Coexistence of superconductivity and antiferromagnetism in single crystals $A_{0.8}Fe_{2-y}Se_2$ (A= K, Rb, Cs, Tl/K and Tl/Rb): evidence from magnetization and resistivity

TL;DR

This study demonstrates the coexistence of high-temperature antiferromagnetism and superconductivity in single crystals of $A_{0.8}Fe_{2-y}Se_2$, revealing potential new physics in unconventional superconductors through resistivity and magnetization measurements.

Contribution

It provides experimental evidence of superconductivity coexisting with antiferromagnetic order at high Neel temperatures in $A_{0.8}Fe_{2-y}Se_2$ crystals, highlighting a novel physical phenomenon.

Findings

Superconductivity observed with 100% Meissner volume fraction.

Antiferromagnetic transition occurs at 500-540 K.

Resistivity increases sharply due to Fe vacancy ordering.

Abstract

We measure the resistivity and magnetic susceptibility in the temperature range from 5 K to 600 K for the single crystals $A$Fe$_{2-y}$Se$_2$ ($A$ = K$_{0.8}$, Rb$_{0.8}$, Cs$_{0.8}$, Tl$_{0.5}$K$_{0.3}$ and Tl$_{0.4}$Rb$_{0.4}$). A sharp superconducting transition is observed in low temperature resistivity and susceptibility, and susceptibility shows 100% Meissner volume fraction for all crystals, while an antiferromagnetic transition is observed in susceptibility at Neel temperature ($T_N$) as high as 500 K to 540 K depending on A. It indicates the coexistence of superconductivity and antiferromagnetism. A sharp increase in resistivity arises from the structural transition due to Fe vacancy ordering at the temperature slightly higher than $T_{\rm N}$. Occurrence of superconductivity in an antiferromagnetic ordered state with so high $T_{\rm N}$ may suggest new physics in this type of unconventional superconductors.