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ICQM members publish an article in Physical Review Letters reporting "Disorder and Metal-Insulator Transitions in Weyl Semimetals"
《物理学评论快报》报道量子材料科学中心在拓扑半金属研究最新成果 “Weyl半金属中的无序和金属绝缘体转变”

A PhD student Chui-Zhen Chen and his advisor Prof. X.C. Xie of The International Center for Quantum Materials (ICQM), School of Physics, Peking University, published an article in “Physics Review Letter with the title: Disorder and Metal-Insulator Transitions in Weyl Semimetals (PRL. 115, 246603). This work was done in collaboration with Pro.Juntao Song from Hebei Normal University, Prof. Hua Jiang from Soochow University, Prof. Qing-feng Sun from ICQM, and Pro. Ziqiang Wang from Boston College.

The newly proposed Weyl semimetal (WSM), a time-reversal symmetry breaking topological quantum state of matter, has been observed recently in real materials (TaAs and NbAs) as well as on optical lattices. It is thus timely to study the effects of disorder, localization, and Anderson type of metal-insulator transitions in WSM systems, both for their direct experimental relevance and for their fundamental value in advancing the understanding of the interplay between randomness and topological order.

C.-Z. Chen et al. do so in this work using both numerical and analytical approaches. Because the WSM has novel gapless excitations, i.e. the Weyl nodes in the bulk and Fermi arcs on the surface, they find an unexpectedly rich phase diagram in the presence of disorder [see Fig.(b)] that is highlighted by the WSM to quantum anomalous Hall insulator transition; the WSM to 3D anomalous Hall metal transition; and the normal band insulator to WSM transition. They address the important issue on the stability of the Weyl nodes and Fermi arcs against weak disorder and obtain the complete phase behaviors for all disorder strengths by calculating the localization length and the Hall conductivity. They propose that the novel disorder-induced phase transitions can be realized on photonic lattices.

 

(a)       The phase diagram of the clean Weyl semimetal Hamiltonian H0  on the tz/m0-mz/m0  plane.

(b)       The disordered Weyl semimetal phase diagram on the W-mz plane. The symbols guided by the solid lines are obtained from the localization length. The blue dashed lines are the phase boundaries determined using the SCBA.

The work was supported by National Basic Research Programs of China, National Natural Science Foundation of China, and Collaborative Innovation Center of Quantum Matter, China.

  最近量子材料科学中心陈垂针博士在谢心澄教授指导下在拓扑半金属研究领域取得重要进展,在物理学评论快报上发表了标题为“Disorder and metal-insulator transitions in Weyl semimetals” 的研究论文[Phys. Rev. Lett. 115, 246603 (2015)]。这项工作与河北师范大学宋俊涛教授, 苏州大学江华教授,量子材料中心的孙庆丰教授以及Boston College 的汪自强教授合作完成。

  Weyl半金属是一种全新的拓扑量子态。 2015年,它在TaAs族材料中被实验证实,并立刻成为基础凝聚态物理和材料科学研究的焦点。 与传统半金属材料不同,Weyl半金属体态能谱中存在成对的零能隙Weyl点,并拥有局域在表面的费米弧。独特的能带结构引发了Weyl半金属中无能隙Weyl点对杂质势散射(无序)稳定性的重要猜测。前人研究表明单个Weyl点非常稳定,即使在强无序极限下也不能发生Anderson局域化。然而,前人理论还发现Weyl点具有类似于动量空间的磁单极性质,这就决定了在实际材料中它们必须成对出现。当一对Weyl点在动量空间中相互靠近或者它们之间引发强的谷间散射的时候,就有可能会被湮灭掉。因此,对成对Weyl点在无序下的局域化性质的研究就显得至关重要。

  最近,陈垂针等人利用精确数值结合理论解析方法系统地研究了无能隙Weyl点和费米弧的对杂质势散射的稳定性。在此基础上,获得了无序引起的Weyl半金属中新奇量子相变物理图像。他们意外发现, 考虑Weyl点成对出现这一特性时, Weyl半金属体系存在异常丰富的相图 [见图b]。特别是,该相图中存在三个难以被传统理论理解的新奇相变: (1)Weyl半金属到三维量子反常霍尔绝缘体的相变;(2)Weyl半金属到三维反常霍尔金属的相变;(iii)普通绝缘体到Weyl半金属的相变。他们得到了无序引发这些相变的物理机制并探讨了在光子晶体中观测到这些相变的可能性。 

  1. 干净Weyl半金属哈密顿量H0关于tz/m0-mz/m0 的相图。
  2. 无序Weyl半金属系统关于 W-mz 的相图, W表示无序强度。相图中实线连接的实心点是通过严格数值方法计算得到的。而蓝色虚线是通过解析方法确定的。

  这个工作得到国家重点基础研究发展规划和国家自然科学基金和量子物质科学协同创新中心的支持。