Finite-temperature effects in magnetic dipole transitions

Finite-temperature effects in electromagnetic transitions in nuclei contribute to many aspects of nuclear structure and astrophysically relevant nuclear reactions. While electric dipole transitions have already been extensively studied, the temperature sensitivity of magnetic transitions remains largely unknown. This work comprises the study of isovector magnetic dipole excitations ($M1$) occurring between spin-orbit (SO) partner states using the recently developed self-consistent finite-temperature relativistic quasiparticle random-phase approximation (FT-RQRPA) in the temperature range from $T=$ 0 to 2 MeV. The $M1$ strength distributions of ${}_{40-60}\mathrm{Ca}$ and ${}_{100-140}\mathrm{Sn}$ isotopic chains exhibit a considerable temperature dependence. The $M1$ strength peaks shift significantly towards the lower energies due to the decrease in SO splitting energies and weakening of the residual interaction, especially above the critical temperatures where the pairing correlations vanish. By exploring the relevant two-quasiparticle configurations contributing to the $M1$ strength of closed- and open-shell nuclei, new proton and neutron excitation channels between SO partners are observed in low- and high-energy regions due to the thermal unblocking effects around the Fermi level. At higher temperatures, we have noticed an interesting result in ${}_{40,60}\mathrm{Ca}$ nuclei, the appearance of $M1$ excitations, which are forbidden at zero temperature due to fully occupied (or fully vacant) spin-orbit partner states.