Friedel-like oscillations from interstitial iron in superconducting $\text{Fe}_{ 1+y}$ $\text{Te}_{0.62}$ $\text{Se}_{0.38}$

Abstract

Using polarized and unpolarized neutron scattering, we show that interstitial Fe in superconducting $\text{Fe}_{1+y}$ $\text{Te}_{1−x}$ $\text{Se}_x$ induces a magnetic Friedel-like oscillation that diffracts at $Q_⊥=(\frac{1}{2} 0)$ and involves > 50 neighboring Fe sites. The interstitial >$2μ_B$ moment is surrounded by compensating ferromagnetic four-spin clusters that may seed double stripe ordering in $\text{Fe}_{1+y}$ $\text{Te}$. A semimetallic five-band model with $(\frac{1}{2} \frac{1}{2})$ Fermi surface nesting and fourfold symmetric superexchange between interstitial Fe and two in-plane nearest neighbors largely accounts for the observed diffraction.