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Novel Quantum Spin Hall Insulator Predicted in Monolayer Bi4Br4


Quantum spin Hall insulators (QSHIs), also known as 2D topological insulators, are special kind of two-dimensional semiconductors that has gapless helical edge states inside the bulk band gap. The edge states are topologically protected against backscattering from non-magnetic disorder, and give rise to a quantized spin Hall conductance in QSHI. QSHI have attracted tremendous interests in condensed matter physics and material science due to its rich physics and promising applications. The quantized spin Hall conductance has only been demonstrated experimentally in HgTe/ CdTe [3] and InAs/GaSb [4] quantum wells, which require extremely low working temperature due to their small bulk band gaps. Novel QSHIs with large gap are desired to realize quantized spin Hall conductance at room temperature.


Recently, Dr. Jin-Jian Zhou, Wanxiang Feng,  Cheng-Cheng Liu and Shan Guan from Prof. Yao's research group at the Beijing Institute of Technology, predicted a novel QSHI in Monolayer Bi4Br4. Based on DFT calculations, they found the monolayer structure of layered semiconductor Bi4Br4 has a non-trivial band gap of about 180 meV, which is far beyond the room temperature condition (~ 26 meV) (Nano Lett. 14, 4767 (2014) ). Except the large-gap, the high feasibility of experimental preparation of Monolayer Bi4Br4 was also demonstrated. The interlayer coupling in Bi4Br4 single crystal is of weak van der Waals-type. Monolayer Bi4Br4 is possible to be exfoliated from bulk Bi4Br4. They found the band gap and low-energy electronic structure are barely affected by the interaction between adjacent Monolayers in Multilayer Bi4Br4, thus the surface of bulk Bi4Br4 as the intrinsic insulating substrate for monolayer Bi4Br4. The stair-stepped edge prepared by nano-fabrication on the cleaved surface of Bi4Br4 can be used to observe topological edge states (New J. Phys. 17, 015004 (2015) ). Another interesting feature is the highly structural anisotropy due to the special 1D molecular chain structure of Monolayer Bi4Br4. Their results indicate the monolayer Bi4Br4 holds great promise for realization of quantized spin Hall conductance at room temperature, which awaiting experiment confirmation.



The work was supported by grants from the National Science Foundation of China and the Ministry of Science and Technology of China.

Release date:2015-10-27