Shannon entropy of optimized proton-neutron pair condensates

TL;DR

This paper investigates the nature of proton-neutron pair condensates in N=Z nuclei using Shannon entropy to identify phase transitions and compare with like-nucleon condensates, revealing distinct entanglement properties.

Contribution

It introduces an explicit formalism for optimizing proton-neutron pair condensates with good particle numbers and analyzes their entanglement entropy, highlighting phase transition possibilities.

Findings

Like-nucleon condensates show high entanglement entropy in semi-magic nuclei.

Proton-neutron condensates tend to resemble Hartree-Fock solutions with low entropy.

Artificial pairing interactions can induce a transition from T=1 to T=0 pairing phases.

Abstract

Proton-neutron pairing and like-nucleon pairing are two different facets of atomic nuclear configurations. While like-nucleon pair condensates manifest their superfluidic nature in semi magic nuclei, it is not absolutely clear if there exists a T=0 proton-neutron pair condensate phase in $N=Z$ nuclei. With an explicit formalism of general pair condensates with good particle numbers, we optimize proton-neutron pair condensates for all $N=Z$ nuclei between $^{16}$O and $^{100}$Sn, given shell model effective interactions. As comparison, we also optimize like-nucleon pair condensates for their semi-magic isotones. Shannon entanglement entropy is a measurement of mixing among pair configurations, and can signal intrinsic phase transition. It turns out the like-nucleon pair condensates for semi-magic nuclei have large entropies signaling an entangled phase, but the proton-neutron pair condensates end up not far from a Hartree-Fock solution, with small entropy. With artificial pairing interaction strengths, we show that the general proton-neutron pair condensate can transit from an entangled T=1 phase to an entangled T=0 phase, i.e. pairing phase transition driven by external parameters. In the T=0 limit, the proton-neutron pair condensate optimized for $^{24}$Mg turns out to be a purely P pair condensate with large entanglement entropy, although such cases may occur in cold atom systems, unlikely in atomic nuclei.