Abstract
-
The recent discovery of possible high-temperature superconductivity
in single-layer FeSe films1,2 has generated significant
experimental and theoretical interest3,4
. In both the cuprate5,6
and the iron-based7–11 high-temperature superconductors, superconductivity
is induced by doping charge carriers into the
parent compound to suppress the antiferromagnetic state. It is
therefore important to establish whether the superconductivity
observed in the single-layer sheets of FeSe—the essential
building blocks of the Fe-based superconductors—is realized
by undergoing a similar transition. Here we report the phase
diagram for an FeSe monolayer grown on a SrTiO3 substrate,
by tuning the charge carrier concentration over a wide range
through an extensive annealing procedure. We identify two
distinct phases that compete during the annealing process:
the electronic structure of the phase at low doping (N phase)
bears a clear resemblance to the antiferromagnetic parent
compound of the Fe-based superconductors, whereas the superconducting
phase (S phase) emerges with the increase in
doping and the suppression of the N phase. By optimizing
the carrier concentration, we observe strong indications of
superconductivity with a transition temperature of 65 ± 5 K.
The wide tunability of the system across different phases
makes the FeSe monolayer ideal for investigating not only
the physics of superconductivity, but also for studying novel
quantum phenomena more generally.