# Hexadecapole fluctuation mechanism for s-wave heavy fermion superconductor CeCu2Si2: Interplay between intra- and inter-orbital Cooper pairs

## TL;DR

This paper proposes a new mechanism for s-wave superconductivity in CeCu2Si2, showing that hexadecapole fluctuations driven by vertex corrections induce strong on-site Coulomb attraction, leading to fully-gapped pairing.

## Contribution

It introduces a multipole fluctuation theory highlighting hexadecapole fluctuations as the origin of pairing in heavy-fermion superconductors, without phonon involvement.

## Key findings

- Hexadecapole fluctuations mediate attractive pairing.
- Superconductivity driven by Coulomb repulsion alone.
- Fully-gapped s-wave state explained by multipole interactions.

## Abstract

In heavy-fermion superconductors, it is widely believed that the superconducting gap function has sign-reversal due to the strong electron correlation. However, recently discovered fully-gapped s-wave superconductivity in CeCu2Si2 has clarified that strong attractive pairing interaction can appear even in heavy-fermion systems. To understand the origin of attractive force, we develop the multipole fluctuation theory by focusing on the inter-multipole many-body interaction called the vertex corrections. By analyzing the periodic Anderson model for CeCu2Si2, we find that hexadecapole fluctuations mediate strong attractive pairing interaction. Therefore, fully-gapped s-wave superconductivity is driven by pure on-site Coulomb repulsion, without introducing electron-phonon interactions. The present theory of superconductivity will be useful to understand rich variety of the superconducting states in heavy fermion systems.