Excited states of zero seniority based on a pair condensate

TL;DR

This paper investigates zero seniority excited states in like-particle systems using pair condensate models, identifying two types of excitations and their relation to experimental data and giant pairing vibrations.

Contribution

It introduces a detailed analysis of two types of zero seniority excited states based on pair condensates, linking them to experimental observations and giant pairing vibrations.

Findings

First type states match well with known J=0 states in $^{108}$Sn.

Excited pair condensate states appear at higher energies without simple eigenstate correspondence.

EPC states at high energy resemble giant pairing vibrations.

Abstract

We study the excited states of zero seniority for various like-particle systems interacting by pairing forces and by general two-body interactions. We consider two types of excitations, generated from a ground state described by a pair condensate. One type is obtained by breaking a pair from the ground state condensate and replacing it by "excited" collective pairs built on time-reversed single-particle orbits. The second type of zero seniority excited states is described by a condensate of identical excited pairs. The structure of these excited states is analysed for the picked fence model and for the valence neutrons of $^{108}$Sn. For a state-depending pairing interaction, the first type of excited states agree well with the J=0 states which are known in $^{108}$Sn. At the same time, these states can be also associated unambiguously with those exact states which are the closest in energy to the experimental levels. The states corresponding to the excited pair condensate appear at low energies, around the energy of the second excited state of the first type, and they do not have a simple correspondence with exact eigenstates. However, at a much higher excitation energy there is an exact state which is similar in structure to an EPC state. It is shown that this EPC state has the features of a giant pairing vibration.