Pairing and specific heat in hot nuclei

TL;DR

This paper investigates the pairing phase transition in hot nuclei using a finite-temperature variation after projection BCS approach, showing smooth changes in pairing gap and specific heat, and comparing results with exact diagonalization and experimental data.

Contribution

It introduces a finite-temperature variation after projection BCS method for studying pairing in nuclei, accurately modeling the smooth transition and analyzing specific heat in finite nuclear systems.

Findings

Pairing gap and specific heat vary smoothly with temperature.

FT-VAP results agree well with exact diagonalization.

Model reasonably reproduces experimental specific heat data.

Abstract

The thermodynamics of pairing phase-transition in nuclei is studied in the canonical ensemble and treating the pairing correlations in a finite-temperature variation after projection BCS approach (FT-VAP). Due to the restoration of particle number conservation, the pairing gap and the specific heat calculated in the FT-VAP approach vary smoothly with the temperature, indicating a gradual transition from the superfluid to the normal phase, as expected in finite systems. We have checked that the predictions of the FT-VAP approach are very accurate when compared to the results obtained by an exact diagonalization of the pairing Hamiltonian. The influence of pairing correlations on specific heat is analysed for the isotopes $^{161,162}$Dy and $^{171,172}$Yb. It is shown that the FT-VAP approach, applied with a level density provided by mean field calculations and supplemented, at high energies, by the level density of the back-shifted Fermi gas model, can approximate reasonably well the main properties of specific heat extracted from experimental data. However, the detailed shape of the calculated specific heat is rather sensitive to the assumption made for the mean field.