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D. C. Zheng

Publications and source records attributed to D. C. Zheng.

14 recordsLinked to original sources
arXiv1996

Non-trivial aspects of the onset of nuclear collectivity: Static moments

We consider several topics concerning static magnetic dipole and electric quadrupole moments ($μ$ and $Q$) as signatures of the onset of nuclear collectivity. Having previously noted that in $^{50}\mbox{Cr}$ there is an abrupt change of sign in $Q$ of yrast states with $J^π=10^+, 12^+$, and $14^+$ relative to lower $J$ states, we discuss whether these states are oblate or prolate. We next show that configuration mixing leads to much larger changes in $Q$ than in $μ$. We then look for other bands of interest in $^{50}\mbox{Cr}$. Finally we discuss the Jolos-von Brentano relationship which relates $Q$ of $2^+_1$ states to $B({\rm E2})$'s for transitions from and to the $2^+_1$ states.

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arXiv1995

Large-basis shell model studies of light nuclei with a multi-valued G-matrix effective interaction

Large-basis shell model studies of low-lying excitations in light nuclei from 4He to 7Li have been performed with a multi-valued G-matrix effective interaction, as recently suggested by Haxton et al.. Calculations were performed relative to the vacuum (``no core") using very large, separable model spaces containing all excitations with unperturbed energies up to 8\hbarΩ. Using G matrices derived from a new Nijmegen potential, we achieve a very satisfactory description of these excitations.

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arXiv1995

Change of shape in the yrast sequence in 50Cr

In shell model calculations for the yrast even spin states of 50Cr, the static quadrupole moments of the low spin states 2+_1 and 4+_1 are negative but those of the high spin states 10+_1, 12+_1 and 14+_1 are positive. Even spin states beyond the single j shell limit, J_max=14, again have negative moments. While the B(E2)'s for the J -> J-2 transitions are strongest along the yrast path for J <= 8, it is found that the transition from the second J=10 state to the first J=8 state (10+_2 -> 8+_1) is much stronger than the 10+_1 -> 8+_1 transition. We also note that while the 16+_1 -> 14+_1 transition is weak, the 16+_1 -> 14+_2 is quite strong.

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arXiv1994

Auxiliary potential in no-core shell-model calculations

The Lee-Suzuki iteration method is used to include the folded diagrams in the calculation of the two-body effective interaction $v^{(2)}_{\rm eff}$ between two nucleons in a no-core model space. This effective interaction still depends upon the choice of single-particle basis utilized in the shell-model calculation. Using a harmonic-oscillator single-particle basis and the Reid-soft-core {\it NN} potential, we find that $v^{(2)}_{\rm eff}$ overbinds $^4\mbox{He}$ in 0, 2, and $4\hbarΩ$ model spaces. As the size of the model space increases, the amount of overbinding decreases significantly. This problem of overbinding in small model spaces is due to neglecting effective three- and four-body forces. Contributions of effective many-body forces are suppressed by using the Brueckner-Hartree-Fock single-particle Hamiltonian.

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arXiv1994

Self weakening of the tensor interaction in a nucleus

We examine several ``landmarks'' for the effects of the tensor interaction on the properties of light nuclei. These were usually discussed in the context of small-space shell-model calculations. We show, using $G$ matrices derived from a realistic nucleon-nucleon potential, that when the model space is small (e.g., $0\hbarω$), these effects are overestimated, indicating that the tensor interaction is too strong. However, when larger spaces are used, there is a diminishing of these effects, which, in general, leads to better agreement with experiment.

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arXiv1994

Large-space shell-model calculations for light nuclei

An effective two-body interaction is constructed from a new Reid-like $NN$ potential for a large no-core space consisting of six major shells and is used to generate the shell-model properties for light nuclei from $A$=2 to 6. (For practical reasons, the model space is partially truncated for $A$=6.) Binding energies and other physical observables are calculated and compare favorably with experiment.

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arXiv1993

Simple approximation for the starting-energy-independent two-body effective interaction with applications to 6Li

We apply the Lee-Suzuki iteration method to calculate the linked-folded diagram series for a new Nijmegen local NN potential. We obtain an exact starting-energy-independent effective two-body interaction for a multi-shell, no-core, harmonic-oscillator model space. It is found that the resulting effective-interaction matrix elements can be well approximated by the Brueckner G-matrix elements evaluated at starting energies selected in a simple way. These starting energies are closely related to the energies of the initial two-particle states in the ladder diagrams. The ``exact'' and approximate effective interactions are used to calculate the energy spectrum of 6Li in order to test the utility of the approximate form.

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arXiv1993

Higher-shell corrections for systems with one and two valence nucleons: A spin-orbit and tensor interaction analysis

It is shown that when higher-shell admixtures are included for systems with two valence particles or holes, there are effects which are quite different from those for one-valence-nucleon systems. For example, for nuclei with one valence particle or hole, there is no first-order correction for the magnetic dipole moment or the Gamow-Teller transition amplitude. However for nuclei with two valence particles or holes, one can get substantial corrections. The effects of the tensor and spin-orbit interactions in core renormalization are emphasized. We find that in 6Li, the spin-orbit interaction causes the quadrupole moment of the J=1+ ground state to be positive, but the tensor interaction causes it to be negative. The G-matrices derived from realistic interactions are employed in these calculations.

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arXiv1993

Nuclear shell-model calculations for 6Li and 14N with different NN potentials

Two ``phase-shift equivalent'' local NN potentials with different parametrizations, Reid93 and NijmII, which were found to give nearly identical results for the triton by Friar et al, are shown to yield remarkably similar results for 6Li and 14N in a (0+2)hw no-core space shell-model calculation. The results are compared with those for the widely used Hamada-Johnson hard-core and the original Reid soft-core potentials, which have larger deuteron D-state percentages. The strong correlation between the tensor strength and the nuclear binding energy is confirmed. However, many nuclear-structure properties seem to be rather insensitive to the details of the NN potential and, therefore, cannot be used to test various NN potentials. (Submitted to Phys. Rev. C on Nov. 9, 1993 as a Brief Report.)

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arXiv1993

Minimizing Effective Many-Body Interactions

A simple two-level model is developed and used to test the properties of effective interactions for performing nuclear structure calculations in truncated model spaces. It is shown that the effective many-body interactions sensitively depend on the choice of the single-particle basis and they appear to be minimized when a self- consistent Hartree-Fock basis is used.

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arXiv1993

Microscopic Calculations of the Spectra of Light Nuclei

We perform large-space shell-model calculations for the low-lying energy spectra of a few light nuclei, $^4\mbox{He}$, $^5\mbox{He}$, $^6\mbox{Li}$ and $^7\mbox{Li}$, in a no-core model space with a realistic effective two-body interaction (Brueckner G-matrix). Our G-matrices are calculated for the Reid-soft-core potential in a harmonic oscillator basis. Single-particle ``-U'' insertions are replaced by two-particle ``-U'' insertions and are included in the G matrix calculations, which sum them to all orders. With the starting energy of the G-matrix chosen to give approximately the experimental binding energy, we obtain nuclear energy spectra which are in reasonable agreement with experiment. We also investigate the dependence of our results on the size of the model space and on the harmonic oscillator basis parameter ($\hbarΩ$).

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arXiv1993

Medium corrections to the tensor interaction in nuclei: Energy difference of T=1 and T=0 J=0- states in 16O

It has been shown in the past that in a shell model calculation, restricted to 1Hbar*Omega 1p-1h configurations, the energy splitting between the lowest J=0-, T=0 and J=0-, T=1 states in 16O is mainly due to the tensor component of the nucleon-nucleon interaction. In this work, we extend the calculation to include 3Hbar*Omega 2p-2h and 3p-3h configurations. We use realistic G matrices derived from various One-Boson-Exchange potentials taking into account the effects of the Dirac-Brueckner-Hartree-Fock approach, i.e., a modification of the Dirac spinors for the nucleons in the medium. It is found that the splitting is rather sensitive to the long-range components of the isovector tensor force. The splitting is reduced considerably by including 2p-2h configurations, in particular.

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arXiv1993

Correlation between the quenching of total GT+ strength and the increase of E2 strength

Relations between the total beta+ Gamow-Teller (GT+) strength and the E2 strength are further examined. It is found that in shell-model calculations for N=Z nuclei, in which changes in deformation are induced by varying the single-particle energies, the total GT+ or GT- strength decreases monotonically with increasing values of the B(E2) from the ground state to the first excited J=2+ state. Similar trends are also seen for the double GT transition amplitude (with some exceptions) and for the spin part of the total M1 strength as a function of B(E2).

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