Recently, Associate Prof. He Hongtao and Associate Prof. Lu Haizhou made a new progress in the research on topological semimetals. They observed the long-sought negative magnetoresistance in the topological Dirac semimetal Cd3As2. The work, titled "Negative Magnetoresistance in Dirac semimetal Cd3As2", has been published in the journal "Nature Communications" (impact factor 11.470).  
The resistance of a solid changes under magnetic fields, known as the magnetoresistance effect. The effect has diversified applications, such as in magnetic storage devices and magnetic sensors. Because of the Lorentz force, a magnetic field usually prevents electrons from moving forward. As a result, the resistance increases with increasing magnetic field in most occasions, leading to the positive magnetoresistance. In nature, negative magnetoresistance is rare in non-ferromagnetic materials. The only known example so far is the weak localization effect, which arises from the quantum interference at extremely low temperatures. In recently discovered topological semimetals, a new mechanism of negative magnetoresistance may emerge. The band structure of a topological semimetal looks like that of graphene, but in three dimensions (Figure 1a), with the conductance and valence bands touching at a finite number of momentum points, dubbed the Weyl points. The Weyl points can host paired monopole and anti-monopole in momentum space (Figure 1b). The nontrivial topological structure may generate an effect much similar to the chiral anomaly in high-energy physics. As a consequence, a magnetic field that is parallel to an electric field can generate a chiral current, effectively giving rise to a semi-classical negative magnetoresistance. 

In this work, the negative magnetoresistance was observed in the topological Dirac semimetalCd3As2. This material has extremely high mobility (10 to the 7 cm squared per volt second) and large non-saturating transverse positive magnetoresistance, thus promising many potential applications. Despite of many previous works on Cd3As2, no negative magnetoresistance has been reported so far, because of the high carrier density in the samples. The negative magnetoresistance origins from the monopole and anti-monopole, so the Fermi energy has to be as close to the Weyl nodes as possible. In other words, one needs low carrier density to observe a prominent negative magnetoresistance. The samples used in this work have the lowest carrier densities known to the material, of the order of 10 to the 16 per cubic cm. At such low carrier densities, the negative magnetoresistance can be observed even at room temperature (Figure 1c). The samples in the experiment were from Prof. Jiannong Wang’s lab at Hong Kong University of Science and Technology (HKUST). The magnetoresistance was measured in Prof. Hongtao He’s Quantum Transport Laboratory at SUSTech. Prof. Haizhou Lu provided the theoretical support to the analysis of the experimental data, based on collaboration works with Prof. Shunqing Shen from the University of Hong Kong. Dr. Hui Li from HKUST, Prof. Hongtao He, and Prof. Haizhou Lu are the first authors with equal contribution.
Also, Prof. Haizhou Lu has joined another collaboration work by Shuang Jia’s group at Peking University and Hasan’s group at Princeton University. The work is on the negative magnetoresistance in TaAs, the first topological Weyl semimetal, and has been accepted recently by Nature Communications.
The research was supported by SUSTech Research Start-up Fund, National Science Foundation of China (grant no.: 11204183 and 11374135), and Young Scholar Project of Thousand Talents Program of China.

 Associate Prof. He Hongtao and Associate Prof. Lu Haizhou

The research was supported by SUSTech Research Start-up Fund, National Science Foundation of China (grant no.: 11204183 and 11374135), and Young Scholar Project of Thousand Talents Program of China.

The link to the article

http://www.nature.com/ncomms/2016/160108/ncomms10301/full/ncomms10301.html