectively), has been studied largely based on the idea of the quantum liquid (à la Landau), where nucleons are quasiparticles moving in a (mean) potential well, with weak “residual” interactions between nucleons. The potential is rigid in general, although it can be anisotropic. While this view was a good starting point, it is time to look into kaleidoscopic aspects of the nuclear structure brought in by underlying dynamics and nuclear forces. (ii) Methods: exotic features as well as classical issues are investigated from fresh viewpoints based on the shell model and nucleon–nucleon interactions. The 70-year progress of the shell–model approach, including effective nucleon–nucleon interactions, enables us to do this. (iii) Results: we go beyond the picture of the solid potential well by activating the monopole interactions of the nuclear forces. This produces notable consequences in key features such as the shell/magic structure, the shape deformation, the dripline, etc. These consequences are understood with emerging concepts such as shell evolution (including type-II), T-plot, self-organization (for collective bands), triaxial-shape dominance, new dripline mechanism, etc. The resulting predictions and analyses agree with experiment. (iv) Conclusion: atomic nuclei are surprisingly richer objects than initially thought.

Cross-Shell Excitation in F and Ne Isotopes around N = 20

Within the framework of the configuration–interaction shell model, the present work applies three effective interactions to investigate the effects of the cross-shell excitation on F and Ne isotopes around N = 20, which are significantly proton–neutron asymmetric, and have different properties compared with the proton–neutron symmetric nuclei. It is shown that cross-shell excitation is necessary in order to reproduce separation energies, neutron drip lines, and low-energy levels of these isotopes. Furthermore, the cross-shell excitation of (0–5)ħω is suggested to be important in the description of 29F and 30Ne. However, the three interactions are insufficient in describing the bound structure of 29,31Ne, and provide inconsistent shell structures and evolutions in the target nuclei. Their cross-shell interactions are suggested to be improved.

The impact of nuclear shape on the emergence of the neutron dripline

Atomic nuclei are composed of a certain number of protons Z and neutrons N. A natural question is how large Z and N can be. The study of superheavy elements explores the large Z limit^1,2, and we are still looking for a comprehensive theoretical explanation of the largest possible N for a given Z-the existence limit for the neutron-rich isotopes of a given atomic species, known as the neutron dripline³. The neutron dripline of oxygen (Z = 8) can be understood theoretically as the result of single nucleons filling single-particle orbits confined by a mean potential, and experiments confirm this interpretation. However, recent experiments on heavier elements are at odds with this description. Here we show that the neutron dripline from fluorine (Z = 9) to magnesium (Z = 12) can be predicted using a mechanism that goes beyond the single-particle picture: as the number of neutrons increases, the nuclear shape assumes an increasingly ellipsoidal deformation, leading to a higher binding energy. The saturation of this effect (when the nucleus cannot be further deformed) yields the neutron dripline: beyond this maximum N, the isotope is unbound and further neutrons 'drip' out when added. Our calculations are based on a recently developed effective nucleon-nucleon interaction⁴, for which large-scale eigenvalue problems are solved using configuration-interaction simulations. The results obtained show good agreement with experiments, even for excitation energies of low-lying states, up to the nucleus of magnesium-40 (which has 28 neutrons). The proposed mechanism for the formation of the neutron dripline has the potential to stimulate further thinking in the field towards explaining nucleosynthesis with neutron-rich nuclei.

Shape Coexistence at Zero Spin in ^{64}Ni Driven by the Monopole Tensor Interaction

The low-spin structure of the semimagic ^{64}Ni nucleus has been considerably expanded: combining four experiments, several 0^{+} and 2^{+} excited states were identified below 4.5 MeV, and their properties established. The Monte Carlo shell model accounts for the results and unveils an unexpectedly complex landscape of coexisting shapes: a prolate 0^{+} excitation is located at a surprisingly high energy (3463 keV), with a collective 2^{+} state 286 keV above it, the first such observation in Ni isotopes. The evolution in excitation energy of the prolate minimum across the neutron N=40 subshell gap highlights the impact of the monopole interaction and its variation in strength with N.