Direct observation of microstructures on superconducting single crystals of KxFe2−ySe2

Potassium-intercalated FeSe has been reported as a superconductor with a superconducting transition temperature (T c) of 30–48 K. However, the relationship among the surface morphology, compositional ratio, and crystal structure has not yet been clarified. This report directly reveals the correspondence among these three characteristics in single crystals with a T c onset of around 44 K by using a microsampling technique. Island-like parts on the surface of the crystals clearly exhibit the K x Fe2Se2 structure with perfect FeSe layers, which is formed in conjunction with the K2Fe4Se5 phase. This results in the appearance of the T c onset at 44 K.