Superconductivity and Magnetic Properties of high-quality single crystals of $A_{x}$Fe$_2$Se$_2$ ($A$ = K and Cs)

TL;DR

This study reports the successful growth and characterization of high-quality single crystals of $A_{x}$Fe$_2$Se$_2$ ($A$ = K, Cs), revealing their superconducting properties, high resistivity, and magnetic anisotropy, contributing to understanding iron-based superconductors.

Contribution

The paper presents the first high-quality single crystals of $A_{x}$Fe$_2$Se$_2$ with detailed superconducting and magnetic property measurements, expanding knowledge of iron selenide superconductors.

Findings

Superconducting transition temperatures of 31 K (K) and 30 K (Cs).

High normal-state resistivity exceeding 160 mΩ·cm and 1300 mΩ·cm.

Large upper critical fields with anisotropy around 3.

Abstract

We successfully grew the high-quality single crystals of $A_{x}$Fe$_2$Se$_2$ ($A$ = K and Cs) by self-flux method. Sharp superconducting transition was observed for both types of crystals. The crystals show the onset superconducting transition temperatures ($T_{\rm c}$) of 31 K and 30 K for K- and Cs-compounds, respectively, with nearly 100% shielding fraction. The crystals show quite high resistivity in the normal state of more than 160 m$\Omega$ cm and 1300 m$\Omega$ cm maximum resistivity for $K_{0.86}Fe_2Se_{1.82}$ and $Cs_{0.86}Fe_{1.66}Se_{2}$ single crystals, respectively. Much larger upper critical field $H_{\rm c2}$ is inferred from low-temperature iso-magnetic-field magnetoresistance in these crystals than in FeSe. The anisotropy $H^{ab}_{\rm c2}$(0)/$H^{c}_{\rm c2}$(0) is around 3 for both of the two materials. Anisotropic peculiar magnetic behavior in normal state has been found for $Cs_{0.86}Fe_{1.66}Se_{2}$