The research group of Professor Feng Miao from Nanjing University, our long-term customer, published the latest results on the regulation of the chiral properties of the second type of Weyl semimetals in Nature Communications.
In 1929, physicist Hermann Weyl predicted a class of massless relativistic particles known as Weyl fermions. It was only in recent years that Weyl fermions were finally discovered in condensed matter physics. The usual Weyl fermions satisfy Lorentz symmetry and are called "Weyl fermions of the first kind". Recent theory predicts that there can be another "second type of Weyl fermions" that break Lorentz symmetry in condensed matter physics, and the corresponding topological materials are called "second type Weyl semimetals". The second class of Weyl semimetals has topologically protected unclosed surface states (Fermi arcs) in the bulk band due to significant tilting of the energy bands near the Weyl point, theoretically predicted to induce anisotropic chiral transport properties Such novel quantum phenomena have aroused widespread research interest.
The research group of Professor Feng Miao of Nanjing University selected the first type of Weyl semimetal predicted by theory: tungsten ditelluride (WTe2) for research. WTe2, a layered transition metal chalcogenide, has recently received extensive attention due to the experimentally observed giant unsaturated magnetoresistance.The tungsten ditelluride (WTe2) material is provided by the new material division of Juna Group。The two crystallographic directions in the plane are perpendicular to each other (a, b axis), resulting in strong in-plane anisotropy due to the formation of tungsten chains in the a-axis direction. The research group took advantage of the layered peelability of WTe2 to obtain samples with different layers, and used the calibrated mask evaporation technology to fabricate high-quality WTe2 thin-film devices, and found that the ideal thickness of the samples for studying the chiral transport properties of WTe2 is 7 -15nm. The researchers observed a significant longitudinal negative magnetoresistance effect (the magnetic field is parallel to the current direction) in samples with the current parallel to the b-axis. The effect is very sensitive to the change of the angle between the magnetic field and the current. The effect is most obvious when the angle is completely parallel (the angle is zero), and the effect can be effectively suppressed by a small angle. The analysis shows that the longitudinal negative magnetoresistance effect is Caused by abnormal chirality. At the same time, the research group used the same fabrication process to prepare a sample with a current parallel to the a-axis (tungsten chain), and found that the chiral anomalous characteristic disappeared, which effectively verified the characteristics of WTe2 as the second type of Weyl semimetal.
In the research of topological electronics, how to effectively control the topological transport characteristics is the key to realize the application. Based on the observation of the chiral anomalous effect, the research group took advantage of the gate voltage controllability of thin-film devices to realize the in-situ adjustment of the Weyl semimetal Fermi energy near the Weyl point for the first time, and realized the chirality in the WTe2 device. Field-effect modulation of transport properties. This work not only provides a universal experimental method for the in situ study of the second type of Weyl fermions in condensed matter physics, but also has important implications for the applied research of topology and chiral electronics.
This work was published in the journal Nature Communications 7, 13142 (2016) under the title "Gate-Tunable Negative Longitudinal Magnetoresistance in the Predicted Type-II Weyl Semimetal WTe2" on October 11, 2016. Wang Yaojia, a doctoral student at the School of Physics, Nanjing University, is the first author of the paper. Professor Miao Feng, and Professor Wan Xiangang and Wang Bogen, who provided theoretical calculations, are the co-corresponding authors of the paper. Academician Xing Dingyu and Professor Lu Haizhou from Southern University of Science and Technology are the co-corresponding authors of the paper. The research provided theoretical support, and Professor Wang Zhenlin provided Raman experimental assistance. This research was supported by the Collaborative Innovation Center for Microstructure Science and Technology, the National Major Scientific Research Program of "Quantum Regulation" of the Ministry of Science and Technology (Young Scientists Special Project), the National Key Basic Research and Development Program of the Ministry of Science and Technology (973 Program), the Jiangsu Provincial Fund for Distinguished Young Scholars, Funded by the National Natural Science Foundation of China.

     In addition to providing integrated services such as graphene materials, equipment, and testing, Juna Group also cooperates with companies such as HQ Graphene in the Netherlands, 2dsemiconductors in the United States, and ACS Materials to provide high-quality graphene and graphene-like two-dimensional crystal materials for global customers.（Such as natural graphite NG, MoS2, WS2, boron nitride crystal, black phosphorus BP, WTe2, ReS2, ReSe2, etc.），And provide customized services（Such as two-dimensional material mechanical peeling technology training, performance testing training such as layer number determination, and laboratory-specific mechanical peeling tape, etc.），To meet the different needs of customers.