Low-temperature STM/STS study on superconducting FeSe films*

By using molecular beam epitaxy (MBE) technique, we have prepared single crystalline and atomically flat FeSe thin films on graphene-terminated SiC substrate. Low temperature scanning tunneling microscopy/spectroscopy (STM/STS) measurements reveal that the local superconducting gap in the quasiparticle density of states remains robust down to two triple layers ($\sim $1.1 nm), and that the FeSe films show a novel thickness-dependent superconductivity transition behavior. We show that the superconductivity of the FeSe films can be manipulated by concise control of their surface structures. \textit{dI/dV} mapping of the vortex lattice confirms that FeSe is a typical type II superconductor. Understanding these properties may help us to unravel the mechanism of the recently discovered Fe-based superconductors and even the long-term studied cuprates.