The Mayorana fermion is an antiparticle, and Italian physicist majorana predicted that it is its own unique elementary particle. Finding the Mayorana fermion is a research hotspot in the field of high energy physics, but no definite evidence of its existence has been found. In solid materials, quasi-particles with properties similar to those of Mayorana fermions exist in specific topological defects, and the annihilation operator of quasi-particles satisfies the self-yoke relation, so it is called majorana zero-energy mode. Majorana zero-energy mode conforms to non-Abelian statistics, and its weaving operation is one of the main paths to realize fault-tolerant topological quantum computation.

Since 20 17, the Gao Hongjun/Ding Hong joint research team of Institute of Physics of Chinese Academy of Sciences/Beijing National Research Center for Condensed Matter Physics has carried out in-depth cooperation in iron-based superconductor series materials. They used several sets of independent ultra-low temperature and high magnetic field scanning tunneling microscope (STM) combined systems designed and assembled by themselves to systematically study the zero-energy mode of majorana. In 20 17, they first observed the pure majorana zero-energy mode in the magnetic vortex on the surface of Fe-based superconductor FeTe0.55Se0.45 (D. Wang et al., Science 362,333 (2018)). The results were verified by independent research teams from Fudan University, RIKEN, University of Illinois, Nanjing University and other institutions. Subsequently, the joint research team of Gao Hongjun/Ding Hong conducted an in-depth study on the phenomenon that the zero-energy mode of majorana only exists in part of the magnetic flux vortex, and found that this is due to the non-uniformity of the chemical potential on the surface of FeTe0.55Se0.45 single crystal, which may lead to the destruction of the strong topological insulation state in some areas, resulting in the loss of the topological surface state of the local surface (L Kong et al., Nature Physics 15,1655). In addition, they also observed the near-quantized platform of majorana zero-energy mode in the flux vortex on the surface of FeTe0.55Se0.45 single crystal topology (S. Zhu et al., Science 367, 189 (2020)), which provided strong evidence for the existence of majorana zero-energy mode in the flux vortex on the surface of FeTe0.55Se0.45 single crystal. In addition, they also observed the majorana zero-energy mode in the magnetic vortex on the surface of CaKFe4As4 single crystal, which extended the study of majorana zero-energy mode to the iron-phosphorus based superconducting family (W. Liu et al., Nature Commun.11,5688 (2020)).

This majorana platform also challenges the basic understanding of defect excitation in topological nontrivial band structure superconductors, and provides new possibilities for creating majorana zero-energy modes under different physical conditions. Generally speaking, there are two kinds of defect states in superconductors: the spin-polarized Yu-Shibuya-Rusinov (YSR) bound state induced by magnetic impurities and the Karoli-de-Gennis-Matron (CdGM) bound state induced by magnetic field. In 20 15, under the guidance of Pan Shuheng, a researcher at the Institute of Physics, Chinese Academy of Sciences, Yin Jiaxin and others found a robust zero-energy bound state on a single iron impurity in the iron-based superconductor Fe 1+x(Te, Se) (J. Yin et al., Nature Physis.11). Some recent theoretical work shows that when the exchange interaction between interstitial iron atoms and the substrate in FeTe0.55Se0.45 single crystal is strong enough, the exchange interaction and spin-orbit coupling will generate an equivalent magnetic field, and the quantum anomalous vortex (QAV) will be induced near the iron atoms. Due to the existence of topological surface state of FeTe0.55Se0.45 single crystal, majorana zero-energy mode can be generated in QAV.

Recently, the research team of academicians (,,money,) and so on. ) further cooperate with the research team of Ding Hong (Yang Fazhi, etc.). ) and Professor Wang Ziqiang from Boston College. They systematically studied the bound States on a single iron atom on the surface of FeTe0.55Se0.45 single crystal by using scanning tunneling microscope with extremely low temperature and strong magnetic field. They deposited a single iron atom on the surface of FeTe0.55Se0.45 single crystal at low temperature, and observed YSR bound state and zero energy mode on the iron atom (Figure 1). Through the scanning tunneling spectrum (dI/dV spectrum) of zero-energy mode Fe atom, it is found that the zero-energy mode does not split with the change of space, and there are bound States with integer quantized energy distribution near the zero-energy mode (Figure 2), which is consistent with the characteristic that the bound States of CdGM in topological flux vortex have integer quantized distribution at quantum limit. Through the temperature change test of zero-energy mode, it is found that the disappearance temperature of zero-energy mode is 4 K, which is consistent with the behavior of majorana zero-energy mode in flux vortex (Figure 2). In the high field, the zero-energy mode does not split and the half-peak width remains unchanged, which shows its stability in the magnetic field (Figure 2), which is obviously different from the phenomenon of YSR bound state splitting in the magnetic field. The above results provide strong evidence for the existence of QAV and majorana zero-energy modes on the surface of FeTe0.55Se0.45 single crystal.

Moreover, they continuously and controllably changed the distance between the needle tip and the iron atom directly above the iron atom, thus effectively changing the exchange interaction between the iron atom and the substrate, thus successfully realizing the reversible transition between the YSR bound state and the majorana zero-energy mode (Figure 3). Moreover, they have observed many times that when the distance between the QAV and the magnetic field-induced flux vortex is very close, the majorana zero-energy mode splits due to mutual coupling, and the width of the split increases with the further increase of the distance between the QAV and the magnetic field-induced flux vortex (Figure 4).

In this study, the spatial distribution, temperature and magnetic field dependence of Mayorana zero-energy mode on the surface of FeTe0.55Se0.45 single crystal iron atom are studied systematically by scanning tunneling microscope under extremely low temperature and strong magnetic field, and its topological properties are revealed. The interaction between QAV and majorana zero-energy modes in the magnetic field-induced flux vortex is observed, which provides strong evidence for the existence of QAV and majorana zero-energy modes, and then the reversible conversion between YSR bound state and majorana zero-energy modes is realized by needle tip manipulation.

This study opens up a new way for further studying the interaction and weaving of majorana zero-energy modes, and promotes the future topological quantum computation. The related results were published online in Nature Communication (NAT) on March 1. Commun. 12， 1348 (202 1))。 Yang Fazhi and Qian are the first authors of * * *. Gao Hongjun, Wang Ziqiang and Ding Hong are journalists. Gu Genda, a researcher at Brookhaven National Laboratory in the United States, provided high-quality single crystal samples. This work was supported by the National Natural Science Foundation (118810, 6 1888 102, 52022 105,/kloc-0). 20 18yfa0305800, 20 19yfa0308500) and China academy of sciences (XDB28000000, XDB07000000,121/kloc.

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