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《Science》:The discovery of the new chiral fermion has scientific significance
2019-03-25
来源：转载自第三方
Researcher Ding Hong from the Institute of Physics of the Chinese Academy of Sciences/National Research Center for Condensed Matter Physics Ding Hong, Qian Tianhe and Associate Researcher Sun Yijie and the collaborators of the Department of Physics, Renmin University of China, and other collaborators found that there is a new type of chiral fermion in CoSi. The experimental results prove the existence of a new type of chiral fermion, which provides an ideal platform for exploring novel physical phenomena caused by chiral fermions.
Chirality refers to the phenomenon that an object cannot overlap with its mirror image. For example, our hands cannot overlap with the right and left hands. Chiral phenomena are widespread in nature and represent an important "symmetry" in physics. In relativistic physics, chirality means that the spin and momentum directions of massless particles are parallel or antiparallel, although the theory predicted the existence of massless chiral fermions, the fermions, 90 years ago, but they are basic particles. The existence has not been confirmed by experiments.
In solids, the physical electronic structure of some materials does not hybridize due to some symmetry or topological protection, and the band is degenerate. This feature is between the metal and the insulator or semiconductor and is a semi-metallic material, also known as a topological semi-metal. Various types of Fermi quasiparticles are excited near the degeneracy point.
To date, experiments have shown that there are three kinds of Fermi quasi-particles in the "solid universe": the quadruple degenerate Dirac fermion, the double degenerate deer fermion, and the triple degenerate fermion. Only the ferm fermion has a chirality, and Dirac Fermi and the triple degenerate ferm itself have no chirality. Qian Tian said, "It is possible to break down the symmetry, such as the external magnetic field, and retreat them into a chiral ferm fermion."
Many of the physical properties exhibited by the new exotic topological quantum material Dirac semi-metal and triple degenerate point semi-metals, such as the negative magnetoresistance effect caused by chiral anomalies and the surface state Fermi arc, are essentially derived from chirality.
"The existing theory has proved that there are many types of chiral fermions and related materials other than fermions in the "solid universe", but direct experimental evidence is still lacking." Qian Tian said that among the many theoretical predictions about the new chiral fermions, the transition metal silicide CoSi is an ideal material for the band structure. For this reason, the material has attracted the attention of many international research teams.
Angle-resolved photoelectron spectroscopy experiments can provide direct evidence, but require atomic-level sample surfaces. Since CoSi is a three-dimensional material, the surface obtained by the conventional cleavage method cannot meet the experimental requirements. Sun Yijie, one of the authors of the paper, and the first author of the thesis, Rao Zhicheng, a doctoral student at the University of Chinese Academy of Sciences, used the method of “polishing-bombardment-annealing” to obtain an atomically leveled surface on the CoSi single crystal sample after a long period of continuous exploration. And in the Shanghai Light Source "Dream Line" observed clear body and surface energy bands. The experimental results show that there are degenerate points of the body energy band at the center and corner of the Brillouin zone, and the chiral fermion at the degenerate point is determined by analyzing the surface state, which is highly consistent with the theoretical calculation results.
Three research teams in Japan, the United States, and the United Kingdom also found evidence of new chiral fermions in CoSi materials, but Chinese scientists have achieved the highest quality results in terms of sample quality and data quality.
"Looking for new things in the physical world, many of them are theoretical predictions first. On this basis, sample preparation and experimental observations are carried out to 'determine', and finally a certain phenomenon or physical state exists or does not exist, but whether it has a certain Some of these properties and even the ultimate practical value require more research." Sun said.
Scientists believe that the new chiral fermions other than the ferm fermions are not only a breakthrough in the field of topological semi-metals, but also provide more ways to explore the physical phenomena related to chiral fermions, with important science. Meaning and application value.
Edited by Suzhou Yacoo Science Co., Ltd.
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