Electric dipole transitions in the relativistic quasiparticle random phase approximation at finite temperature

Finite temperature results in various effects on the properties of nuclear structure and excitations of relevance for nuclear processes in hot stellar environments. Here we introduce the self-consistent finite temperature relativistic quasiparticle random phase approximation (FT-RQRPA) based on relativistic energy density functional with point coupling interaction for describing the temperature effects in electric dipole (E1) transitions. We perform a study of E1 excitations in the temperature range $T=$ 0-2 MeV for the selected closed- and open-shell nuclei ranging from $^{40}$Ca to $^{60}$Ca and $^{100}$Sn to $^{140}$Sn by including both thermal and pairing effects. The isovector giant dipole resonance strength is slightly modified for the considered range of temperature, while new low-energy peaks emerge for $E<$12 MeV with non-negligible strength in neutron-rich nuclei at high temperatures. The analysis of relevant two-quasiparticle configurations discloses how new excitation channels open due to thermal unblocking of states at finite temperature. The study also examines the isospin and temperature dependence of electric dipole polarizability ($\alpha_D$), resulting in systematic increase in the values of $\alpha_D$ with increasing temperature, with a more pronounced effect observed in neutron-rich nuclei. The FT-RQRPA introduced in this work will open perspectives for microscopic calculation of $\gamma$-ray strength functions at finite temperatures relevant for nuclear reaction studies.