npj: 新型拓扑氧化物助力拓扑电子态万花筒

来源：知社学术圈

新型拓扑电子材料的发现带来了发现新物理的机会。迄今，很多拓扑材料已经由理论提出并被实验验证。可惜的是，拓扑氧化物的存在仍旧缺少令人信服的实验证据。

福建师范大学的张薇副教授与中国科学院物理研究所的翁红明研究员、武汉大学的余睿教授等合作，运用第一性原理计算和对称性分析相结合的方法，提出在烧绿石结构的Tl2Nb2O6+x（0≤x≤ 1.0）中，氧含量在实验上的连续可调会导致多种不同的拓扑电子态。通过改变氧含量和/或调节晶体对称性，可以得到拓扑绝缘体、狄拉克半金属和三重简并点半金属等。这些由氧的固溶体驱动的拓扑相变是独特的、合乎物理的，其本质是源于T1+和Tl3+的价态改变。该研究有望丰富拓扑电子材料的知识，促进其在氧化物中的应用前景。另外，在同一系列中实现多种拓扑电子态，将促进不同拓扑电子态间的转变、耦合机制的研究，这有望产生新的物理及有趣的输运性质。

该文近期发表于npj Computational Materials 5: 105 (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。

Topological phases in pyrochlore thallium niobate Tl2Nb2O6+x

Wei Zhang,Kaifa Luo, Zhendong Chen,Ziming Zhu, Rui Yu, Chen Fang &Hongming Weng

The discovery of new topological electronic materials brings a chance to uncover new physics. Up to now, many materials have been theoretically proposed and experimentally proved to host different kinds of topological states.Unfortunately, there is little convincing experimental evidence for the existence of topological oxides. The reason is that oxidation of oxygen leads to ionic crystal in general and makes band inversion unlikely. In addition, the realization of different topological states in a single material is quite difficult, but strongly needed for exploring topological phase transitions. In this work, using first-principles calculations and symmetry analysis, we propose that the experimentally tunable continuous solid solution of oxygen in pyrochlore Tl2Nb2O6+x (0 ≤ x ≤ 1.0) leads to various topological states. Topological insulator, Dirac semimetal, and triply degenerate nodal point semimetal can be realized in it via changing the oxygen content and/or tuning the crystalline symmetries. When x = 1, it is a semimetal with quadratic band touching point at Fermi level. It transits into a Dirac semimetal or a topological insulator depending on the in-plane strain. When x = 0.5, the inversion symmetry is spontaneously broken inTl2Nb2O6.5 , leading to triply degenerate nodal points. When x = 0, Tl2Nb2O6 becomes a trivial insulator with a narrow band gap. These topological phase transitions driven by solid solution of oxygen are unique and physically plausible due to the variation of valence state of Tl+ and Tl3+. This topological oxide will be promising for studying correlation induced topological states and potential applications.