Coexistence of superconductivity and antiferromagnetism in single crystals $A_{0.8}F{e}_{2-y}S{e}_2$ (A= K, Rb, Cs, Tl/K and Tl/Rb): evidence from magnetization and resistivity

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 $T_{\mathrm{N}}$. Occurrence of superconductivity in an antiferromagnetic ordered state with so high $T_{\mathrm{N}}$ may suggest new physics in this type of unconventional superconductors.

5 pages, 4 figures