Z. P. Li

Impurity effect of Lambda hyperon on collective excitations of nuclear core in MgΛ25

Abstract Taking the ground state rotational band in Mg 24 as an example, we investigate the impurity effect of Λ hyperon on collective excitations of atomic nuclei in the framework of non-relativistic energy density functional theory. To this end, we take into account correlations related to the restoration of broken symmetries and fluctuations of collective variables by solving the eigenvalue problem of a five-dimensional collective Hamiltonian for quadrupole vibrational and rotational degrees of freedom. The parameters of the collective Hamiltonian are determined with constrained mean-field calculations for triaxial shapes using the SGII Skyrme force. We compare the low-spin spectrum for Mg 24 with the spectrum for the same nucleus inside Mg Λ 25 . It is found that the Λ hyperon stretches the ground state band and reduces the B ( E 2 : 2 1 + → 0 1 + ) value by ∼ 9 % , mainly by softening the potential energy surface towards the spherical shape, even though the shrinkage effect on the average proton radius is only ∼ 0.5 % .

Beyond relativistic mean-field studies of low-lying states in neutron-deficient krypton isotopes

Background: Neutron-deficient krypton isotopes are of particular interest due to the coexistence of oblate and prolate shapes in low-lying states and the transition of the ground state from one dominate shape to another as a function of neutron number. Moreover, the onset of large E2 transition strength around 76Kr indicates the erosion of the N=40 subshell gap. Purpose: A detailed interpretation of these phenomena in neutron-deficient Kr isotopes requires the use of a method going beyond a mean-field approach that permits the determination of spectra and transition probabilities. The aim of this work is to provide a systematic calculation of low-lying states in the even-even 68 -86Kr isotopes and to understand the shape coexistence phenomenon and the onset of large collectivity around N=40 from beyond relativistic mean-field studies. Method: The starting point of our method is a set of relativistic mean-field plus BCS wave functions generated with a constraint on triaxial deformations (β,γ). The excitation energies and electric multipole transition strengths of low-lying states are calculated by solving a five-dimensional collective Hamiltonian (5DCH) with parameters determined by the mean-field wave functions. To examine the role of triaxiality, a configuration mixing of both particle-number- and angular-momentum-projected axially deformed states is also carried out within the exact generator coordinate method based on the same energy density functional. Results: The energy surfaces, the excitation energies of 02+, 21+, and 22+ states, as well as the E0 and E2 transition strengths are compared with the results of similar 5DCH calculations but with parameters determined by the nonrelativistic mean-field wave functions, as well as with the available data. The results show a picture of oblate-triaxial-prolate shape transition. Coexistence of low-lying excited 0+ states is found to be a common feature in the neutron-deficient Kr isotopes. The underlying mechanism responsible for the shape coexistence is discussed. Conclusions: The main features of the low-spin spectra and the systematics of excitation energies and transition strengths in the neutron-deficient Kr isotopes are reproduced very well. The effects of dynamic correlations and triaxiality turn out to have important influences on the balance between the competing oblate and prolate states. An exact treatment of configuration mixing of particle-number- and angular-momentum-projected triaxial states is highly demanded to pin down these effects.

Simultaneous quadrupole and octupole shape phase transitions in Thorium

The evolution of quadrupole and octupole shapes in Th isotopes is studied in the framework of nuclear Density Functional Theory. Constrained energy maps and observables calculated with microscopic collective Hamiltonians indicate the occurrence of a simultaneous quantum shape phase transition between spherical and quadrupole-deformed prolate shapes, and between non-octupole and octupole-deformed shapes, as functions of the neutron number. The nucleus Th-224 is closest to the critical point of a double phase transition. A microscopic mechanism of this phenomenon is discussed in terms of the evolution of single-nucleon orbitals with deformation

Enhanced collectivity in neutron-deficient Sn isotopes in energy functional based collective Hamiltonian

Abstract The low-lying collective states in Sn isotopes are studied by a five-dimensional collective Hamiltonian with parameters determined from the triaxial relativistic mean-field calculations using the PC-PK1 energy density functional. The systematics for both the excitation energies of 2 1 + states and B ( E 2 ; 0 1 + → 2 1 + ) values are reproduced rather well, in particular, the enhanced E2 transitions in the neutron-deficient Sn isotopes with N 66 . We show that the gradual degeneracy of neutron levels 1 g 7 / 2 and 2 d 5 / 2 around the Fermi surface leads to the increase of level density and consequently the enhanced paring correlations from N = 66 to 58. It provokes a large quadrupole shape fluctuation around the spherical shape, and leads to an enhanced collectivity in the isotopes around N = 58 .

Covariant description of shape evolution and shape coexistence in neutron-rich nuclei at N≈60

The shape evolution and shape coexistence phenomena in neutron-rich nuclei at N≈ 60, including Kr, Sr, Zr, and Mo isotopes, are studied in the covariant density functional theory (DFT) with the new parameter set PC-PK1. Pairing correlations are treated using the BCS approximation with a separable pairing force. Sharp rising in the charge radii of Sr and Zr isotopes at N= 60 is observed and shown to be related to the rapid changing in nuclear shapes. The shape evolution is moderate in neighboring Kr and Mo isotopes. Similar as the results of previous Hartree-Fock-Bogoliubov (HFB) calculations with the Gogny force, triaxiality is observed in Mo isotopes and shown to be essential to reproduce quantitatively the corresponding charge radii. In addition, the coexistence of prolate and oblate shapes is found in both Sr98 and Zr100. The observed oblate and prolate minima are related to the low single-particle energy level density around the Fermi surfaces of neutron and proton respectively. Furthermore, the 5-dimensional (5D) collective Hamiltonian determined by the calculations of the PC-PK1 energy functional is solved for Sr98 and Zr100. The resultant excitation energy of 02+ state and E0 transition strength Ρ2(E0;02+→01+) are in rather good agreement with the data. It is found that the lower barrier height separating the two competing minima along the γ deformation in Zr100 gives rise to the larger Ρ2(E0;02+→01+) than that in Sr98.

Anatomy of molecular structures in 20 Ne

Abstract We present a beyond mean-field study of clusters and molecular structures in low-spin states of 20Ne with a multireference relativistic energy density functional, where the dynamical correlation effects of symmetry restoration and quadrupole–octupole shapes fluctuation are taken into account with projections on parity, particle number and angular momentum in the framework of the generator coordinate method. Both the energy spectrum and the electric multipole transition strengths for low-lying parity-doublet bands are better reproduced after taking into account the dynamical octupole vibration effect. Consistent with the finding in previous studies, a rotation-induced dissolution of the α + O 16 molecular structure in 20Ne is predicted.

Beyond relativistic mean-field approach for nuclear octupole excitations

We report the first beyond-mean-field study of low-lying parity-doublet states in 224Ra by extending the multireference relativistic energy density functional method to include dynamical correlations related to symmetry restoration and quadrupole-octupole shape fluctuation with a generator coordinate method combined with parity, particle-number, and angular-momentum projections. We clarify full microscopically that the origin of spin-dependent parity splitting in low-spin states is related to the octupole shape stabilization of positive-parity states, the dominated shapes of which drift gradually to that of negative-parity ones.

Triaxially deformed relativistic point-coupling model for

The impurity effect of hyperon on atomic nuclei has received a renewed interest in nuclear physics since the first experimental observation of appreciable reduction of

\Lambda

E2

hypernuclei: a quantitative analysis of hyperon impurity effect on nuclear collective properties

transition strength in low-lying states of hypernucleus