Laboratory observation of (Li,Fe)OHFeSe superconductors and monolayer FeSe thin films

Iron-based superconductors as the second type of high-temperature superconductors after the copper oxide superconductors, the superconducting mechanism is an important issue in condensed matter physics. Most iron-based superconductors have a hole-type Fermi surface at the center of the Brillouin zone and an electronic Fermi surface at the top corner of the Brillouin zone. A common mechanism of superconductivity (Fermi surface "nesting") suggests that the scattering of electrons between electron-type and hole-type Fermi surfaces is the main cause of electron pairing and superconductivity in iron-based superconductors.

In early 2012, Xue Qikun's research group of Tsinghua University and the Ma Xucun team of the surface laboratory of the Institute of Physics, Chinese Academy of Sciences reported the possible presence of high-temperature superconductivity in the liquid nitrogen area of ​​a single-layer FeSe thin film grown on a SrTiO3 substrate. Phys. Lett. 29 (2012) 037402]. The origin of high-temperature superconductivity in monolayer FeSe/SrTiO3 thin films, especially the effect of interface on superconductivity enhancement, has become another hot topic in superconductor research. Institute of Physics, Chinese Academy of Sciences / Beijing National Laboratory for Condensed Matter Physics Zhou Xingjiang, National Key Laboratory of Superconductivity, and subsequently observed 65 K superconducting transitions from the energy gap measurement of a single-layer FeSe/SrTiO3 superconducting thin film. Discovered its simple, unique electronic structure: There is no hole-type Fermi surface in the center of the Brillouin zone, but only an electronic Fermi surface at the top corner of the Brillouin zone [Nature Communications 3 (2012) 931; Nature Materials 12 (2013) 605 ]. These results, combined with the Fermi-like results previously observed in the AxFe2-ySe2 superconductors, pose serious challenges to the Fermi surface nesting theory for the superconducting mechanism of iron-based superconductors. However, the interfacial effect and the complexity of the phase separation involved in the single-layer FeSe/SrTiO3 thin film and the AxFe2-ySe2 superconductor make it possible for some people to think that they are different from the vast majority of bulk iron-based superconductors. The special mechanism of superconductivity, and then questioned whether they constitute sufficient evidence to exclude the nesting theory of Fermi surfaces.

Associate Professors Zhao Xing, He Shaolong, Ph.D. Liang Aiji, Hu Yong, and Liu Defa of the Zhou Xingjiang Research Group, and Zhao Zhongxian, a fellow of the Chinese Academy of Sciences, Dong Xiaoli, a researcher, and Lu Zhongyi and Liu Kai, professors of the Renmin University of China, used high resolution angular resolution photoelectron spectroscopy technology. , Took the lead in studying the electronic structure and superconducting energy gap of newly discovered (Li,Fe)OHFeSe superconductors, and obtained important results for understanding the superconducting mechanism of iron-based superconductors and the high-temperature superconductivity of single FeSe/SrTiO3 films. The origin of sex provides important information.

Their study found that the electronic structure of the (Li,Fe)OHFeSe superconductor is very similar to that of the single-layered FeSe/SrTiO3 superconducting thin film and the AxFe2-ySe2 superconductor (Figure 1): there is no hole type around the center of the Brillouin zone. The Fermi surface, and there is an electronic Fermi surface only at the top corner of the Brillouin area. A further comparison shows that the energy band structures of the three also show striking similarities (Figure 2). The superconducting energy gap of (Li,Fe)OHFeSe is nearly isotropic, and there is no node with an energy gap of zero (Fig. 3, Fig. 4), which is also associated with a single layer FeSe/SrTiO3 superconducting thin film and AxFe2-ySe2 superconductor The characteristics of the superconducting energy gap are similar.

The (Li,Fe)OHFeSe new superconductors and the single-layer FeSe/SrTiO3 superconducting films and AxFe2-ySe2 superconductors show similar electronic structures and superconducting energy gap structures, suggesting that they may have a common superconducting mechanism. The generation of high-temperature superconductivity and the interfacial effect of FeSe/SrTiO3 superconducting thin films provide important information. Unlike single-layer FeSe/SrTiO3 superconducting films and AxFe2-ySe2 superconductors, (Li,Fe)OHFeSe superconductors are single-phase pure materials and have a very high superconducting temperature (~42K). A direct comparison of most bulk iron-based superconductors offers the possibility. In the (Li,Fe)OHFeSe superconductor, the unique electronic structure of the electron-only Fermi surface is observed, and the electron scattering between the hole-type Fermi surface and the electron-type Fermi surface can be completely ruled out as superconductivity in the iron-based superconductor. The Fermi Surface Nesting Theory of Electron Matching Origin

Related research results were published on the recent Nature Communications [Lin Zhao et al., Common Electronic Origin of Superconductivity in (Li, Fe) OHFeSe Bulk Superconductor and Single-Layer FeSe/SrTiO3 Films, Nature Communications 7, 10608 (2016)].

The above research work has been funded by the National Natural Science Foundation of China, the Ministry of Science and Technology and the Chinese Academy of Sciences Pilot B project fund.

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