Recently, top journal Physical Review Letters (PRL) published an article titled “Forbidden Backscattering and Resistance Dip in the Quantum Limit as a Signature for Topological Insulators,” first authored by Class of 2018 alumni Chen Yiyuan. Associate Professor Lu Hai-Zhou from Department of Physics is the corresponding author, while Professor Xie Xincheng from Peking University co-authored the article as well. Chen was the second undergraduate from Lu’s research team whose paper was published on PRL. In April, PRL published “Rules for Phase Shifts of Quantum Oscillations in Topological Nodal-Line Semimetals” whose first author is Class of 2017 alumni Li Cequn.

Associate professor Lu Haizhou (left) and undergrad student Chen Yiyuan (right).

Recently graduated from the Department of Physics, Chen Yiyuan joined Professor Lu Hai-Zhou’s research group in his junior year and completed most of the theoretical computation in his final year. In August, Chen will begin his Ph.D. studies at the University of Florida. The success of Chen Yiyuan and Lu Hai-Zhou’s research project can be attributed to the Department of Physics’ education reform, which was initiated by Associate Professor Ye Fei. As a result, Professor Lu Hai-Zhou introduced the content of Landau Level which describes the charged particles’ motions in the magnetic field in his class Quantum Mechanics II for junior year students, which prepared undergraduates for professional research and publication opportunities.

Chen Yiyuan said that the study of topological phases had been expanded to topological material with intrinsic properties. For instance, the 2016 Nobel Prize in Physics was awarded to David J. Thouless (University of Washington), F. Duncan M. Haldane (Princeton University) and J. Michael Kosterlitz (Brown University) for their “theoretical discoveries of topological phase transitions and topological phases of matter.”

When introducing the background of his research, Chen pointed out that Identifying topological insulators and semimetals often focuses on their surface states, using spectroscopic methods such as angle-resolve photoemission spectroscopy or scanning tunneling microscopy. In contrast, studying the topological properties of topological insulators from their bulk-state transport is more accessible in most labs but seldom addressed. In his work, Chen Yiyuan shows that, in the quantum limit of a topological insulator, the backscattering between the only two states on the Fermi surface of the lowest Landau band can be forbidden at a critical magnetic field. The conductivity is determined solely by the backscattering between the two states, leading to a resistance dip that may serve as a signature for topological insulator phases. More importantly, this forbidden backscattering mechanism for the resistance dip is irrelevant to details of disorder scattering. The theory can be applied to revisit the experiments on Pb1−xSnxSe, ZrTe5, and Ag2Te families, and will be particularly useful for controversial small-gap materials at the boundary between topological and normal insulators.

Figure 1 in the paper. (a) The energy spectrum of topological insulator in absence of magnetic field. (b) Landau bands and the quantum limit of topological insulator in a strong magnetic field. (c) Experiment published in Phys. Rev. Lett. 119, 106602 (2017). (d) Comparison between theory and experiment in the paper.

This work is supported by many foundations including the National Key R & D Program under Ministry of Science and Technology, National Natural Science Foundation of China, foundation for Construction of a High level University of Technology in Guangdong Province, Shenzhen Key Laboratories, and Initiative Research Funding by SUSTech.

Article Link: https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.121.036602

Original Chinese Report: Department of Physics

Academic Paper: Chen Yiyuan, Lu Hai-Zhou, and Xie Xincheng

Translation and Adaptation: Fan Yining & Department of Physics

Proofreading: Chris Edwards