Shape evolution of 72,74Kr with temperature in covariant density functional theory

The rich phenomena of deformations in neutron-deficient krypton isotopes, such as shape evolution with neutron number and shape coexistence, have attracted the interest of nuclear physicists for decades. It is interesting to study such shape phenomena using a novel way, e.g. by thermally exciting the nucleus. In this work, we develop the finite temperature covariant density functional theory for axially deformed nuclei with the treatment of pairing correlations by the BCS approach, and apply this approach for the study of shape evolution in 72,74Kr with increasing temperature. For 72Kr, with temperature increasing, the nucleus firstly experiences a relatively quick weakening in oblate deformation at temperature T-0.9 MeV, and then changes from oblate to spherical at T-2.1 MeV. For 74Kr, its global minimum is at quadrupole deformation β2--0.14 and abruptly changes to spherical at T-1.7 MeV. The proton pairing transition occurs at critical temperature 0.6 MeV following the rule Tc =0.6△p （0）, where △p（0） is the proton pairing gap at zero temperature. The signatures of the above pairing transition and shape changes can be found in the specific heat curve. The single-particle level evolutions with temperature are presented.