What is a Cooper pair ?

TL;DR

This paper examines the fundamental nature of Cooper pairs in the BCS-BEC crossover, addressing ongoing controversies and clarifying their role in correlated Fermi systems, especially in ultracold atomic gases.

Contribution

It provides a fundamental discussion on what constitutes a Cooper pair in correlated Fermi systems, clarifying theoretical ambiguities in the context of ultracold gases.

Findings

Highlights the controversy over the nature of Cooper pairs.

Discusses the elementary understanding of Cooper pairs in correlated systems.

Addresses recent experimental observations of preformed pairs.

Abstract

Recently, the nature of Cooper pairs in the BCS-BEC crossover has regained attention due to the observation of a large fraction of preformed fermion pairs on the BCS side of the Feshbach resonance in ultracold atomic Fermi gases. While several theoretical explanations were proposed, the interpretations are still controversial. The root of the controversy is understanding what represents a Cooper pair in a correlated Fermi system. This paper discusses these issues at the most elementary level.