BCS ansatz, Bogoliubov approach to superconductivity and Richardson-Gaudin exact wave function

TL;DR

This paper explores the mathematical foundations of the BCS wave function in superconductivity, comparing it with the exact Richardson-Gaudin solutions, and identifies different pairing regimes affecting the superconducting gap.

Contribution

It analytically compares the BCS ansatz with Richardson-Gaudin solutions, revealing new pairing regimes and their impact on the superconducting gap and physical properties.

Findings

BCS ansatz matches the ground state energy in the large system limit.

Identification of super dilute and super dense pairing regimes.

Existence of a density-dependent superconducting gap.

Abstract

The Bogoliubov approach to superconductivity provides a strong mathematical support to the wave function ansatz proposed by Bardeen, Cooper and Schrieffer (BCS). Indeed, this ansatz --- with all pairs condensed into the same state --- corresponds to the ground state of the Bogoliubov Hamiltonian. Yet, this Hamiltonian only is part of the BCS Hamiltonian. As a result, the BCS ansatz definitely differs from the BCS Hamiltonian ground state. This can be directly shown either through a perturbative approach starting from the Bogoliubov Hamiltonian, or better by analytically solving the BCS Schr\"{o}dinger equation along Richardson-Gaudin exact procedure. Still, the BCS ansatz leads not only to the correct extensive part of the ground state energy for an arbitrary number of pairs in the energy layer where the potential acts --- as recently obtained by solving Richardson-Gaudin equations analytically --- but also to a few other physical quantities such as the electron distribution, as here shown. The present work also considers arbitrary filling of the potential layer and evidences the existence of a super dilute and a super dense regime of pairs, with a gap \emph{different} from the usual gap. These regimes constitute the lower and upper limits of density-induced BEC-BCS cross-over in Cooper pair systems.