Pairing in excited nuclei: a review.

The present review summarizes the recent studies on the thermodynamic properties of pairing in many-body systems including superconductors, metallic nanosized clusters and/or grains, solid-state materials, focusing on the excited nuclei, that is nuclei at finite temperature and/or angular momentum formed via heavy-ion fusion, $\alpha$-induced fusion reactions, or inelastic scattering of light particles on heavy targets. Because of the finiteness of the systems, several interesting effects of pairing such as nonvanishing pairing gap, smoothing of superfluid-normal phase transition, first and second order phase transitions, pairing reentrance, etc., will be discussed in detail. Influences of exact and approximate thermal pairing on some nuclear properties such as temperature-dependent width of the giant dipole resonance, total level density, and radiative strength function of the $\gamma$-rays emission will be also analyzed. Finally, the first experimental evidence of the pairing reentrance phenomenon in a $^{104}$Pd nucleus as well as its solid-state counterpart of ferromagnets under strong magnetic field will be presented.