Spin-aligned neutron-proton pairs in $N=Z$ nuclei

A study is carried out of the role of the aligned neutron-proton pair with angular momentum J=9 and isospin T=0 in the low-energy spectroscopy of the $N=Z$ nuclei $^{96}$Cd, $^{94}$Ag, and $^{92}$Pd. Shell-model wave functions resulting from realistic interactions are analyzed in terms of a variety of two-nucleon pairs corresponding to different choices of their coupled angular momentum $J$ and isospin $T$. The analysis is performed exactly for four holes ($^{96}$Cd) and carried further for six and eight holes ($^{94}$Ag and $^{92}$Pd) by means of a mapping to an appropriate version of the interacting boson model. The study allows the identification of the strengths and deficiencies of the aligned-pair approximation.