Origin of the Higher-$T_\rm{c}$ Phase in the K$_x$Fe$_{2-y}$Se$_2$ System

TL;DR

This study investigates the origin of the higher-temperature superconducting phase in K$_x$Fe$_{2-y}$Se$_2$, revealing that Fe-vacancy ordering at around 270°C influences the emergence of a 44 K superconducting phase.

Contribution

It demonstrates that Fe-vacancy ordering at specific temperatures is linked to the higher $T_c$ phase, providing insights into phase formation mechanisms in this material.

Findings

Higher quenching temperatures produce surface mesh-like morphology with sharp 32 K superconductivity.

Unquenched crystals show a 44 K onset transition and island-like Fe-rich regions.

Fe-vacancy order forms around 270°C, correlating with the higher $T_c$ phase.

Abstract

Single crystals of K$_x$Fe$_{2-y}$Se$_2$ are prepared by quenching at various temperatures. The crystals obtained at higher quenching temperatures have a surface morphology with mesh-like texture. They show a sharp superconducting transition at $T_\rm{c}$ ~32 K with a large shielding volume fraction. On the other hand, the crystals prepared without quenching show an onset superconducting transition at ~44 K and a zero resistivity around ~33 K, and they possess island-like regions on the surface with a larger amount of Fe incorporation. In-situ high-temperature single crystal X-ray diffraction measurements tell us the Fe-vacancy ordered phase is generated at a temperature region around 270 {\deg}C via iron diffusion. The creation of this Fe-vacancy ordered phase may become a driving force of the growth of the higher $T_\rm{c}$ phase. The superconductivity at ~44 K is attributed to a metallic phase with no Fe-vacancy.