Scanning tunneling microscopy study of K-doped iron selenide superconductor film by MBE

The alkali-doped iron selenide superconductors have generated considerable excitements as well as confusions, regarding the delicate interplay between Fe vacancies, magnetism and superconductivity. We have grown high-quality K$_{x}$Fe$_{2-y}$Se$_{2}$ thin film with (001) surface orientation on STO substrate by molecular beam epitaxy. The scanning tunneling microscopy (STM) measurement demonstrates that there are two superconducting phases: striped KFe$_{2}$Se$_{2}$ in adjacent to the phase with $\surd 5 \times \surd $5 Fe vacancy order and doped KFe$_{2}$Se$_{2}$ with Fe and K vacancies. Both phases have a superconducting gap of 9 meV. These findings elucidate the existing controversies on the role of $\surd 5 \times \surd $5 Fe vacancy order in superconducting K$_{x}$Fe$_{2-y}$Se$_{2}$. Based on the atomic level information by STM, we will discuss the mechanism of the two different superconducting phases.