2x2 structure and charge inhomogeneity at the surface of superconducting BaFe2-xCoxAs2 (x=0-0.32)

The structure and electronic property at the cleaved (001) surfaces of the newly discovered pnictide superconductors BaFe2-xCoxAs2 with x ranging from 0 to 0.32 are systematically investigated by scanning tunneling microscopy. A 2x2 surface structure is revealed to dominate for all the compounds and is identified to be Ba layer with half Ba atoms lifted off by combination with theoretical simulation. Charge inhomogeneity is observed at this 2x2 surface associated with an energy gap of about 30 meV for all the compounds.