Roles of proton-neutron interactions in alpha-like four-nucleon correlations

An extended pairing plus QQ force model, which has been shown to successfully explain the nuclear binding energy and related quantities such as the symmetry energy, is applied to study the alpha-like four-nucleon correlations in 1f7/2 shell nuclei. The double difference of binding energies, which displays a characteristic behavior at N ≈ Z, is interpreted in terms of the alpha-like correlations. Important roles of proton-neutron interactions forming the alpha-like correlated structure are discussed. 21.10Dr;21.60-n;21.60Cs;21.60.Gx Typeset using REVTEX 1 Recently, the present authors [1,2] have proposed a schematic interaction for N ≈ Z nuclei in 1f7/2 and 1g9/2 shell regions and have applied it to explain double differences of binding energies discussed in Refs. [3,4]. The double difference of binding energies of neighboring even-even nuclei was discussed to investigate α-like four-nucleon correlations in early work [5,6]. The calculations in Refs. [1,2] showed that the schematic interaction is applicable to the 1f7/2 shell nuclei where the α-like four-nucleon correlations are very important [7–12]. Our interaction, which is composed of the isospin-invariant monopole pairing (P0), quadrupole pairing (P2), quadrupole-quadrupole (QQ) forces and J-independent isoscalar proton-neutron (p-n) force (V τ=0 πν ), is useful to study the roles of interactions. The purpose of this note is to investigate cooperation of the P0+P2+QQ force and V τ=0 πν , or competition of the p-n and like-nucleon (p-p, n-n) interactions, in the α-like four-nucleon correlations. It is well known that the single j space (f7/2) n is a reasonable model space for the 1f7/2 shell nuclei [13]. We adopted the (f7/2) n model with the P0 +P2 +QQ+V τ=0 πν interaction in Refs. [1,2]. Our effective Hamiltonian which describes valence nucleons outisde the doublyclosed-shell core Ca is given by H = ǫ7/2 n̂+ VPQ + V τ=0 πν , (1) VPQ = V (P0) + V (P2) + V (QQ) = −g0(2j + 1) ∑ κ A†001κA001κ −G2 ∑