Direct observation of microstructures on superconducting single crystals of K_xFe_2-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_xFe₂Se₂ 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. (author)