arXiv subjects

Z. P. Li

Publications and source records attributed to Z. P. Li.

At least 37 records · Page 2Linked to original sources

arXiv2019

Superheavy nuclei in microscopic collective Hamiltonian approach: the impact of beyond mean field correlations on the ground state and fission properties

The impact of beyond mean field effects on the ground state and fission properties of superheavy nuclei has been investigated in a five-dimensional collective Hamiltonian based on covariant density functional theory. The inclusion of dynamical correlations reduces the impact of the $Z=120$ shell closure and induces substantial collectivity for the majority of the $Z=120$ nuclei which otherwise are spherical at the mean field level (as seen in the calculations with the PC-PK1 functional). Thus, they lead to a substantial convergence of the predictions of the functionals DD-PC1 and PC-PK1 which are different at the mean field level. On the contrary, the predictions of these two functionals remain distinctly different for the $N=184$ nuclei even when dynamical correlations are included. These nuclei are mostly spherical (oblate) in the calculations with PC-PK1 (DD-PC1). Our calculations for the first time reveal significant impact of dynamical correlations on the heights of inner fission barriers of superheavy nuclei with soft potential energy surfaces, the minimum of which at the mean field level is located at spherical shape. These correlations affect the fission barriers of the nuclei, which are deformed in the ground state at the mean field level, to a lesser degree.

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arXiv2018

Shape evolution and coexistence in neutron-deficient Nd and Sm nuclei

The evolution of shapes and low-energy shape coexistence is analyzed in neutron-deficient Nd and Sm nuclei, using a five-dimensional quadrupole collective Hamiltonian (5DCH). Deformation energy surfaces, calculated with the relativistic energy density functional PC-PK1 and a separable finite-range pairing interaction, exhibit a transition from spherical shapes near $N=80$, to $γ-$soft shapes, and to prolate deformed minima in lighter isotopes. The corresponding 5DCH model calculation, based on the self-consistent mean-field potentials, reproduces the empirical isotopic trend of characteristic collective observables, and predicts significantly different deformations for the first two $0^+$ states in the $N=74$ isotones $^{134}$Nd and $^{136}$Sm. In addition to bands based on the triaxial $γ-$soft ground state, in excellent agreement with data, the occurrence of a low-energy rotational band is predicted, built on the prolate deformed ($β\sim0.4$) excited state $0^+_2$.

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arXiv2018

Microscopic description of triaxiality in Ru isotopes with covariant energy density functional theory

The triaxiality in nuclear low-lying states has attracted great interests for many years. Recently, the reduced transition probabilities for levels near the ground state in $^{110}$Ru have been measured and provided strong evidences for a triaxial shape of this nucleus. The aim of this work is to provide a microscopic study of low-lying states for the Ru isotopes with $A\sim100$ and to examine in detail the role of triaxiality, and the evolution of quadrupole shapes with the isospin and spin degrees of freedom. The low-lying excitation spectra and transition probabilities of even-even Ru isotopes are described at the beyond mean-field level by solving a five-dimensional collective Hamiltonian with parameters determined by constrained self-consistent mean-field calculations based on the relativistic energy density functional PC-PK1. The calculated energy surfaces, low-energy spectra, intraband and interband transition rates, as well as some characteristic collective observables, such as $E(4_{\rm g.s.}^+)/E(2^+_{\rm g.s.})$, $E(2^+_γ)/E(4^+_{\rm g.s.})$, $B(E2; 2^+_{\rm g.s.}\to 0^+_{\rm g.s.})$, and $γ$ band staggerings are in a good agreement with the available experimental data. The main features of the experimental low-lying excitation spectra and electric transition rates are well reproduced, and thus strongly support the onset of triaxiality in the low-lying excited states of the Ru isotopes around $^{110}$Ru.

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arXiv2018

Nuclear quantum shape-phase transitions in odd-mass systems

Microscopic signatures of nuclear ground-state shape phase transitions in odd-mass Eu isotopes are explored starting from excitation spectra and collective wave functions obtained by diagonalization of a core-quasiparticle coupling Hamiltonian based on energy density functionals. As functions of the physical control parameter -- the number of nucleons -- theoretical low-energy spectra, two-neutron separation energies, charge isotope shifts, spectroscopic quadrupole moments, and $E2$ reduced transition matrix elements accurately reproduce available data, and exhibit more pronounced discontinuities at neutron number $N=90$, compared to the adjacent even-even Sm and Gd isotopes. The enhancement of the first-order quantum phase transition in odd-mass systems can be attributed to a shape polarization effect of the unpaired proton which, at the critical neutron number, starts predominantly coupling to Gd core nuclei that are characterized by larger quadrupole deformation and weaker proton pairing correlations compared to the corresponding Sm isotopes.

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arXiv2017

Spectroscopy of reflection-asymmetric nuclei with relativistic energy density functionals

Quadrupole and octupole deformation energy surfaces, low-energy excitation spectra and transition rates in fourteen isotopic chains: Xe, Ba, Ce, Nd, Sm, Gd, Rn, Ra, Th, U, Pu, Cm, Cf, and Fm, are systematically analyzed using a theoretical framework based on a quadrupole-octupole collective Hamiltonian (QOCH), with parameters determined by constrained reflection-asymmetric and axially-symmetric relativistic mean-field calculations. The microscopic QOCH model based on the PC-PK1 energy density functional and $δ$-interaction pairing is shown to accurately describe the empirical trend of low-energy quadrupole and octupole collective states, and predicted spectroscopic properties are consistent with recent microscopic calculations based on both relativistic and non-relativistic energy density functionals. Low-energy negative-parity bands, average octupole deformations, and transition rates show evidence for octupole collectivity in both mass regions, for which a microscopic mechanism is discussed in terms of evolution of single-nucleon orbitals with deformation.

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arXiv2017

Microscopic core-quasiparticle coupling model for spectroscopy of odd-mass nuclei

Predictions of the spectroscopic properties of low-lying states are critical for nuclear structure studies, but are problematic for nuclei with an odd nucleon due to the interplay of the unpaired single particle with nuclear collective degrees of freedom. In this work, a microscopic core-quasiparticle coupling (CQC) model based on the covariant density functional theory is developed that contains the collective excitations of even-mass cores and spherical single-particle states of the odd nucleon as calculated from a quadrupole collective Hamiltonian combined with a constrained triaxial relativistic Hartree-Bogoliubov model. Predictions of the new model for excitation energies, kinematic and dynamic moments of inertia, and transition rates are shown to be in good agreement with results of low-lying spectroscopy measurements of the axially deformed odd-proton nucleus $^{159}$Tb and the odd-neutron nucleus $^{157}$Gd. Future studies with additional nuclei are planned.

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arXiv2017

Microscopic study of induced fission dynamics of $^{226}$Th with covariant energy density functionals

Static and dynamic aspects of the fission process of $^{226}$Th are analyzed in a self-consistent framework based on relativistic energy density functionals. Constrained relativistic mean-field (RMF) calculations in the collective space of axially symmetric quadrupole and octupole deformations, based on the energy density functional PC-PK1 and a $δ$-force pairing, are performed to determine the potential energy surface of the fissioning nucleus, the scission line, the single-nucleon wave functions, energies and occupation probabilities, as functions of deformation parameters. Induced fission dynamics is described using the time-dependent generator coordinate method in the Gaussian overlap approximation. A collective Schrödinger equation, determined entirely by the microscopic single-nucleon degrees of freedom, propagates adiabatically in time the initial wave packet built by boosting the ground-state solution of the collective Hamiltonian for $^{226}$Th. The position of the scission line and the microscopic input for the collective Hamiltonian are analyzed as functions of the strength of the pairing interaction. The effect of static pairing correlations on the pre-neutron emission charge yields and total kinetic energy of fission fragments is examined in comparison with available data, and the distribution of fission fragments is analyzed for different values of the initial excitation energy.

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arXiv2017

Global analysis of quadrupole shape invariants based on covariant energy density functionals

Coexistence of different geometric shapes at low energies presents a universal structure phenomenon that occurs over the entire chart of nuclides. Studies of the shape coexistence are important for understanding the microscopic origin of collectivity and modifications of shell structure in exotic nuclei far from stability. The aim of this work is to provide a systematic analysis of characteristic signatures of coexisting nuclear shapes in different mass regions, using a global self-consistent theoretical method based on universal energy density functionals and the quadrupole collective model. The low-energy excitation spectrum and quadrupole shape invariants of the two lowest $0^{+}$ states of even-even nuclei are obtained as solutions of a five-dimensional collective Hamiltonian (5DCH) model, with parameters determined by constrained self-consistent mean-field calculations based on the relativistic energy density functional PC-PK1, and a finite-range pairing interaction. The theoretical excitation energies of the states: $2^+_1$, $4^+_1$, $0^+_2$, $2^+_2$, $2^+_3$, as well as the $B(E2; 0^+_1\to 2^+_1)$ values, are in very good agreement with the corresponding experimental values for 621 even-even nuclei. Quadrupole shape invariants have been implemented to investigate shape coexistence, and the distribution of possible shape-coexisting nuclei is consistent with results obtained in recent theoretical studies and available data. The present analysis has shown that, when based on a universal and consistent microscopic framework of nuclear density functionals, shape invariants provide distinct indicators and reliable predictions for the occurrence of low-energy coexisting shapes. This method is particularly useful for studies of shape coexistence in regions far from stability where few data are available.

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arXiv2015

Anatomy of molecular structures in $^{20}$Ne

We present a beyond mean-field study of clusters and molecular structures in low-spin states of $^{20}$Ne 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 $α+^{16}$O molecular structure in $^{20}$Ne is predicted.

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arXiv2015

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.

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arXiv2015

Global study of beyond-mean-field correlation energies in covariant energy density functional theory using a collective Hamiltonian method

We report the first global study of dynamic correlation energies (DCEs) associated with rotational motion and quadrupole shape vibrational motion in a covariant energy density functional (CEDF) for 575 even-even nuclei with proton numbers ranging from $Z=8$ to $Z=108$ by solving a five-dimensional collective Hamiltonian, the collective parameters of which are determined from triaxial relativistic mean-field plus BCS calculation using the PC-PK1 force. After taking into account these beyond mean-field DCEs, the root-mean-square (rms) deviation with respect to nuclear masses is reduced significantly down to 1.14 MeV, which is smaller than those of other successful CEDFs: NL3* (2.96 MeV), DD-ME2 (2.39 MeV), DD-ME$δ$ (2.29 MeV) and DD-PC1 (2.01 MeV). Moreover, the rms deviation for two-nucleon separation energies is reduced by $\sim34\%$ in comparison with cranking prescription.

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arXiv2015

Triaxially deformed relativistic point-coupling model for $Λ$ hypernuclei: a quantitative analysis of hyperon impurity effect on nuclear collective properties

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 $E2$ transition strength in low-lying states of hypernucleus $^{7}_Λ$Li. Many more data on low-lying states of $Λ$ hypernuclei will be measured soon for $sd$-shell nuclei, providing good opportunities to study the $Λ$ impurity effect on nuclear low-energy excitations. We carry out a quantitative analysis of $Λ$ hyperon impurity effect on the low-lying states of $sd$-shell nuclei at the beyond-mean-field level based on a relativistic point-coupling energy density functional (EDF), considering that the $Λ$ hyperon is injected into the lowest positive-parity ($Λ_s$) and negative-parity ($Λ_p$) states. We adopt a triaxially deformed relativistic mean-field (RMF) approach for hypernuclei and calculate the $Λ$ binding energies of hypernuclei as well as the potential energy surfaces (PESs) in $(β, γ)$ deformation plane. We also calculate the PESs for the $Λ$ hypernuclei with good quantum numbers using a microscopic particle rotor model (PRM) with the same relativistic EDF. The triaxially deformed RMF approach is further applied in order to determine the parameters of a five-dimensional collective Hamiltonian (5DCH) for the collective excitations of triaxially deformed core nuclei. Taking $^{25,27}_Λ$Mg and $^{31}_Λ$Si as examples, we analyse the impurity effects of $Λ_s$ and $Λ_p$ on the low-lying states of the core nuclei...

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arXiv2014

Covariant density functional analysis of shape evolution in $N =40$ isotones

The structure of low-lying excitation states of even-even $N=40$ isotones is studied using a five-dimensional collective Hamiltonian with the collective parameters determined from the relativistic mean-field plus BCS method with the PC-PK1 functional in the particle-hole channel and a separable paring force in the particle-particle channel. The theoretical calculations can reproduce not only the systematics of the low-lying states along the isotonic chain but also the detailed structure of the spectroscopy in a single nucleus. We find a picture of spherical-oblate-prolate shape transition along the isotonic chain of $N=40$ by analyzing the potential energy surfaces. The coexistence of low-lying excited $0^+$ states has also been shown to be a common feature in neutron-deficient $N=40$ isotones.

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arXiv2014

A microscopic benchmark-study of triaxiality in low-lying states of 76Kr

We report on a seven-dimensional generator coordinate calculation in the two deformation parameters $β$ and $γ$ together with projection on three-dimensional angular momentum and two particle numbers for the low-lying states in $^{76}$Kr. These calculations are based on covariant density functional theory. Excellent agreement is found with the data for the spectrum and the electric multipole transition strengths. This answers the important question of dynamic correlations and triaxiality in a fully microscopic way. We find that triaxial configurations dominate both the ground state and the quasi $γ$-band. This yields a different picture from the simple interpretation in terms of "coexistence of a prolate ground state with an oblate low-lying excited state", which is based on the measured sign of spectroscopic quadrupole moments. This study also provides for the first time a benchmark for the collective Hamiltonian in five dimensions. Moreover, we point out that the staggering phase of the $γ$-band is not a safe signature for rigid triaxiality of the low-energy structure.

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arXiv2014

Global dynamical correlation energies in covariant density functional theory: cranking approximation

The global dynamical correlation energies for 575 even-even nuclei with proton numbers ranging from Z=8 to Z=108 calculated with the covariant density functional theory using the PC-PK1 parametrization are presented. The dynamical correlation energies include the rotational correction energies obtained with the cranking approximation and the quadrupole vibrational correction energies. The systematic behavior of the present correlation energies is in good agreement with that obtained from the projected generator coordinate method using the SLy4 Skyrme force although our values are systematically smaller. After including the dynamical correlation energies, the root-mean-square deviation predicted by the PC-PK1 for the 575 even-even nuclei masses is reduced from 2.58 MeV to 1.24 MeV.

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arXiv2014

Low-energy structure and anti-bubble effect of dynamical correlations in 46Ar

The low-energy structure of 46Ar is of particular interest due to the possible coexistence of different shapes and the possible existence of proton bubble structure. In this work, we apply a beyond relativistic mean-field approach to study the low-energy structure of 46Ar. Correlations beyond the mean field are introduced by configuration mixing of both particle-number and angular-momentum projected axially deformed mean-field states. The low-lying spectroscopy and charge density in laboratory frame are calculated and an excellent agreement with available data is achieved. Even though an evident proton bubble structure is shown in the spherical state of 46Ar, it eventually disappears after taking into account the dynamical correlation effects. Moreover, our results indicate that the existence of proton bubble structure in argon isotopes is unlikely.

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arXiv2013

Effect of pairing correlations on nuclear low-energy structure: BCS and general Bogoliubov transformation

Low-lying nuclear states of Sm isotopes are studied in the framework of a collective Hamiltonian based on covariant energy density functional theory. Pairing correlation are treated by both BCS and Bogoliubov methods. It is found that the pairing correlations deduced from relativistic Hartree-Bogoliubov (RHB) calculations are generally stronger than those by relativistic mean-field plus BCS (RMF+BCS) with same pairing force. By simply renormalizing the pairing strength, the diagonal part of the pairing field is changed in such a way that the essential effects of the off-diagonal parts of the pairing field neglected in the RMF+BCS calculations can be recovered, and consequently the low-energy structure is in a good agreement with the predictions of the RHB model.

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arXiv2013

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 $^{224}$Th 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.

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