arXiv:nucl-th/0201009v4 9 May 2002 Interpretation of the Wigner Energy as due to RPA Correlations Kai Neerg˚ard Næstved Gymnasium og HF, Nyg˚ardsvej 43, DK-4700 Næstved, Denmark, neergard@inet.uni2.dk Abstract In a schematic model with equidistant fourfold degenerate single-nucleon levels, a conventional isovector pairing force and a symmetry force, the RPA correlation energy rises almost linearly with the isospinT, thus producing a Wigner term in accordance with the empirical proportionality of the symmetry energy toT(T+ 1). Key words: symmetry energy, Wigner energy, isovector pairing force, symmetry force, isobaric invariance, RPA, Goldstone mode PACS-numbers: 21.10.Dr, 21.60.Jz, 27.40.+z Nearly symmetric nuclei have an extra binding, the so-called Wigner energy, that is not described by the quadratic symmetry term in the semi-empirical mass formula [1]. This is explained in various ways in the literature. Counting the even bonds among supermultiplet degenerate nucleon orbitals, Wigner estimates that the isospin-dependent part of the interaction energy in the ground state of a doubly even nucleus is proportional toT(T+ 4), whereT is the isospin [2]. Talmi proves that for seniority-conserving forces such as the pairing force acting in a singlej-shell, this part of the interaction energy is proportional toT(T+ 1) [3], and Bohr and Mottelson point out that this isospin-dependence arises in general from an interaction proportional to the scalar product of the isospins of the interacting nucleons [4]. A symmetry en- ergy proportional toT(T+ 1) also fits the empirical masses well [5]. Myers and Swiatecki attribute the extra binding of nearly symmetric nuclei to the interaction of neutrons and protons in overlapping orbitals [6]. Shell model calculations with realistic forces are succesful in reproducing the measured binding energies [7], whereas with Skyrme forces, no Wigner term appears in Hartre-Fock-Bogolyubov calculations and only a small one in Hartree-Fock calculations [8]. However, by invoking a particular isoscalar pairing force that Preprint submitted to Elsevier Preprint
breaks geometric symmetries, Satula and Wyss obtain a significant Wigner en- ergy in approximately number-projected Bogolyubov calculations [9]. In finite- temperature Bogulyubov calculations with a Yamaguchi force, R¨opkeet al. get at low temperature in the local density approximation a contribution from neutron-proton pairing to the binding energies in theA= 40 isobaric chain with a maximum atN−Z=−1 and a quite irregular dependence on the asymmetry [10]. The RPA is the leading order correction to the Hartree-Fock-Bogolyubov ap- proximation. I have therefore calculated the RPA correlation energy from the schematic Hamiltonian H=H0−GP†·P+κ 2T2,H0=∑ kστ ǫka† kστakστ. In this expression, the indexkστlabels orthonormal nucleon orbitals, andakστ are the corresponding annihilation operators.kσtakes the valueskandkso that the orbitalkis obtained from the orbitalkby time-reversal, andτis ‘n’ for a neutron orbital or ‘p’ for a proton orbital.Pis the pair annihilation isovector, andTdenotes the total isospin. The former has the coordinates Px= (−Pn+Pp)/√2,Py=−i(Pn+Pp)/√2,Pz=Pnp, Pτ=∑ k akτakτ,Pnp=∑ k (akpakn+aknakp)/√2. The single-nucleon energyǫktakes Ω equidistant values separated byη, and Gandκare coupling constants. To describe states with a given numberAv=∑ kστa† kστakστof valence nucleons and a given isospin, I employ the Routhian R=H−λAv−µTz. It is just for convenience thatTzis chosen here as the isospin-coordinate to be constrained. SinceHis isobarically invariant, one could equivalently constrain the projection ofTon any axis in isospace. Following Marshalek [11] I base the RPA on the Hartree-Bogolyubov (not Fock) self-consistent state derived fromR. This is the Bogolyubov vacuum that minimizesE0−λ〈Av〉 −µ〈Tz〉, where E0=〈H0〉 − |∆|2 G+κ 2〈T〉2,∆=−G〈P〉. At the minimum one has〈Tx〉=〈Ty〉= 0. For large values ofµa product of neutron and proton BCS states is expected. Since this state is invariant under 2
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