(100) at low doping

Monolayer FeSe grown on SrTiO₃(001) has been established to host a high-temperature superconducting state, with the gap-opening temperature between 60 and 70 K. On the other hand, it was shown that monolayer FeSe exhibits an insulating gap when the electron doping level is below 0.09 electron per iron atom. This insulating gap is temperature independent, in contrast to the gap-opening phenomenology in superconducting monolayer FeSe. Here we grow monolayer FeSe on rutile TiO₂(100), and show that the gap opens below 40 K with an electron doping of 0.08 electron per iron atom. This doping level is lower than the previously reported doping threshold for superconductivity. We will discuss the origin of this gap.