Nuclear superfluidity at finite temperature

TL;DR

This paper develops a finite-temperature theoretical framework for nuclear superfluidity, incorporating dynamical interactions and emergent bosonic modes, enabling analysis of pairing gaps beyond traditional BCS theory in medium-heavy nuclei.

Contribution

It introduces a truncated dynamical kernel approach with coupled fermionic and bosonic propagators to study superfluid pairing at finite temperature.

Findings

Derived equations for fermionic propagators including dynamical effects.

Applied framework to medium-heavy nuclei 68Ni and 44,46Ca.

Analyzed temperature dependence of the superfluid pairing gap.

Abstract

The equation of motion for the two-fermion two-time correlation function in the pairing channel is considered at finite temperature. Within the Matsubara formalism, the Dyson-type Bethe-Salpeter equation (Dyson-BSE) with the frequency-dependent interaction kernel is obtained. Similarly to the case of zero temperature, it is decomposed into the static and dynamical components, where the former is given by the contraction of the bare interaction with the two-fermion density and the latter is represented by the double contraction of the four-fermion two-time correlation function, or propagator, with two interaction matrix elements. The dynamical kernel with the four-body propagator, being formally exact, requires approximations to avoid generating prohibitively complicated hierarchy of equations. We focus on the approximation where the dynamical interaction kernel is truncated on the level of two-body correlation functions, neglecting the irreducible three-body and higher-rank correlations. Such a truncation leads to the dynamical kernel with the coupling between correlated fermionic pairs, which can be interpreted as emergent bosonic quasibound states, or phonons, of normal and superfluid nature. The latter ones are, thus, the mediators of the dynamical superfluid pairing. In this framework, we obtained the closed system of equations for the fermionic particle-hole and particle-particle propagators. This allows us to study the temperature dependence of the pairing gap beyond the Bardeen-Cooper-Schrieffer approximation, that is implemented for medium-heavy nuclear systems. The cases of 68Ni and 44,46Ca are discussed in detail.