(Center for Superconducting Physics and Materials, National Laboratory for Solid State Microstructures, Department of Physics, Nanjing University, Nanjing 210093, China)
Keywords:
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CLC:
PACS:
TH142.8
DOI:
10.7502/j.issn.1674-3962.2013.09.01
DocumentCode:
Abstract:
Since the discovery in 2008 of superconductivity at 26K in fluorine-doped LaFeAsO, the research has been tuned to a new direction on high temperature superconductivity. So far, several types of iron-based superconductors with different structures have been discovered, with the highest transition temperature to date being 56K. By the end of 2010, the iron-chalcogenide superconductor KxFe2-ySe2 with Tc ~ 32K was discovered. This system shows a set of distinctive properties as compared with other iron-pnictide compounds. Both angle resolved photoemission spectroscopy experiments and band structure calculations indicate that the hole pockets are missing at the Fermi energy. This greatly challenges the widely perceived picture that the superconducting pairing is established by exchanging antiferromagnetic (AF) spin fluctuations and the electrons are scattered between the electron and hole pockets. Later, it was found that this material separates into two phases-a dominant AF insulating phase with a K2Fe4Se5 structure, and a minority superconducting phase. A recent paper of Hai-Hu Wen抯 group identifies a three-dimensional network of superconducting filaments within this material and present evidence for the existence of K2Fe7Se8 which may be the possible parent phase for superconductivity. This 278 phase has a Fe structure of a single Fe vacancy out of every eight Fe-sites arranged in a parallelogram structure. In this paper, we review the status of research in this rapidly growing field. Crystal growth and quenching technique, Fe-vacancy orders and the block-AF state, phase separation and the hunt for the superconducting phase, pairing symmetry and the gap structure are introduced and discussed. In the end, important issues are listed as perspective for future research