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理解物质不同相的基础物理学是凝聚态物理学最重要的目标之一。理解未发现相的有效途径是研究相变的本质。拓扑相和拓扑相变是与Landau相变不同的新现象，由于其不寻常的基本物理特性以及在下一代电子设备中的潜在应用前景，吸足了眼球。因此，在真实材料中寻找新型拓扑相并研究其相变成为凝聚态物理和材料科学的研究前沿。传统的拓扑相变描述了从拓扑平凡到拓扑非平凡态的演化，但两个拓扑非平凡绝缘态之间的非常规拓扑相变如何进行，尚不得而知。
来自中国台湾两所大学的Tay-Rong Chang和Horng-Tay Jeng等提出了由Dirac无间隙态介导的两个拓扑非平凡绝缘态之间的非常规拓扑相变体系，该体系源于非对称型晶体对称性，不同于传统的拓扑相变。KHgX（X = As，Sb，Bi）族是第一个实验上实现的拓扑非同态晶体绝缘体，其中拓扑表面态以莫比乌斯扭曲连接为特征。他们基于第一原理计算，通过引入两个新相来使KHgX的拓扑相图多样化。通过施加应力，KHgX经历拓扑绝缘体-金属的转变，从拓扑非同态晶体绝缘体相转变为Cm = -2的DSM相，在非对称晶体结构中的非平凡镜Chern数Cm = -3。通过对称性破坏，DSM相转换为另一个新的拓扑非同态晶体绝缘体相，其中Cm = -3主导着QSH效应。表面能带的连通性的变化，提供了拓扑相变的直接证明，而且要实现这些预测的新拓扑相，操纵带隙是其关键。
该文近期发表于npj Computational Materials 5: 65 (2019)，英文标题与摘要如下，点击左下角“阅读原文”可以自由获取论文PDF。
Unconventional topological phase transition in non-symmorphic material KHgX (X = As, Sb, Bi)
Chin-Shen Kuo, Tay-Rong Chang, Su-Yang Xu & Horng-Tay Jeng
Traditionally topological phase transition describes an evolution from topological trivial to topological nontrivial state. Originated from the non-symmorphic crystalline symmetry, we propose in this work an unconventional topological phase transition scheme between two topological nontrivial insulating states mediated by a Dirac gapless state, differing from the traditional topological phase transition. The KHgX (X = As, Sb, Bi) family is the first experimentally realized topological non-symmorphic crystalline insulator (TNCI), where the topological surface states are characterized by the Mobius-twisted connectivity. Based on first-principles calculations, we present a topological insulator–metal transition from TNCI into a Dirac semimetal (DSM) via applying an external pressure on KHgX. We find an unusual mirror Chern number Cm = −3 for the DSM phase of KHgX in the non-symmorphic crystal structure, which is topologically distinct from the traditional DSM such as Na3Bi and Cd3As2. Furthermore, we predict a new TNCI phase in KHgX via symmetry breaking. The topological surface states in this new TNCI phase display zigzag connectivity, different from the unstressed one. Our results offer a comprehensive study for understanding how the topological surface states evolve from a quantum.
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