  Up to now, there have been two material families, the cuprates and the
iron-based compounds with high-temperature superconductivity (HTSC). An
essential open question is whether the two classes of materials share the same
essential physics. In both, superconductivity (SC) emerges when an
antiferromagnetical (AFM) ordered phase is suppressed. However, in cuprates,
the repulsive interaction among the electrons is so strong that the parent
compounds are "Mott insulators." By contrast, all iron-based parents are
metallic. One perspective is that the iron-based parents are weakly correlated
and that the AFM arises from a strong "nesting" of the Fermi surfaces. An
alternative view is that the electronic correlations in the parents are still
sufficiently strong to place the system close to the boundary between
itinerancy and electronic localization. A key strategy to differentiate theses
views is to explore whether the iron-based system can be tuned into a Mott
insulator. Here we identify an insulating AFM in (Tl,K)FexSe2 by introducing
Fe-vacancies and creating superconductivity in the Fe-planar. With the
increasing Fe-content, the AFM order is reduced. When the magnetism is
eliminated, a superconducting phase with Tc as high as 31K (and a Tc onset as
high as 40K) is induced. Our findings indicate that the correlation effect
plays a crucial role in the iron-based superconductors. (Tl,K)FexSe2,
therefore, represents the first Fe-based high temperature superconductor near
an insulating AFM.
