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

We measure the resistivity and magnetic susceptibility in the temperature range from 5 K to 600 K for the single crystals AFe 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 the fully shielding fraction shows bulk susceptibility for all the crystals, while an antiferromagnetic transition is observed in susceptibility at Néel temperature ( T N ) as high as 500 K to 540 K depending on A. This indicates the coexistence of superconductivity and antiferromagnetism in these intercalated iron selenides. A sharp increase in resistivity arises from the structural transition due to Fe vacancy ordering at temperature slightly higher than T N . Occurrence of superconductivity in an antiferromagnetic ordered state with so high T N may suggest new physics in this type of unconventional superconductors.