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

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.

5 pages, 4 figures