Overview on the physics and materials of the new superconductor K(x)Fe(2-y)Se2

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 K(1-x)Fe(2-y)Se(2) was discovered. A naive counting of the electrons in the system would lead to the 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 antiferromagnetic (AF) spin fluctuations and that the electrons are scattered between the electron and hole pockets. Later, it was found that both potassium and iron might be deficient in K(1-x)Fe(2-y)Se(2), yielding to a picture of phase separation. In this picture the superconducting phase and the AF phase may separate spatially into different regions. This generates further curiosity about what the real superconducting phase is, what the relationship is between the superconducting phase and the AF phase, and what the parent state is 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 unresolved issues as perspectives for future research.