The discovery of new high-temperature superconducting phase in helium iron arsenic
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Phase diagram of LaFeAsO1-xFx. AF, SCI, and SCII represent antiferromagnetic, superconducting region I, and superconducting region II, respectively. The superconducting transition temperature Tc is determined by the susceptibility transition, and the structural phase transition temperatures Ts and Xs are determined by NMR experiments and electron microscope experiments.
In the past century, superconductors (especially high-temperature superconductors) have attracted the interest of countless physicists and material scientists. This is not only because the superconducting phenomenon is rich in physics, but also because of its broad application prospects in industry and gradually entering people's daily lives. The current discovery of high-temperature superconductors has two major families, one is copper oxide and the other is iron-based compounds. The common feature is that high-temperature superconductors appear near the antiferromagnetically ordered states. Therefore, many people believe that magnetic (spin) fluctuations promote the electronic pairing in these materials. For a long time, exploration of high-temperature superconductors has been carried out under such a guiding ideology.
Recently, scientists from the Institute of Physics, Chinese Academy of Sciences/Beijing National Laboratory for Condensed Matter Physics (CMM) discovered a new high-temperature superconducting phase away from antiferromagnetically ordered iron-based materials. Moreover, the superconducting maximum transition temperature Tc exceeds the superconducting phase of the same substance near the magnetic order, reaching 41K (defined by resistivity data).
Iron-based compound superconductors were originally discovered by the Hosono team at the Tokyo Institute of Technology in Japan in 2008. The parent material LaFeAsO has an antiferromagnetic ordered phase, and after replacing some oxygen with fluorine, the magnetic order is suppressed and superconductivity appears. The superconducting transition temperature Tc varies with fluorine doping amount x, forming an arched superconducting region, where x can only reach 0.2. When x = 0.06, the Tc of the sample is the highest, reaching 27 K. At this time, there is a strong low-energy spin fluctuation. This naturally leads people to believe that in iron-based superconductors, superconductivity is caused by spin fluctuations.
Recently, the Institute of Physics, Chinese Academy of Sciences/Beijing National Laboratory for Condensed Matter State Superconducting National Key Laboratory of Superconductivity, Zheng Guoqing Research Group (SC9 Group), Associate Researcher Yang Jie, etc., cooperated with Zhao Zhongxian and Li Jianqi, academicians of the Chinese Academy of Sciences, through the high pressure. Sample preparation technology has synthesized a series of highly doped samples of LaFeAsO1-xFx. The doping amount of x can reach 0.75, which is far beyond people's knowledge. Through the measurements of resistivity, magnetic susceptibility and nuclear magnetic resonance, it was found that the superconducting transition temperature Tc forms a new superconducting region with x. At the optimal doping x = 0.5-0.55, the Tc is even higher than the original x = 0.06. (Tc defined by resistivity data is 41K. Tc defined by susceptibility is 30K).
What is even more amazing is that in this newly discovered superconducting region, the physical properties are completely different from the previously reported first superconducting region. First, the NMR spin-lattice relaxation rate 1/T1T measurement showed no evidence of spin-flick behavior. Second, a new type of structural phase transition was found in the sample through NMR and electron microscopy. Four-fold rotation occurred when the phase transition occurred. The symmetry is destroyed, and the temperature at which the phase change occurs varies with the doping amount and the line can just extend to the optimum doping. At the same time, the resistivity shows a linear temperature change near the optimum doping, indicating a new quantum fluctuation.
This work implies that HTS may be a very universal phenomenon. In addition to the spin fluctuations discussed in recent 30 years, other mechanisms such as orbital fluctuations may also cause HTS. This achievement has provided a new clue for exploring HTS. The results of the study were published in the form of Express in Chin. Phys. Lett., 32, 107401 (2015).
The study was supported by the "973" project of the Ministry of Science and Technology, the Class B pilot project of the Chinese Academy of Sciences and the National Natural Science Foundation of China.
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