Spin-aligned neutron-proton pair mode in atomic nuclei

TL;DR

This paper reveals a novel spin-aligned neutron-proton pairing mode in the nucleus $^{92}$Pd, which dominates its low-energy structure and differs from traditional pairing schemes, with distinctive spectral features and transition probabilities.

Contribution

It demonstrates that spin-aligned neutron-proton pairs dominate the low-lying states in $^{92}$Pd, replacing conventional pairing models in $N=Z$ nuclei near $^{100}$Sn.

Findings

Low-lying states are nearly equally spaced in energy.

Transition probabilities $I ightarrow I-2$ are approximately constant.

Spin-aligned neutron-proton pairing replaces isovector pairing in $^{92}$Pd.

Abstract

Shell model calculations using realistic interactions reveal that the ground and low-lying yrast states of the $N=Z$ nucleus $^{92}_{46}$Pd are mainly built upon isoscalar neutron-proton pairs each carrying the maximum angular momentum J=9 allowed by the shell $0g_{9/2}$ which is dominant in this nuclear region. This structure is different from the ones 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 $I\rightarrow I-2$ is approximately constant and strongly selective. This unique mode is shown to replace normal isovector pairing as the dominating coupling scheme in $N=Z$ nuclei approaching the doubly-magic nucleus $^{100}$Sn.