Coexistence of quartets and pairs in even-even N > Z nuclei

We analyse the structure of the ground states of even-even N > Z nuclei with nucleons moving in the same major shell and interacting via realistic two-body forces of shell model type. We describe the ground states of these nuclei as a product of two terms, one representing the proton-neutron subsystem with an equal number of protons and neutrons and the other one associated with the excess neutrons. The first term is expressed by means of quartets made of two neutrons and two protons coupled to total isospin T = 0. The second term is represented as a neutron pair condensate. Quartets and pairs are constructed variationally. The accuracy of this approximation is discussed for nuclei with valence nucleons in the sd and pf major shells.(c) 2023 Elsevier B.V. All rights reserved.