Recently, Prof. Wen Haihu of the School of Physics of the School of Physics of our school has made new progress in superconducting research: A new superconducting parent phase, K2Fe7Se8, has been discovered in the iron-based material KxFe2-ySe2. Its work has been published as an article in May 2013. Posted in Nature Communication (4,1897,2013).
In late 2010, the new superconductor series KxFe2-ySe2 was discovered. The initial band structure calculations show that this material does not have a hole-type Fermi surface near the Fermi energy, which strongly challenges the widely accepted iron-based superconducting pairing image, which has attracted great interest of physicists. Shortly afterwards, it was found that there was an Fe vacancy in this material and was separated into two phases: an anti-ferromagnetic insulating phase with a K2Fe4Se5 structure and a superconducting phase. Professor Wen Haihu's group noticed the existence of phase separation earlier in the world and found that with different quenching treatments, the superconducting properties of the samples changed greatly. The existence of phase separation makes the related research on this material very difficult, and there is a great controversy in the academic community about the problem of what superconducting phase of KxFe2-ySe2 or its maternal phase is.
Recently, the Institute of Physics and the "Superconducting Physics and Materials Research Center" of our university have made new progress in superconducting research. Professor Wen Haihu and Ding Xiaxin prepared samples after different heat treatment and studied KxFe2 in depth through transmission, magnetization, X-ray diffraction, scanning electron microscopy (cooperated with Professor Ma Guobin, etc.), scanning tunneling microscope, and other measurement methods. The relationship between the microstructure and superconductivity in the ySe2 sample confirms that the superconducting phase of the material exists in the form of a three-dimensional network of filaments, and various means have been used to prove that the parent phase of superconductivity is formed by Fe vacancies. The ordered parallelogram structure consists of the K2Fe7Se8 phase. This is a completely new phase, with eight iron atoms having a missing atom. Once this work was reported, it attracted the attention of many theorists who used calculations to understand why this new phase might lead to superconductivity. This work was published as an article in May 2013 in Nature Communication, 4, 1897, (2013).
The main results of this study show backscattered images of the cleaved planes of three typical samples. The researchers found that the surface morphology of the samples obtained through three different heat treatment methods is very different. They also observed the structure of the topmost K atom layer on the KxFe2-ySe2 surface through a scanning tunneling microscope, and the Fe vacancy is located at the bottom layer, showing this structure.
The above work received funding support from the fund's key projects, the Ministry of Science and Technology's 973 project and the Ministry of Education’s 985 program.
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