Possible existence of pygmy dipole resonance built on excited states in a neutron-rich ${}^{80}$Ge nucleus

Abstract

The pygmy dipole resonance (PDR) at the low-lying tail of the giant dipole resonance (GDR) is an interesting research subject as it carries important information about the nuclear surface with mixed isoscalar and isovector vibrations in $N>Z$ systems. The present paper investigates the possible existence of the PDR built on excited states in a neutron-rich ${}^{80}$Ge nucleus using the phonon damping model (PDM) with and without pairing correlation at zero and finite temperatures. The results obtained within the PDM with exact pairing (EP+PDM) reveal the appearance of two enhanced $\gamma$-transitions $E_{\gamma }=7.25$ and 7.35 MeV at $T=0.6$ MeV, which carry the PDR nature and hence implying the possible existence of PDR built on excited states in this nucleus. These two $\gamma$-energies nicely match with a recent experimental observation, thus indicating the crucial role of the inclusion of exact pairing solution in the precise description of low-lying $\gamma$-transitions. The partition function-based analysis shows that this predicted PDR at $T=$ 0.6MeV is mainly contributed by the first $2^{+}$ excited state of the ${}^{80}$Ge nucleus. The isospin mixing at the nuclear surface is also observed in the investigated PDR at $T=0.6-0.7$ MeV. The primary mechanism underlying the emergence of the hot PDR is found due to the coupling of GDR phonon to non-collective particle-particle and hole-hole configurations at finite temperatures within the PDM framework, along with the shift in particle-hole excitation energies due to thermal pairing correlations