Reentrance of proton-neutron pairing in hot nuclear systems
We develop a generalized finite-temperature proton-neutron BCS framework using the superoperator formalism, incorporating both isovector and isoscalar monopole pairing channels. Numerical calculations for a schematic equidistant multilevel model and realistic even-even Ge isotopes demonstrate the emergence of proton-neutron ($pn$) pairing reentrance in even-even asymmetric ($N>Z$) nuclei with preexisting like-nucleon pairing correlations. This nonmonotonic behavior arises from thermal excitations that partially lift Pauli blocking of single-particle orbitals near the chemical potentials, thereby enlarging the phase space for $pn$ pair formation. We uncover a delicate interplay between thermal unblocking and like-nucleon pairing, which can either suppress or enhance $pn$ correlations depending on temperature and shell filling. A qualitative analysis of Fermi charge-exchange strength functions in hot $^{72}$Ge, which neglects the residual interaction between thermal quasiparticles, suggests that $pn$ pairing reentrance may alter the transition strength distribution around $T\approx1$~MeV. This indicates that finite-temperature $pn$ correlations could potentially impact stellar weak-interaction rates in $rp$-process and supernova environments.