Two-dimensional Cs-vacancy superstructure in iron-based superconductor $Cs_{0.8}Fe_{1.6}Se_2$

TL;DR

This study combines neutron and x-ray scattering with modeling to identify and characterize superlattice phases in Cs0.8Fe1.6Se2, revealing distinct vacancy orderings linked to magnetic and superconducting properties.

Contribution

It provides the first detailed structural solutions for the superlattice phases in Cs0.8Fe1.6Se2, highlighting the vacancy arrangements and their relation to magnetic and superconducting phases.

Findings

Identification of $\sqrt{5} imes\sqrt{5}$ and $\sqrt{2} imes\sqrt{2}$ superlattice phases.

Cs vacancies form a $\sqrt{2} imes\sqrt{2}$ superlattice within the plane.

Majority phase is antiferromagnetic, minority phase is superconducting.

Abstract

Single crystal neutron diffraction is combined with synchrotron x-ray scattering to identify the different superlattice phases present in $Cs_{0.8}Fe_{1.6}Se_2$. A combination of single crystal refinements and first principles modelling are used to provide structural solutions for the $\sqrt{5}\times\sqrt{5}$ and $\sqrt{2}\times\sqrt{2}$ superlattice phases. The $\sqrt{5}\times\sqrt{5}$ superlattice structure is predominantly composed of ordered Fe vacancies and Fe distortions, whereas the $\sqrt{2}\times\sqrt{2}$ superlattice is composed of ordered Cs vacancies. The Cs vacancies only order within the plane, causing Bragg rods in reciprocal space. By mapping x-ray diffraction measurements with narrow spatial resolution over the surface of the sample, the structural domain pattern was determined, consistent with the notion of a majority antiferromagnetic $\sqrt{5}\times\sqrt{5}$ phase and a superconducting $\sqrt{2}\times\sqrt{2}$ phase.