Nearly ferromagnetic metal state in the collapsed tetragonal phase of YFe$_2$(Ge,Si)$_2$

TL;DR

This study investigates the magnetic and superconducting properties of YFe$_2$Ge$_{x}$Si$_{2-x}$ compounds, revealing ferromagnetic fluctuations and impurity effects that influence superconductivity in the collapsed tetragonal phase.

Contribution

It provides the first nuclear magnetic resonance analysis of YFe$_2$Ge$_{x}$Si$_{2-x}$, highlighting ferromagnetic fluctuations and impurity effects near a quantum critical point.

Findings

Strong ferromagnetic spin fluctuations in Fe(Ge,Si) layers

Layers are coupled antiferromagnetically

Magnetic impurity effects influence superconductivity

Abstract

The surprising discovery of tripling the superconducting critical temperature of KFe$_2$As$_2$ at high pressures issued an intriguing question of how the superconductivity in the collapsed tetragonal phase differs from that in the non-collapsed phases of Fe-based superconductors. Here we report $^{89}$Y nuclear magnetic resonance study of YFe$_2$Ge$_{x}$Si$_{2-x}$ compounds whose electronic structure is similar to that of iron-pnictide collapsed tetragonal phases already at ambient pressure. Fe(Ge,Si) layers show strong ferromagnetic spin fluctuations whereas layers are coupled antiferromagnetically -- both positioning the studied family close to a quantum critical point. Next, localized moments attributed either to Fe interstitial or antisite defects may account for magnetic impurity pair-breaking effects thus explaining the substantial variation of superconductivity among different YFe$_2$Ge$_2$ samples.