Tunable Topological Energy Bands in 2D Dialkali‐Metal Monoxides

Abstract

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.

Two-dimensional topological semimetals*

“…Similar spinless Weyl points have been identified in several other 2D materials, such as graphynes [40], some 2D boron allotropes [41], germanene [42], and group-Va monolayers with the phosphorene structure [36]. It was noted that in some of these examples, the Weyl points appear on the high-symmetry path, like in βgraphyne [40], germanene on Al(111) [42], and strained monolayer Na 2 O [35]. Moreover, in strained phosphorene structures, Weyl points at generic k were reported [36].…”

“…In 2D, higher-order lines appear unlikely, but higher-order points do exist. For example, twofold nodal points with quadratic dispersion, called double Weyl points, have been reported in several 2D materials [33][34][35].…”

Higher-Order Nodal Points in Two Dimensions

“…The only type known so far is the so-called double Weyl point [39], which has twofold degeneracy and quadratic band splitting. It was found in a few 2D materials, including the blue phosphorene oxide [39], monolayer Mg 2 C [40], and monolayer Na 2 O and K 2 O [41]. However, many fun-damental questions are yet to be answered.…”

“…Third, the QNPs only exist in the absence of SOC, which hence just correspond to the double Weyl points in Refs. [39][40][41]. There is no QNP in 2D systems with SOC.…”

Strong Electron–Phonon Coupling in 3D WN and Coexistence of Intrinsic Superconductivity and Topological Nodal Line in Its 2D Limit

“…3D-to-2D crossover can induce quantum phase transition depending on symmetry, terminations and thickness of the 2D films, as been demonstrated in group-V monolayers [19] and dialkali-metal monoxides. [20] Specifically, Cd 3 As 2 , a typical Dirac semimetals in its bulk form, becomes trivial insulator below seven monolayers [21] and a dual 2D topological insulator in the monolayer limit. [22] Another Dirac semimetals Na 3 Bi, transform to topological insulator in monolayer and double layer films.…”