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The discovery of new superconducting materials with higher superconducting transition temperature and understanding of the mechanism of high temperature superconductivity are two important directions of today's superconducting research. The highest superconducting temperature of iron-based superconductors discovered in 2008 reached 55K. Recently, Xue Qikun's research group of the Department of Physics of Tsinghua University and Ma Xucun's research group of the Institute of Physics of the Chinese Academy of Sciences have successfully grown FeSe thin films on SrTiO3 substrates, and found that there may be a temperature close to liquid nitrogen (77K) in a single-layer FeSe thin film. ) Signs of superconductivity change. This work, on the one hand, may break the record of the highest superconducting temperature of 55K for iron-based superconductors; on the other hand, because the superconducting temperature is much higher than the superconducting transition temperature of the bulk FeSe at normal pressure ~ 8K and can be achieved under high pressure Tc ~ 36.7K, this result is quite unexpected. What is the reason why the single-layer FeSe thin film grown on the SrTiO3 substrate exhibits such strange superconducting characteristics? In response to this problem, the understanding of the superconducting mechanism of iron-based superconductors and how to improve the superconductivity of materials The transition temperature has important significance.
Institute of Physics, Chinese Academy of Sciences / Beijing National Laboratory for Condensed Matter Physics (Preparation) State Key Laboratory of Superconductivity, Zhou Xingjiang Research Group Liu Defa, Mou Daixiang, He Junfeng, Zhao Lin and others and Surface Laboratory Ma Xucun Research Group and Tsinghua University Xue Qikun Research Zhang Wenhao and Ou Yunbo of the group used high-resolution angle-resolved photoelectron spectroscopy experimental methods to study the electronic structure of single-layer FeSe thin films grown on SrTiO3 substrates. First, they found that the electronic structure of the single-layer FeSe film is significantly different from other existing iron-based superconductors, and has a very simple Fermi surface (Figure 1). In the center of the Brillouin zone, there is no band that can pass through the Fermi energy. Only at the corner of the Brillouin zone, there is an electronic Fermi surface (Figure 2). Second, they observed the opening of the superconducting energy gap on the Fermi surface near the M point at low temperatures. By measuring the change of the energy gap with temperature, it was found that the energy gap was closed at around 55K, indicating that the superconducting transition temperature of the sample was around 55K (Figure 3). The measurement of the superconducting energy gap with the momentum change shows that the energy gap is basically isotropic (~ 15 meV) (Figure 4). Since this is an ideal two-dimensional system, it can be directly determined that there are no gap nodes in the system.
Single-layer FeSe has such a simple Fermi surface, but also has such a high superconducting temperature, which provides important information for understanding the superconducting mechanism of iron-based superconductors. On the one hand, superconductivity is observed in this ideal two-dimensional system, indicating that the three-dimensional interlayer coupling has no obvious effect on the generation of iron-based high-temperature superconductivity. On the other hand, among the existing types of iron-based superconductors, several hole-type energy bands and Fermi surfaces generally exist near the center of the Brillouin zone, and these Fermi surfaces are considered to be closely related to the generation of superconductivity Related. Combined with the previous research on the AxFe2-ySe2 superconductor, the results of the single-layer FeSe further indicate that the existence of the hole-type Fermi surface near the Γ point is not necessarily a necessary condition for the superconductivity of the iron-based superconductor, while Electronic Fermi noodles are very important. On the one hand, this information captures the key factors for the generation of iron-based superconductors, and on the other hand, it simplifies theoretical analysis and processing.
The research results were published in the recent "Nature-Communications" magazine [Nature Communications 3, 931 (2012)]. Related work was recommended to Journal Club for Condensed Matter Physics, and received expert reviews.
This work was supported by the National Natural Science Foundation of China and the 973 Program of the Ministry of Science and Technology.
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