Observation of a first-order pairing phase transition in atomic nuclei
  Experimental nuclear level densities at excitation energies below the neutron threshold follow closely a constant-temperature shape. This dependence is unexpected and poorly understood. In this work, a fundamental explanation of the observed constant-temperature behavior in atomic nuclei is presented for the first time. It is shown that the experimental data portray a first-order phase transition from a superfluid to an ideal gas of non-interacting quasiparticles. Even-even, odd-$A$, and odd-odd level densities show in detail the behavior of gap- and gapless superconductors also observed in solid-state physics. These results and analysis should find a direct application to mesoscopic systems such as superconducting clusters. 
L. G. Moretto, A. C. Larsen, F. Giacoppo, M. Guttormsen, S. Siem, and   A. V. Voinov
nucl-th nucl-ex