Ab initio computations from $^{78}$Ni towards $^{70}$Ca along neutron number $N=50$

TL;DR

This study uses ab initio coupled-cluster calculations with chiral forces to explore the evolution of nuclear structure from $^{78}$Ni to $^{70}$Ca, revealing a diminishing of the $N=50$ magic number and increased deformation in neutron-rich nuclei.

Contribution

It provides the first ab initio predictions of deformation and low-lying spectra for neutron-rich $N=50$ nuclei approaching $^{70}$Ca, highlighting the erosion of magicity.

Findings

Predicted low-lying rotational band in $^{78}$Ni consistent with recent data.

Deformation observed in $^{76}$Fe, $^{74}$Cr, and $^{72}$Ti ground states.

Flattening of the potential energy landscape in $^{70}$Ca indicating reduced rigidity.

Abstract

We present coupled-cluster computations of nuclei with neutron number $N=50$ "south" of $^{78}$Ni using nucleon-nucleon and three-nucleon forces from chiral effective field theory. We find an erosion of the magic number $N=50$ toward $^{70}$Ca manifesting itself by an onset of deformation and increased complexity in the ground states. For $^{78}$Ni, we predict a low-lying rotational band consistent with recent data, which up until now has been a challenge for ab initio nuclear models. Ground states are deformed in $^{76}$Fe, $^{74}$Cr, and $^{72}$Ti, although the spherical states are too close in energy to unambiguously identify the shape of the ground state within the uncertainty estimates. In $^{70}$Ca, the potential energy landscape from quadrupole-constrained Hartree-Fock computations flattens, and the deformation becomes less rigid. We also compute the low-lying spectra and $B({\rm E2})$ values for these neutron-rich $N=50$ nuclei.