Doping dependence of the electronic structure and superconductivity of FeSe thin films by Cobalt dopant

Both superconductivity and structural transition/orbital order are found to be suppressed by Co doping in FeSe single crystal. We have grown FeSe thin films on STO substrates using MBE, and studied the doping dependence of the electronic structure and superconductivity of FeSe thin films by Cobalt dopant. For single layer FeSe, The electron pockets at M point are enlarged when doped with cobalt, and the Tc is slightly increased. When the 2.5 UC, 3.25 UC and 4 UC FeSe sample are electron doped by cobalt, the nematic order is suppressed and superconductivity can be detected. For the multi-layer FeSe thin films, the electronic structure turns out to be alike of single layer FeSe when they are effectively Co doped. But it is not superconducting at the largest doping level, which indicates that the interface effect plays an important role for the high Tc in 1UC FeSe/STO system.