Block Antiferromagnetism and Checkerboard Charge Ordering in Alkali-doped Iron Selenides $R$$_{1-x}$Fe$_{2-y}$Se$_2$

TL;DR

This study uses first-principles calculations to reveal a checkerboard phase with block antiferromagnetic order in alkali-doped iron selenides, explaining experimental observations and unifying phase understanding.

Contribution

It identifies the checkerboard phase as the ground state in these materials, linking magnetic order, charge order, and superconductivity in a unified framework.

Findings

Checkerboard phase is the ground state without iron vacancies.

Block AFM order coexists with small charge density wave.

Results explain experimental 2x2 ordered patterns and hidden orders.

Abstract

By performing first-principles electronic structure calculations and analyzing effective magnetic model of alkali-doped iron selenides, we show that the materials without iron vacancies should approach a novel checkerboard phase in which each four Fe sites group together in tetragonal structure. The checkerboard phase is the ground state with a block antiferromagnetic (AFM) order and a small charge density wave order in the absence of superconductivity. Both of them can also coexist with superconductivity. The results explain mysterious $2\times 2$ ordered patterns and hidden orders observed in various different experiments, clarify the missing link between AFM and superconducting phases, suggest that the block-AFM state is the parent state, and unify the understanding of various observed phases in alkali-doped iron selenides.