Observation of a first-order pairing phase transition in atomic nuclei

TL;DR

This paper reveals that atomic nuclei exhibit a first-order phase transition from superfluid to a normal state, explaining the constant-temperature behavior of nuclear level densities at certain energies.

Contribution

It provides the first fundamental explanation linking nuclear level density behavior to a phase transition, bridging nuclear physics and condensed matter concepts.

Findings

Nuclear level densities follow a constant-temperature shape at certain energies.

Experimental data indicate a first-order phase transition in nuclei.

Results have implications for mesoscopic systems like superconducting clusters.

Abstract

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.