arXiv subjects
Publications and source records attributed to J. Meng.
The recently developed model of three-dimensional angular momentum projection plus generator coordinate method on top of triaxial relativistic mean-field states has been applied to study the low-lying states of $^{30}$Mg. The effects of triaxiality on the low-energy spectra and E0 and E2 transitions are examined.
The recently developed structure model that uses the generator coordinate method to perform configuration mixing of angular-momentum projected wave functions, generated by constrained self-consistent relativistic mean-field calculations for triaxial shapes (3DAMP+GCM), is applied in a systematic study of ground states and low-energy collective states in the even-even magnesium isotopes $^{20-40}$Mg. Results obtained using a relativistic point-coupling nucleon-nucleon effective interaction in the particle-hole channel, and a density-independent $δ$-interaction in the pairing channel, are compared to data and with previous axial 1DAMP+GCM calculations, both with a relativistic density functional and the non-relativistic Gogny force. The effects of the inclusion of triaxial degrees of freedom on the low-energy spectra and E2 transitions of magnesium isotopes are examined.
The framework of relativistic energy density functionals is extended to include correlations related to the restoration of broken symmetries and to fluctuations of collective variables. The generator coordinate method is used to perform configuration mixing of angular-momentum projected wave functions, generated by constrained self-consistent relativistic mean-field calculations for triaxial shapes. The effects of triaxial deformation and of $K$-mixing is illustrated in a study of spectroscopic properties of low-spin states in $^{24}$Mg.
The rapid transition between spherical and $γ$-soft shapes in Ba and Xe nuclei in the mass region $A \geq 130$ is analyzed using excitation spectra and collective wave functions obtained by diagonalization of a five-dimensional Hamiltonian for quadrupole vibrational and rotational degrees of freedom, with parameters determined by constrained self-consistent relativistic mean-field calculations for triaxial shapes. The results reproduce the characteristic evolution of excitation spectra and E2 transition probabilities and, in general, a good agreement with available data is obtained. The calculated spectra display fingerprints of a second-order shape phase transition that can approximately be described by analytic solutions corresponding to the E(5) dynamical symmetry.
The potential energy surfaces of even-even $^{146-156}$Sm are investigated in the constrained reflection-asymmetric relativistic mean-field approach with parameter set PK1. It is shown that the critical-point candidate nucleus $^{152}$Sm marks the shape/phase transition not only from U(5) to SU(3) symmetry, but also from the octupole-deformed ground state in $^{150}$Sm to the quadrupole-deformed ground state in $^{154}$Sm. By including the octupole degree of freedom, an energy gap near the Fermi surface for single-particle levels in $^{152}$Sm with $β_2 = 0.14 \sim 0.26$ is found, and the important role of the octupole deformation driving pair $ν2f_{7/2}$ and $ν1i_{13/2}$ is demonstrated.
The one-pion exchange current corrections to isoscalar and isovector magnetic moments of double-closed shell nuclei plus and minus one nucleon with $A=15,17,39$ and 41 have been studied in the relativistic mean field (RMF) theory and compared with previous relativistic and non-relativistic results. It has been found that the one-pion exchange current gives a negligible contribution to the isoscalar magnetic moments but a significant correction to the isovector ones. However, the one-pion exchange current doesn't improve the description of nuclear isovector magnetic moments for the concerned nuclei.
Microscopic signatures of nuclear ground-state shape phase transitions in Nd isotopes are studied using excitation spectra and collective wave functions obtained by diagonalization of a five-dimensional Hamiltonian for quadrupole vibrational and rotational degrees of freedom, with parameters determined by constrained self-consistent relativistic mean-field calculations for triaxial shapes. As a function of the physical control parameter -- the number of nucleons, energy gaps between the ground state and the excited vibrational states with zero angular momentum, isomer shifts, and monopole transition strengths, exhibit sharp discontinuities at neutron number N=90, characteristic of a first-order quantum phase transition.
The single-particle spectrum obtained from the relativistic mean field (RMF) theory is used to extract the shell correction energy with the Strutinsky method. Considering the delicate balance between the plateau condition in the Strutinsky smoothing procedure and the convergence for the total binding energy, the proper space sizes used in solving the RMF equations are investigated in detail by taking 208Pb as an example. With the proper space sizes, almost the same shell correction energies are obtained by solving the RMF equations either on basis space or in coordinate space.
Quantum fluctuations concerning the shape of nuclei are treated within the framework of covariant density functional theory. Long range correlations beyond mean field are taken into account by configuration mixing of wave functions with triaxial shapes and the restoration of spontaneously broken rotational symmetries through three-dimensional angular momentum projection. The "controversial" nucleus 16C is treated as an example and it is found that its ground state has a triaxial shape but with large shape fluctuations. They are of crucial importance for a proper description of the spectroscopic properties of such nuclei.
The multipole response of nuclei at temperatures T=0-2 MeV is studied using a self-consistent finite-temperature RPA (random phase approximation) based on relativistic energy density functionals. Illustrative calculations are performed for the isoscalar monopole and isovector dipole modes and, in particular, the evolution of low-energy excitations with temperature is analyzed, including the modification of pygmy structures. Both for the monopole and dipole modes, in the temperature range T=1-2 MeV additional transition strength appears at low energies because of thermal unblocking of single-particle orbitals close to the Fermi level. A concentration of dipole strength around 10 MeV excitation energy is predicted in $^{60,62}$Ni, where no low-energy excitations occur at zero temperature. The principal effect of finite temperature on low-energy strength that is already present at zero temperature, e.g. in $^{68}$Ni and $^{132}$Sn, is the spreading of this structure to even lower energy and the appearance of states that correspond to thermally unblocked transitions.
The electric dipole response of $^{140}$Ce is investigated using the fully consistent relativistic quasiparticle random phase approximation. By analyzing the isospin structure of the E1 response, it is shown that the low-energy (pygmy) strength separates into two segments with different isospin character. The more pronounced pygmy structure at lower energy is composed of predominantly isoscalar states with surface-peaked transition densities. At somewhat higher energy the calculated E1 strength is primarily of isovector character, as expected for the low-energy tail of the giant dipole resonance. The results are in qualitative agreement with those obtained in recent $(γ,γ')$ and $(α,α'γ)$ experiments, and provide a simple explanation for the splitting of low-energy E1 strength into two groups of states with different isospin structure and radial dependence of the corresponding transition densities.
The configuration-fixed constrained triaxial relativistic mean-field approach is extended by including time-odd fields and applied to study the candidate multiple chiral doublets (MKiD) nucleus 106Rh. The energy contribution from time-odd fields and microscopical evaluation of center-of-mass correction as well as the modification of triaxial deformation parameters beta, gamma due to the time-odd fields are investigated. The contributions of the time-odd fields to the total energy are 0.1-0.3 MeV and they modify slightly the gamma values. However, the previously predicted multiple chiral doublets still exist.
Based on a relativistic mean-field theory with an effective point coupling between the nucleons, three-dimensional angular momentum projection is implemented for the first time to project out states with designed angular momentum from deformed intrinsic states generated by triaxial quadrupole constraints. The same effective parameter set PC-F1 of the effective interaction is used for deriving the mean field and the collective Hamiltonian. Pairing correlations are taken into account by the BCS method using both monopole forces and zero range d-forces with strength parameters adjusted to experimental even-odd mass differences. The method is applied successfully to the isotopes 24Mg, 30Mg, and 32Mg.
The analysis of shape transitions in Nd isotopes, based on the framework of relativistic energy density functionals and restricted to axially symmetric shapes in Ref. \cite{PRL99}, is extended to the region $Z = 60$, 62, 64 with $N \approx 90$, and includes both $β$ and $γ$ deformations. Collective excitation spectra and transition probabilities are calculated starting from a five-dimensional Hamiltonian for quadrupole vibrational and rotational degrees of freedom, with parameters determined by constrained self-consistent relativistic mean-field calculations for triaxial shapes. The results reproduce available data, and show that there is an abrupt change of structure at N=90 that can be approximately characterized by the X(5) analytic solution at the critical point of the first-order quantum phase transition between spherical and axially deformed shapes.
The framework of relativistic energy density functionals is extended to include correlations related to restoration of broken symmetries and fluctuations of collective variables. A new implementation is developed for the solution of the eigenvalue problem of a five-dimensional collective Hamiltonian for quadrupole vibrational and rotational degrees of freedom, with parameters determined by constrained self-consistent relativistic mean-field calculations for triaxial shapes. The model is tested in a series of illustrative calculations of potential energy surfaces and the resulting collective excitation spectra and transition probabilities of the chain of even-even gadolinium isotopes.
A particle rotor model is developed which couples several valence protons and neutrons to a rigid triaxial rotor core. It is applied to investigating the chirality in odd-$A$ nucleus $^{135}$Nd with $πh_{11/2}^2\otimesνh^{-1}_{11/2}$ configuration for the first time in a fully quantal approach. For the two chiral sister bands, the observed energies and the $B(M1)$ and $B(E2)$ values for the in-band as well as interband transitions are reproduced excellently. Root mean square values of the angular momentum components and their probability distributions are used for discussing in detail the chiral geometry of the aplanar rotation and its evolution with angular momentum. Chirality is found to change from a soft chiral vibration to nearly static chirality at spin $I=39/2$ and back to another type of chiral vibration at higher spin.
A full three-dimensional angular momentum projection on top of a triaxial relativistic mean-Geld calculation is implemented for the first time. The underlying Lagrangian is a point coupling model and pairing correlations are taken into account by a monopole force. This method is applied for the low-lying excited states in 24Mg. Good agreement with the experimental data is found for the ground state properties. A minimum in the potential energy surface for the 2+ state, with beta = 0.55, gamma = 10 deg, is used as the basis to investigate the rotational energy spectrum as well as the corresponding B(E2) transition probabilities as compared to the available data.
A new mass table calculated by the relativistic mean field approach with the state-dependent BCS method for the pairing correlation is applied for the first time to study r-process nucleosynthesis. The solar r-process abundance is well reproduced within a waiting-point approximation approach. Using an exponential fitting procedure to find the required astrophysical conditions, the influence of mass uncertainty is investigated. R-process calculations using the FRDM, ETFSI-Q and HFB-13 mass tables have been used for that purpose. It is found that the nuclear physical uncertainty can significantly influence the deduced astrophysical conditions for the r-process site. In addition, the influence of the shell closure and shape transition have been examined in detail in the r-process simulations.