Spin-aligned neutron-proton pair mode in atomic nuclei

Abstract

Shell-model calculations using realistic interactions reveal that the ground and low-lying yrast states of the $N=Z$ nucleus ${}_{46}^{92}$Pd are mainly built upon isoscalar neutron-proton pairs, each carrying the maximum angular momentum $J=9$ allowed by the shell $0{g}_{9/2}$, which is dominant in this nuclear region. This structure is different from that found in the ground and low-lying yrast states of all other even-even nuclei studied so far. The low-lying spectrum of excited states generated by such correlated neutron-proton pairs has two distinctive features: (i) the levels are almost equidistant at low energies and (ii) the transition probability $I\ensuremath{\rightarrow}I\ensuremath{-}2$ is approximately constant and strongly selective. This unique mode is shown to replace normal isovector pairing as the dominant coupling scheme in $N=Z$ nuclei approaching the doubly magic nucleus ${}^{100}$Sn.