  Since the discovery of high temperature superconductivity in iron pnictides
in early 2008, many iron-based superconductors with different structures have
been discovered, with the highest transition temperature to date being 57 K. By
the end of 2010, another kind of new superconductor, the Fe-based chalcogenide
K1-xFe2-ySe2 was discovered. A naive counting of the electrons in the system
would lead to a conclusion that the system is heavily electron overdoped (~0.4
e/Fe). Band structure calculations further support this speculation and predict
that the hole pockets which are found in the iron pnictides may be missing.
This greatly challenges the widely perceived picture that the superconducting
pairing is established by exchanging anti-ferromagnetic spin fluctuations and
the electrons are scattered between the electron and hole pockets. Later, it
was found that both potassium and iron might be deficient in K1-xFe2-ySe2,
yielding to a picture of phase separation. In this picture the superconducting
phase and the antiferromagnetic (AF) phase may phase separate spatially into
different regions. This generates further curiosity about what is the real
superconducting phase, what is the relationship between the superconducting
phase and the AF phase, and what is the parent state for the superconducting
phase. We propose a "spider web" model for the phase separation, which can
explain both the transport and magnetic data. In this paper, we review the
status of research in this rapidly growing field and list the important and
unsettled issues as perspectives for future research.
