Electronic and magnetic phase diagram in K$_x$Fe$_{2-y}$Se$_2$ superconductors

TL;DR

This study maps the electronic and magnetic phase diagram of K$_x$Fe$_{2-y}$Se$_2$ superconductors, revealing two antiferromagnetic insulating phases and showing that superconductivity emerges from these parent compounds, suggesting a shared mechanism with cuprates.

Contribution

It provides the first detailed phase diagram of K$_x$Fe$_{2-y}$Se$_2$, identifying two distinct AFM insulating phases and their relation to superconductivity, highlighting a common origin with cuprate superconductors.

Findings

Superconductivity is sandwiched between two AFM insulating phases.

Two distinct Fe vacancy order superstructures are identified.

Superconductivity originates from AFM insulating parent compounds.

Abstract

The correlation and competition between antiferromagnetism and superconductivity are one of the most fundamental issues in all of high temperature superconductors. The superconductivity in high temperature cuprate superconductors arises from suppressing an antiferromagnetic (AFM) Mott insulator phase by doping1 while that in iron-pnictide high temperature superconductors arises from AFM semimetals and can coexist with AFM orders2-9. This key difference marked in their phase diagrams has raised many intriguing debates about whether the two materials can be placed in the same category to understand the mechanism of superconductivity. Recently, superconductivity at 32 K has been reported in iron-chalcogenide superconductors AxFe2-ySe2 (A=K, Rb, and Cs)10-12, which have the same structure as that of iron-pnictide AFe2As2 (A=Ba, Sr, Ca and K)13-15. Here, we report electronic and magnetic phase diagram of KxFe2-ySe2 system as a function of Fe valence. We find two AFM insulating phases and reveal that the superconducting phase is sandwiched between them, and give direct evidence that the superconductivity in AxFe2-ySe2 originates from the AFM insulating parent compounds. The two insulating phases are characterized by two distinct superstructures caused by Fe vacancy orders with modulation wave vectors of q1=(1/5, 3/5, 0) and q2=(1/4, 3/4, 0), respectively. These experimental results strongly indicate that iron-based superconductors and cuprates share a common origin and mechanism of superconductivity.