Approaching itinerant magnetic quantum criticality through a Hund's coupling induced electronic crossover in the YFe${}_2$Ge${}_2$ superconductor

Abstract

Here, by conducting a systematic ${}^{89}$Y NMR study, we explore the nature of the magnetic ground state in a newly discovered iron-based superconductor YFe${}_2$Ge${}_2$. An incoherent-to-coherent crossover due to the Hund's coupling induced electronic correlation is revealed below the crossover temperature $T^{\ast }\sim 75\pm 15\mathrm{K}$. During the electronic crossover, both the Knight shift ($K$) and the bulk magnetic susceptibility ($\chi$) exhibit a similar nonmonotonic temperature dependence, and a so-called Knight shift anomaly is also revealed by a careful $K$-$\chi$ analysis. Such an electronic crossover has been also observed in heavily hole-doped pnictide superconductors \emph{A}Fe${}_2$As${}_2$ (\emph{A} = K, Rb, and Cs), which is ascribed to the Hund's coupling induced electronic correlation. Below $T^{\ast }$, the spin-lattice relaxation rate divided by temperature $(1/T_{1}T)$ shows a similar suppression as the Knight shift, suggesting the absence of critical spin fluctuations. This seems to be in conflict with a predicted magnetic quantum critical point (QCP) near this system. However, considering a $\mathbf{q}$-dependent "filter" effect on the transferred hyperfine field, a predominant spin fluctuation with A-type correlation would be perfectly filtered out at ${}^{89}$Y sites, which is consistent with the recent inelastic neutron scattering results. Therefore, our results confirm that, through a Hund's coupling induced electronic crossover, the magnetic ground state of YFe${}_2$Ge${}_2$ becomes close to an itinerant magnetic QCP with A-type spin fluctuations. In addition, the possible superconducting pairing due to spin fluctuations is also discussed.

6 pages, 4 figures, supplementary information available upon request