Showing papers by "Jie Meng published in 2010"

New parametrization for the nuclear covariant energy density functional with a point-coupling interaction

Abstract: A new parametrization PC-PK1 for the nuclear covariant energy density functional with nonlinear point-coupling interaction is proposed by fitting to observables of 60 selected spherical nuclei, including the binding energies, charge radii, and empirical pairing gaps. The success of PC-PK1 is illustrated in the description of infinite nuclear matter and finite nuclei including the ground-state and low-lying excited states. In particular, PC-PK1 provides a good description for the isospin dependence of binding energy along either the isotopic or the isotonic chain, which makes it reliable for application in exotic nuclei. The predictive power of PC-PK1 is also illustrated for the nuclear low-lying excitation states in a five-dimensional collective Hamiltonian in which the parameters are determined by constrained calculations for triaxial shapes.

Neutron halo in deformed nuclei

Abstract: Halo phenomena in deformed nuclei are investigated within a deformed relativistic Hartree Bogoliubov (DRHB) theory. These weakly bound quantum systems present interesting examples for the study of the interdependence between the deformation of the core and the particles in the halo. Contributions of the halo, deformation effects, and large spatial extensions of these systems are described in a fully self-consistent way by the DRHB equations in a spherical Woods-Saxon basis with the proper asymptotic behavior at a large distance from the nuclear center. Magnesium and neon isotopes are studied and detailed results are presented for the deformed neutron-rich and weakly bound nucleus (44)Mg. The core of this nucleus is prolate, but the halo has a slightly oblate shape. This indicates a decoupling of the halo orbitals from the deformation of the core. The generic conditions for the occurrence of this decoupling effects are discussed.

Configuration mixing of angular-momentum-projected triaxial relativistic mean-field wave functions

Abstract: 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 {sup 24}Mg.

Relativistic Hartree-Fock-Bogoliubov theory with density dependent meson-nucleon couplings

Abstract: Relativistic Hartree-Fock-Bogoliubov (RHFB) theory with density-dependent meson-nucleon couplings is presented. The integrodifferential RHFB equations are solved by expanding the different components of the quasiparticle spinors in the complete set of eigensolutions of the Dirac equations with Woods-Saxon potentials. Using the finite-range Gogny force D1S as an effective interaction in the pairing channel, systematic RHFB calculations are performed for Sn isotopes and $N=82$ isotones. It is demonstrated that an appropriate description of both mean field and pairing effects can be obtained within RHFB theory with finite range Gogny pairing forces. Better systematics are also found in the regions from the stable to the neutron-rich side with the inclusion of Fock terms, especially in the presence of $\ensuremath{\rho}$-tensor couplings.

Open problems in understanding the nuclear chirality

Abstract: Open problems in the interpretation of the observed pair of near-degenerate ΔI = 1 bands with the same parity as the chiral doublet bands are discussed. The ambiguities for the existing fingerprints of the chirality in atomic nuclei and problems in existing theory are discussed, including the description of quantum tunneling in the mean field approximation as well as the deformation, core polarization and configuration of the particle rotor model (PRM). Future developments of the theoretical approach are anticipated.

Nuclear halo structure and pseudospin symmetry

Abstract: Nuclear halo structure and conservation of relativistic symmetry are studied within the framework of the relativistic Hartree-Fock-Bogoliubov theory. Giant halos as well as ordinary ones are found in cerium isotopes close to the neutron drip line. Bridged by the $T=0$ channel, the conservation of pseudospin symmetry (PSS) plays an essential role in stabilizing the neutron halo structures. The Fock terms, especially the $\ensuremath{\rho}$-tensor couplings, not only play a significant role in PSS conservation but also present substantial contributions to the $T=0$ channel, by which the necessity of Fock terms is well demonstrated.

Microscopic description of spherical to γ-soft shape transitions in Ba and Xe nuclei

Abstract: The rapid transition between spherical and γ-soft shapes in Ba and Xe nuclei in the mass region A>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.

Open Problems in Understanding the Nuclear Chirality

Abstract: Open problems in the interpretation of the observed pair of near degenerate $\Delta I = 1$ bands with the same parity as the chiral doublet bands are discussed. The ambiguities for the existing fingerprints of the chirality in atomic nuclei and problems in existing theory are discussed, including the description of quantum tunneling in the mean field approximation as well as the deformation, core polarization and configuration of particle rotor model (PRM). Future developments of the theoretical approach are prospected.

Spin symmetry in Dirac negative-energy spectrum in density-dependent relativistic Hartree-Fock theory

Abstract: The spin symmetry in the Dirac negative-energy spectrum and its origin are investigated for the first time within the density-dependent relativistic Hartree-Fock (DDRHF) theory. Taking the nucleus 16O as an example, the spin symmetry in the negative-energy spectrum is found to be a good approximation and the dominant components of the Dirac wave functions for the spin doublets are nearly identical. In comparison with the relativistic Hartree approximation where the origin of spin symmetry lies in the equality of the scalar and vector potentials, in DDRHF the cancellation between the Hartree and Fock terms is responsible for the better spin symmetry properties and determines the subtle spin-orbit splitting. These conclusions hold even in the case when significant deviations from the G -parity values of the meson-antinucleon couplings occur.

Relativistic energy density functionals: Low-energy collective states of Pu 240 and Er 166

Abstract: The empirical relativistic density-dependent, point-coupling energy density functional, adjusted exclusively to experimental binding energies of a large set of deformed nuclei with A≈150– 180 and A≈230–250, is tested with spectroscopic data for 166Er and 240Pu. Starting from constrained self-consistent triaxial relativistic Hartree-Bogoliubov calculations of binding energy maps as functions of the quadrupole deformation in the β–γ plane, excitation spectra and E2 transition probabilities are calculated as solutions of the corresponding microscopic collective Hamiltonian in five dimensions for quadrupole vibrational and rotational degrees of freedom and compared with available data on low-energy collective states.

Spin symmetry in Dirac negative energy spectrum in density-dependent relativistic Hartree-Fock theory

Abstract: The spin symmetry in the Dirac negative energy spectrum and its origin are investigated for the first time within the density-dependent relativistic Hartree-Fock (DDRHF) theory. Taking the nucleus $^{16}$O as an example, the spin symmetry in the negative energy spectrum is found to be a good approximation and the dominant components of the Dirac wave functions for the spin doublets are nearly identical. In comparison with the relativistic Hartree approximation where the origin of spin symmetry lies in the equality of the scalar and vector potentials, in DDRHF the cancellation between the Hartree and Fock terms is responsible for the better spin symmetry properties and determines the subtle spin-orbit splitting. These conclusions hold even in the case when significant deviations from the G-parity values of the meson-antinucleon couplings occur.

Single-particle resonances in a deformed Dirac equation

Abstract: Single-particle bound and resonant states are obtained by solving the Dirac equation with quadrupole-deformed Woods-Saxon potential in coordinate space with the coupled-channel approach. Taking the m{sup {pi}}=1/2{sup +} resonant states at deformation {beta}=0.1 as examples, the roles of their spherical components have been investigated based on the behaviors of the eigenphases and the corresponding probabilities weighted by the scalar spherical potential. It is shown that the realization of the m{sup {pi}}=1/2{sup +} resonances is supported mainly by the lnot =0 components, and the mixture of the s{sub 1/2} component can lead to the disappearance of some resonances at finite energy. The dominance of the lnot =0 component (d{sub 3/2}) in the small-energy region guarantees the continuation of a certain resonance to the corresponding bound state.

Octupole degree of freedom for the critical-point candidate nucleus Sm152 in a reflection-asymmetric relativistic mean-field approach

Abstract: The potential energy surfaces of even-even {sup 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 {sup 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 {sup 150}Sm to the quadrupole-deformed ground state in {sup 154}Sm. By including the octupole degree of freedom, an energy gap near the Fermi surface for single-particle levels in {sup 152}Sm with beta{sub 2}=0.14approx0.26 is found and the important role of the octupole deformation driving pair nu2f{sub 7/2} and nu1i{sub 13/2} is demonstrated.

Avoid the Tsunami of the Dirac sea in the Imaginary Time Step method

Abstract: The discrete single-particle spectra in both the Fermi and Dirac sea have been calculated by the imaginary time step (ITS) method for the Schrodinger-like equation after avoiding the "tsunami" of the Dirac sea, i.e. the diving behavior of the single-particle level into the Dirac sea in the direct application of the ITS method for the Dirac equation. It is found that by the transform from the Dirac equation to the Schrodinger-like equation, the single-particle spectra, which extend from the positive to the negative infinity, can be separately obtained by the ITS evolution in either the Fermi sea or the Dirac sea. Identical results with those in the conventional shooting method have been obtained via the ITS evolution for the equivalent Schrodinger-like equation, which demonstrates the feasibility, practicality and reliability of the present algorithm and dispels the doubts on the ITS method in the relativistic system.

Lambda and anti-lambda hypernuclei in relativistic mean-field theory

Abstract: Several aspects about Λ-hypernuclei in the relativistic mean field theory, including the effective Λ-nucleon coupling strengths based on the successful effective nucleon-nucleon interaction PK1, hypernuclear magnetic moment and -hypernuclei, have been presented. The effect of tensor coupling in Λ-hypernuclei and the impurity effect of to nuclear structure have been discussed in detail.

Tensor coupling effects on spin symmetry in anti-Lambda spectrum of hypernuclei

Abstract: The effects of $\bar\Lambda\bar\Lambda\omega$-tensor coupling on the spin symmetry of $\bar{\Lambda}$ spectra in $\bar{\Lambda}$-nucleus systems have been studied with the relativistic mean-field theory. Taking $^{12}$C+$\bar{\Lambda}$ as an example, it is found that the tensor coupling enlarges the spin-orbit splittings of $\bar\Lambda$ by an order of magnitude although its effects on the wave functions of $\bar{\Lambda}$ are negligible. Similar conclusions has been observed in $\bar{\Lambda}$-nucleus of different mass regions, including $^{16}$O+$\bar{\Lambda}$, $^{40}$Ca+$\bar{\Lambda}$ and $^{208}$Pb+$\bar{\Lambda}$. It indicates that the spin symmetry in anti-lambda-nucleus systems is still good irrespective of the tensor coupling.

Lambda and Anti-Lambda Hypernuclei in Relativistic Mean-field Theory

Abstract: Several aspects about $\Lambda$-hypernuclei in the relativistic mean field theory, including the effective $\Lambda$-nucleon coupling strengths based on the successful effective nucleon-nucleon interaction PK1, hypernuclear magnetic moment and $\bar\Lambda$-hypernuclei, have been presented. The effect of tensor coupling in $\Lambda$-hypernuclei and the impurity effect of $\bar\Lambda$ to nuclear structure have been discussed in detail.

Single-particle resonances in a deformed relativistic potential

Abstract: The positive-parity single-neutron levels in an axially-deformed relativistic quadrupole Woods-Saxon potential are analyzed. Neutron states are obtained as the solutions of the corresponding single-particle Dirac equation, using the coupled-channels method in the coordinate space. The evolution of the levels close to the continuum threshold and, in particular, the occurrence of single-neutron resonant states as the functions of the axial deformation parameter 0 a (c) 1/2 beta a (c) 1/2 0.5, are examined using the eigenphase representation. Calculations are performed for different values of the radius of the potential (R/r (0))(3), corresponding to a variation of the mass number A.

Coexistence of collective and noncollective structures in Sn118

Abstract: Excited states of $^{118}\mathrm{Sn}$ have been populated in the $^{116}\mathrm{Cd}$($^{7}\mathrm{Li}$,$1p4n$) reaction with a beam energy of $50$ MeV. The previously known level scheme is extended in the low-spin region as well as to higher spins. The systematic behavior of the intruder deformed bands in the Sn isotopes is discussed. Total Routhian surface calculations and configuration-fixed constrained triaxial relativistic mean-field approaches are employed for analysis of the nuclear structure of $^{118}\mathrm{Sn}$.

Exotic Magnetic Rotation in 22 F

Abstract: The rotation structure in exotic neutron-rich nucleus 22F is investigated based on the configuration πd5/2⊗νd5/2−1 with the newly developed tilted axis cranking relativistic mean field theory in a fully microscopic and self-consistent way. The possible existence of magnetic rotation is suggested for 22F via investigating the spectra, the relation between the rotational frequency and the angular momentum, the electromagnetic transition probabilities B(M1) and B(E2) together with the shears mechanism characteristic of magnetic rotation. The effect of the nuclear current is also discussed by comparing the calculation results with and without currents.

Isospin symmetry-breaking corrections for superallowed β decay in relativistic RPA approaches

Abstract: The isospin symmetry-breaking corrections δc obtained with the self-consistent relativistic RPA approaches are presented. It is shown that the proper treatment of the Coulomb mean field is very important for extracting these corrections. The nucleus-independent Ft value, the Vud matrix element and the unitarity of the Cabibbo-Kobayashi-Maskawa matrix are discussed. The effects of neutron-proton mass difference on the isospin symmetry-breaking corrections are also investigated.

New aspects of chiral symmetry breaking in atomic nucleus

Abstract: The significant progresses of the chirality in atomic nuclei are briefly reviewed for both experimental and theoretical sides. A particle rotor model is developed which couples several valence protons and neutrons to a rigid triaxial rotor core and applied to investigating the chirality in odd-A nucleus 135Nd. Static chirality has been shown to be a transient phenomenon surrounding by chiral vibrations. It is found that the B(M1) staggering is associated strongly with the characters of nuclear chirality, i.e., the staggering is weak in chiral vibration region while strong in the static chirality region.

Convergence for Imaginary Time Step evolution in the Fermi and Dirac seas

Abstract: The convergence for the Imaginary Time Step (ITS) evolution with time step is investigated by performing the ITS evolution for the Schrodinger-like equation and the charge-conjugate Schrodinger-like equation deduced from Dirac equation for the single proton levels of 12C in both the Fermi and Dirac seas. For the guaranteed convergence of the ITS evolution to the “exact” results, the time step should be smaller than a “critical” time step Δtc for a given single-particle level. The “critical” time step Δtc is more sensitive to the quantum numbers |κ| than to the energy of the single-particle level. For the single-particle levels with the same κ, their “critical” time steps are in the same order. For the single-particle levels with similar energy, a relatively small (large) “critical” time step for larger (smaller) |κ| is needed. These conclusions can be used in the future self-consistent calculation to optimize the evolution procedure.

New Band Structures in A∼110 Neutron‐Rich Nuclei

Abstract: The high spin states of neutron‐rich nuclei in A∼110 region have been carefully investigated by measuring prompt γ‐γ‐γ coincident measurements populated in the spontaneous fission of 252Cf with the Gammasphere detector array. Many new collective bands have been discovered. In this proceeding paper, we introduce some interesting new band structures recently observed by our cooperative groups, that is, the one‐phonon‐ and two‐phonon γ‐vibrational bands in odd‐A 103Nb, 105Mo and 107Tc, the chiral doublet bands in even‐even 106Mo, 110Ru and 112Ru, and the pseudospin partner bands with in 108Tc. The characteristics of these band structures have been discussed.The high spin states of neutron‐rich nuclei in A∼110 region have been carefully investigated by measuring prompt γ‐γ‐γ coincident measurements populated in the spontaneous fission of 252Cf with the Gammasphere detector array. Many new collective bands have been discovered. In this proceeding paper, we introduce some interesting new band structures recently observed by our cooperative groups, that is, the one‐phonon‐ and two‐phonon γ‐vibrational bands in odd‐A 103Nb, 105Mo and 107Tc, the chiral doublet bands in even‐even 106Mo, 110Ru and 112Ru, and the pseudospin partner bands with in 108Tc. The characteristics of these band structures have been discussed.

One-pion exchange current corrections for nuclear magnetic moments in relativistic mean field theory

Abstract: 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.

Low-lying states in $^{30}$Mg: a beyond relativistic mean-field investigation

Abstract: 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.