  The recent discovery of superconductivity with relatively high transition
temperature Tc in the layered iron-based quaternary oxypnictides La[ O1-xFx]
FeAs was a real surprise. The excitement generated can be seen by the number of
subsequent works published within a very short period of time. Although there
exists superconductivity in alloy that contains Fe element, LaOMPn (with M= Fe,
Ni; and Pn=P and As) is the first system where Fe-element plays the key role to
the occurrence of superconductivity. LaOMPn has a layered crystal structure
with an Fe-based plane. It is quite natural to ask whether there exists other
Fe based planar compounds that exhibit superconductivity. Here we report the
observation of superconductivity with zero resistance transition temperature at
8K in the PbO-type alpha-FeSe compound. Although FeSe has been studied quite
extensively, a key observation is that the clean superconducting phase exists
only in those samples prepared with intentional Se deficiency. What is truly
striking, is that this compound has the same, perhaps simpler, planar crystal
sublattice as the layered oxypnictides. Furthermore, FeSe is, compared with
LaOFeAs, much easier to handle and fabricate. In view of the abundance of
compounds with PbO type structure, this result opens a new route to the search
for unconventional superconductors.
