The recent discovery of high-temperature superconductivity in iron-based compounds has
attracted much attention . How to further increase the superconducting transition temperature
( Tc ) and how to understand the superconductivity mechanism are two prominent issues facing
the current study of iron-based superconductors. The latest report of high- Tc superconductivity
in a single-layer FeSe is therefore both surprising and signifi cant. Here we present investigations
of the electronic structure and superconducting gap of the single-layer FeSe superconductor. Its
Fermi surface is distinct from other iron-based superconductors, consisting only of electron-like
pockets near the zone corner without indication of any Fermi surface around the zone centre.
Nearly isotropic superconducting gap is observed in this strictly two-dimensional system. The
temperature dependence of the superconducting gap gives a transition temperature Tc ~ 55 K.
These results have established a clear case that such a simple electronic structure is compatible
with high- Tc superconductivity in iron-based superconductors.