Crystal structure, physical properties and superconductivity in $A_{x}$Fe$_2$Se$_2$ single crystals

TL;DR

This study investigates the relationship between structure, composition, and superconductivity in $A_x$Fe$_2$Se$_2$ single crystals, revealing that superconductivity is closely linked to the c-axis lattice constant and is robust despite Fe and Se vacancies.

Contribution

It provides new insights into how alkali atom content and lattice parameters influence superconductivity in $A_x$Fe$_2$Se$_2$ crystals, highlighting the robustness of superconductivity against vacancies.

Findings

Superconductivity correlates with the c-axis lattice constant.

Superconductivity is unaffected by Fe and Se vacancies.

High normal-state resistivity is due to defects in FeSe layers.

Abstract

We studied the correlation among structure and transport properties and superconductivity in the different $A_x$Fe$_2$Se$_2$ single crystals ($A$ = K, Rb, and Cs). Two sets of (00$l$) reflections are observed in the X-ray single crystal diffraction patterns, and arise from the intrinsic inhomogeneous distribution of the intercalated alkali atoms. The occurrence of superconductivity is closely related to the {\sl c}-axis lattice constant, and the $A$ content is crucial to superconductivity. The hump observed in resistivity seems to be irrelevant to superconductivity. There exist many deficiencies within the FeSe layers in $A_x$Fe$_2$Se$_2$, while their $T_{\rm c}$ does not change so much. In this sense, superconductivity is robust to the Fe and Se vacancies. Very high resistivity in the normal state should arise from such defects in the conducting FeSe layers. $A_x$Fe$_2$Se$_2$ ($A$ = K, Rb, and Cs) single crystals show the same susceptibility behavior in the normal state, and no anomaly is observed in susceptibility at the hump temperature in resistivity. The clear jump in specific heat for Rb$_x$Fe$_2$Se$_2$ and K$_x$Fe$_2$Se$_2$ single crystals shows the good bulk superconductivity in these crystals.