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 Fe1+y Te1−x Sex induces a magnetic Friedel-like oscillation that diffracts at Q⊥=(120) and involves > 50 neighboring Fe sites. The interstitial >2μB moment is surrounded by compensating ferromagnetic four-spin clusters that may seed double stripe ordering in Fe1+y Te. A semimetallic five-band model with (1212) Fermi surface nesting and fourfold symmetric superexchange between interstitial Fe and two in-plane nearest neighbors largely accounts for the observed diffraction.