Phase separation near the charge neutrality point in FeSe$_{1-x}$Te$_{x}$ crystals with x $<$ 0.15

TL;DR

This paper investigates phase separation in FeSe$_{1-x}$Te$_{x}$ crystals with low tellurium content, revealing structural instability linked to iron plane bond properties and electronic factors, which may influence iron-based superconductor behavior.

Contribution

It provides new insights into the structural and electronic instability mechanisms in FeSe$_{1-x}$Te$_{x}$ near charge neutrality, highlighting the role of bond polarity and magnetic exchange changes.

Findings

Phase separation into different iron stoichiometry phases.

Structural instability related to iron-iron bond distances.

Potential implications for other iron-based superconductors.

Abstract

Our study of FeSe$ _ {1-x}$Te$ _ {x}$ crystals with x $<$ 0.15 shows that the phase separation in these compositions occurs into phases with a different stoichiometry of iron. This phase separation may indicate structural instability of the iron plane in the studied range of compositions. To explain it, we discuss the bond polarity and the peculiarity of the direct $d$ exchange in the iron plane in the framework of the basic phenomenological description such as the Bethe-Slater curve. With this approach, when the distance between iron atoms is close to the value at which the sign of the magnetic exchange for some $d$ orbitals changes, structural and electronic instability can occur. Anomalies in the crystal field near the point of charge neutrality can also be a significant component of this instability. A similar instability of the iron plane may also be an important factor for other series of iron-based superconductors.