2D materials with nontrivial energy bands are highly desirable for exploring various topological phases of matter, as low dimensionality opens unprecedented opportunities for manipulating the quantum states. Here, it is reported that monolayer (ML) dialkali-metal monoxides, in the well-known 2H-MoS2 type lattice, host multiple symmetry-protected topological phases with emergent fermions, which can be effectively tuned by strain engineering. Based on first-principles calculations, it is found that in the equilibrium state, ML Na2O is a 2D double Weyl semimetal, while ML K2O is a 2D pseudospin-1 metal. These exotic topological states exhibit a range of fascinating effects, including universal optical absorbance, super Klein tunneling, and super collimation effect. By introducing biaxial or uniaxial strain, a series of quantum phase transitions between 2D double Weyl semimetal, 2D Dirac semimetal, 2D pseudospin-1 metal, and semiconductor phases can be realized. The results suggest monolayer dialkali-metal monoxides as a promising platform to explore fascinating physical phenomena associated with novel 2D emergent fermions.

中文翻译：

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二维二碱金属一氧化物中的可调谐拓扑能带。

具有非平凡能带的二维材料非常适合探索物质的各种拓扑相，因为低维为操纵量子态提供了前所未有的机会。据报道，单层（ML）二碱金属一氧化物在著名的 2H-MoS2 型晶格中具有多个具有涌现费米子的对称保护拓扑相，可以通过应变工程进行有效调节。基于第一性原理计算，发现在平衡状态下，ML Na2O是二维双Weyl半金属，而ML K2O是二维赝自旋1金属。这些奇特的拓扑态表现出一系列令人着迷的效应，包括通用光学吸收、超级克莱因隧道效应和超级准直效应。通过引入双轴或单轴应变，可以实现二维双外尔半金属、二维狄拉克半金属、二维赝自旋1金属和半导体相之间的一系列量子相变。结果表明，单层二碱金属一氧化物是探索与新型二维涌现费米子相关的迷人物理现象的有前途的平台。