The two subfamilies in the iron-based superconductor family have the similar structure of FeSe4 and FeSe4 tetrahedral layers as their respective superconducting primitives. However, the experimental performance of the FeY-based superconductor AyFe2-xSe2 (A= alkali metal ion) precursor phase and normal state is different from the FeAs-based system, leading to question whether the high-temperature superconductivity of the two iron-based systems has the common physical origin. . Clarifying this issue is of great significance for exploring the physical mechanisms of various types of high-temperature superconductors. However, the experimental difficulty is that the antiferromagnetic insulation 245 phase in AyFe2-xSe2 tends to coexist with the superconducting phase, which hinders the observation of intrinsic properties. In 2014, Fe0.8-based new high-temperature superconductor Li0.8Fe0.2OHFeSe was reported, and relevant experimental research turned to be turning.
Recently, Dong Xiaoli, a researcher in the SC4 group of the National Key Laboratory of Superconductivity of the Chinese State Academy of Sciences for Condensed Matter Physics/National Laboratory for Condensed Matter Physics, cooperated with SC2 group Jin Kui and Professor Zhang Guangming of Tsinghua University in the new high-temperature superconducting system Li1- New progress was made in the study of xFexOHFe1-ySe.
Based on a series of powder samples prepared by hydrothermal synthesis, the complete phase diagram of Li1-xFexOHFe1-ySe for the first time in the high TC system shows that both the antiferromagnetic SDW phase region and the superconducting phase region are physically similar to the FeAs-based system. . The experimental results reveal that FeSe-based and FeAs-based high-temperature superconductors actually have similar backgrounds for electron interactions, so their high-temperature superconductivity has a common physical mechanism. The related results were published in JACS137, 66 (2015).
Ion exchange technology was successfully applied to synthesize large-size (Li0.84Fe0.16) OHFe0.98Se superconducting single crystals (TC=42K, size greater than 10mm) for the first time, and the material did not have 245 phases that plagued experimental observations. The breakthrough of single-crystal samples makes it possible to reveal its highly two-dimensional intrinsic electronic properties. It was found that the normal state below the characteristic temperature T* (=120K) exhibits anomalous magnetization and linear temperature dependence of resistance and highly two-dimensional electron transport; in the superconducting state, the upper critical field has strong anisotropy . The Hall coefficient shows that electron carriers dominate charge transport, and the contribution of hole-type carriers is extremely reduced below T*. Experiments show that the pairing of superconductors is likely related to two-dimensional antiferromagnetic spin fluctuations. Similar antiferromagnetic fluctuation characteristics are also common in FeAs-based and copper oxide high-temperature superconductors. Comparing different types of FeSe-based superconductors, we can see that the higher the critical temperature of the superconducting system, the stronger the two-dimensional crystal and electronic structure; especially the high TC and highly two-dimensional (Li0.84Fe0.16) OHFe0.98Se single crystals. Similar to FeSe monolayer interface superconductors (TC up to ~65K) on SrTiO3 substrates. Therefore, two-dimensional electronic interactions are crucial for high-temperature superconductivity. The related results are published as Editors' Suggestion in PRB 92,064515 (2015).
The main partners of the above-mentioned work are: Zhao Zhongxian, Zhou Fang (SC4 group) and Yuan Jie (SC2 group); SC4 group doctors who undertake related experiments include Zhou Huaxue, Yuan Dongna, Huang Yulong, and Mao Yiyuan; and they are responsible for the determination of related physical properties. Among them are: A06 group researcher Yang Yuxin (electron microscopy), Peking University researcher Sun Junliang (single crystal structure refinement) and EX1 research associate Zheng Ping (high temperature magnetism). The above work was supported by the National Natural Science Foundation of China, the relevant projects of the Ministry of Science and Technology, and the Class B pilot project of the Chinese Academy of Sciences.
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