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

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.