Abstract
We measure the resistivity and magnetic susceptibility in the temperature range from 5 K to 600 K for the single crystals AFe2−ySe2 (A=K0.8, Rb0.8, Cs0.8, Tl0.5K0.3 and Tl0.4Rb0.4). 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 (TN) 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 TN. Occurrence of superconductivity in an antiferromagnetic ordered state with so high TN may suggest new physics in this type of unconventional superconductors.