Spin and Valence-Fluctuation Mediated Superconductivity in Pressurized Fe and CeCu2(Si/Ge)2

TL;DR

This paper reviews evidence for valence-fluctuation and spin-fluctuation mediated superconductivity in pressurized Ce-based compounds and iron, highlighting the distinct and overlapping regimes of these phenomena and their unconventional nature.

Contribution

It provides a comparative analysis of valence and spin fluctuation mechanisms in CeCu2Si2, CeCu2Ge2, and iron under pressure, emphasizing their roles in superconductivity.

Findings

Valence and magnetic critical points are well separated in CeCu2Si2 and CeCu2Ge2.

Superconductivity in iron appears between 15-30 GPa with Tc up to 3 K.

Unconventional superconductivity likely mediated by ferromagnetic spin fluctuations.

Abstract

We review the evidence supporting valence-fluctuation mediated superconductivity in CeCu2Si2 and CeCu2Ge2, where Tc reaches 2.4 K at high pressure. In these systems the valence and magnetic critical points, at p_V and p_c respectively, are well separated. Characteristic signatures associated with both phenomena are distinct. In contrast, the valence and spin fluctuation regions appear much closer in most Ce based compounds. Concerning d-transition metals, superconductivity in pure iron emerges in the pressure window 15-30 GPa with the onset of Tc up to almost 3 K. All relevant observations point to unconventional superconductivity, likely mediated by ferromagnetic spin fluctuations.