Direct Observation of Microstructures on Superconducting Single Crystals of K$_x$Fe$_{2-y}$Se$_2$

TL;DR

This study directly observes the microstructures of K$_x$Fe$_{2-y}$Se$_2$ single crystals, revealing how surface morphology and composition relate to their superconducting properties, especially around 44 K.

Contribution

It provides direct microstructural evidence linking surface features, composition, and crystal phases to superconductivity in K$_x$Fe$_{2-y}$Se$_2$ crystals.

Findings

Island-like surface regions have K$_x$Fe$_2$Se$_2$ structure with perfect FeSe layers.

These regions form alongside the K$_2$Fe$_4$Se$_5$ phase.

The observed microstructure correlates with a $T_c$ onset of around 44 K.

Abstract

Potassium intercalated FeSe has been reported as a superconductor with superconducting transition temperatures ($T_{\rm c}$'s) of 30-48 K. However, there is no clear answer to identify the relationship between the surface morphology, compositional ratio and its crystal structure. This report directly reveals the correspondence among these three characteristics in single crystals with a $T_{\rm c}$ onset of around 44 K using a micro-sampling technique. Island-like parts on the surface of the crystals clearly have the K$_x$Fe$_2$Se$_2$ structure with perfect FeSe layers, which is formed in conjunction with the K$_2$Fe$_4$Se$_5$ phase. This results in the appearance of the $T_{\rm c}$ onset of 44 K.