Elucidating the magnetic and superconducting phases in the alkali metal intercalated iron chalcogenides

TL;DR

This study uses neutron scattering, spectroscopy, and resistivity measurements to clarify the relationships among magnetic and superconducting phases in alkali metal intercalated iron chalcogenides, highlighting the role of iron content and phase miscibility gaps.

Contribution

It reveals how iron content stabilizes different magnetic phases and how sulfur substitution influences superconductivity and metallic behavior, providing new insights into phase relationships.

Findings

Iron content stabilizes specific magnetic phases.

Sulfur substitution decreases electronic correlations.

Miscibility gaps exist between phases.

Abstract

The complex interdigitated phases have greatly frustrated attempts to document the basic features of the superconductivity in the alkali metal intercalated iron chalcogenides. Here, using elastic neutron scattering, energy-dispersive x-ray spectroscopy, and resistivity measurements, we elucidate the relations of these phases in Rb$_{1-\delta}$Fe$_y$Se$_{2-z}$S$_z$. We find: i) the iron content is crucial in stabilizing the stripe antiferromagnetic (AF) phase with rhombic iron vacancy order ($y\approx1.5$), the block AF phase with $\sqrt{5}\times \sqrt{5}$ iron vacancy order ($y\approx1.6$), and the iron vacancy-free phase ($y\approx2$); ii) the superconducting phase ($z=0$) evolves into a metallic phase ($z>1.5$) with sulfur substitution due to the progressive decrease of the electronic correlation strength. Both the stripe AF phase and the block AF phase are Mott insulators. Our data suggest that there are miscibility gaps between these three phases. The existence of the miscibility gaps in the iron content is the key to understanding the relationship between these complicated phases.