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Recently, 2D ferroelectric (FE) heterostructures have become a subject of great interest due to their potential device applications and the underlying physics involved. In this study, the first‐principles calculations are employed to examine the FE control of electronic structures in 2D FE heterostructures, specifically In2Se3/h‐BN and CuInP2S6 (CIPS)/h‐BN. In these results, it is demonstrated that by reversing the polarization of the FE layers, the band alignment of the heterostructures can be interconverted between type II and type I. For In2Se3/h‐BN, the variation of out‐of‐plane polarization can be attributed to the hindrance and facilitation of charge transfer from h‐BN to In2Se3 by the intrinsic electric field of the In2Se3 monolayer. For CIPS/h‐BN heterostructures, the higher transferred charge in the Cdn configuration due to the presence of built‐in electric fields and the stronger interfacial interaction in the Cdn configuration result in a higher polarization value compared to the Cdn configuration. Moreover, the carrier mobility of the heterostructures can also be effectively modulated by the FE polarization. In these findings, the potential significance of FE heterostructures with tunable band alignment and bandgap is highlighted in the development of nanoscale optoelectronic devices. [ABSTRACT FROM AUTHOR]

Topological nodal‐line semimetals have received extensive attention in recent years, and they exhibit a variety of shapes under various crystallographic symmetry protections, including nodal rings, nodal chains, and nodal knots. By using symmetry analysis and an effective force constant model, it is proposed that a class of 2D hourglass nodal nets protected by nonsymmorphic symmetry can exist in the space group P42/ncm, where the glide operations play a crucial role in the formation of the nets. Further effective k⋅p model analysis shows that the connect points of the nets are Dirac points that are forced by the symmetries. Based on first‐principle calculations, it is predicted that this nodal net can be realized in the phonon dispersion of 3D Sn. The Berry phase and nontrivial surface states further support its nontrivial topological characteristics. Herein, the studies expand the nodal structure in the study of nodal line in phononic systems and offer a solid foundation for subsequent experimental investigation on this challenging topological state. [ABSTRACT FROM AUTHOR]

Nonsymmorphic symmetries play a significant role in the emergence of topological nontrivial phases in some 2D materials. Herein, a new 2D material P21/m‐MoTe2 with nontrivial topological properties and nonsymmorphic symmetries is proposed. With the help of first‐principles calculations, it is found that it is dynamically stable, and has relatively high energetic stability. When spin–orbit coupling (SOC) is ignored, this material is a semimetal with two intersects appearing on the high‐symmetry line Γ–Y. Detailed symmetry analysis reveal that they are protected by the screw operator C˜2y. After SOC is turned on, it turns into a topological insulator with a global bandgap of 94 meV. The large gap ensures that the topological nontrivial phase is robust to an external perturbation. The discovery of nonsymmorphic P21/m‐MoTe2 expands the family of transition metal dichalcogenides and provides a new platform to study topological phases in 2D nonsymmorphic materials. [ABSTRACT FROM AUTHOR]

Dirac cones in 2D carbon allotropes are generally protected by symmorphic symmetry operations. Herein, it is proposed that TPH‐graphene has Dirac cones protected by nonsymmorphic mirror symmetries. This monolayer is composed of tetragonal, pentagonal, and heptagonal carbon rings and exhibits not only good mechanical and dynamic stability but also high energetic stability. The structure has anisotropic mechanical properties, and its Young's modulus is close to that of graphene, up to 291 N m−1. The Dirac points on high‐symmetry lines Γ−X and X−S are protected by the nonsymmorphic mirror operations {m010|0,1/2,0} and {m100|0,1/2,0}, respectively. The edge states obtained from semi‐infinite structures confirm the nontrivial topological nature of these cones. The work reveals that TPH‐graphene is an excellent candidate to study nonsymmorphic‐symmetry‐protected Dirac cones in 2D carbon materials. [ABSTRACT FROM AUTHOR]

Valleytronics has gradually attracted people's attention for providing new degrees of freedom to future electronic devices. Herein, it is proposed that valley polarization can be realized in 2D ferromagnetic FeCl with a square lattice. The first‐principles calculations without spin–orbital coupling (SOC) show that 2D FeCl is a spin‐polarized semimetal material, and the crossing points around the Fermi level are protected by mirror symmetry. When considering SOC, local bandgaps open around the Fermi level, and further calculations reveal that the Berry curvatures are opposite in sign around these gaps along with different high‐symmetry directions. By integrating the Berry curvature around these local extreme points, the Chern numbers of two valleys are ≈ −1/4 and 1/4, respectively, and the corresponding valley Chern number Cv is approximately equal to 1/2. This indicates that FeCl has the potential to realize the valley Hall effect (VHE) and will be a promising material in future valleytronics. [ABSTRACT FROM AUTHOR]

Based on first-principles calculation, we found a new type of three-dimensional (3D) topological semimetal: β-PbO2. When spin-orbit coupling (SOC) is ignored, β-PbO2 behaves as a node-line semimetal, but in the presence of SOC, the system turns into 3D Dirac semimetal, and the Dirac points are protected by C4 crystal symmetry. Moreover, the surface states and Fermi arcs on the semi-infinite edge clearly reveal its topological nature. Therefore, β-PbO2 provides a flexible platform for exploring exotic physical phenomena of Dirac semimetals. [ABSTRACT FROM AUTHOR]

The distribution of the Berry curvatures reported violates this rule because FeCl has C SB 4 sb symmetry, which requires the same signs for the Berry curvatures around the valleys B and B'. For 2D FeCl (see B Figure b 2c herein), the bands of the different valleys (B and B') have the same energies for unstrained FeCl and the energy degeneracy is protected by the C SB 4 sb rotation symmetry and I M i SB x( I y i ) sb mirror symmetries of the point group D SB 4h sb . SP [ sp 2,3 SP ] sp Valleys B and B' are related by C SB 4 sb symmetry, which leads to similar Berry curvature distributions around B and B'. Although the energy bands from the MLWFs match with the results of first-principles calculations, the C SB 4 sb symmetry is broken for low-energy electronic states and the Berry curvatures. [Extracted from the article]

The discovery of two new GeP monolayers (GeP‐2 and GeP‐4) with structural similarity to the experimentally synthesized C2/m (GeP‐3) phase, that their structures can be decomposed to hypothetical (2,2), (4,4), and (3,3) GeP nanotubes, respectively, is reported. The calculated total energies show that both GeP‐2 and GeP‐4 possess comparable stabilities to the experimentally realized GeP‐3, and they are confirmed to be dynamically stable according to their phonon vibrational spectrum. The current study also shows that three homojunctions (GeP‐2 and GeP‐4, GeP‐2 and GeP‐3, and GeP‐3 and GeP‐4) with perfect boundary can be naturally formed in GeP‐2, GeP‐3, and GeP‐4. The HSE06‐based electronic properties show that GeP‐2, GeP‐3, GeP‐4, and their homojunctions possess suitable band alignment and type‐II features for sunlight‐driven water‐splitting at both pH = 0 and pH = 7. These low‐energy GeP monolayers are desirable targets and highly expected to be synthesized in future experiments in view of their structural compatibility to GeP‐3 and the potential applications in water‐splitting. [ABSTRACT FROM AUTHOR]

Based on first‐principles calculations and effective model analysis, the long‐ago synthesized material YPtP is identified as a topological semimetal (TSM) material for studying the interaction between Weyl and triply degenerate fermions. When the spin–orbit coupling (SOC) is ignored, a triply degenerate point (TDP) and two types of ring‐shaped gapless nodal lines coexist in YPtP. When SOC is considered, the TDP turns to two type‐II TDPs, and the nodal lines open the bandgap and one of them transforms into 12 pairs of Weyl points. Thus, YPtP is a ternary TSM with the coexistence of type‐II TDPs and Weyl points. The surface states and Fermi arcs in YPtP further verify the topological characteristics of YPtP. The results herein provide a new platform for exploring ternary topological materials and the interaction between the two different fermions. [ABSTRACT FROM AUTHOR]

Two‐dimensional (2D) transition‐metal halides have attracted great interest owing to their versatile applications in electronics, optoelectronics, and renewable energy storage/conversions. Using first‐principles calculations, it is proposed that a new series of 2D transition‐metal halide CdX (X = F, Cl, Br, and I) monolayers with honeycomb lattice structure show topological properties. When omitting the spin–orbital coupling (SOC) effect, all of them behave as Dirac semimetal whose Fermi surface is composed of two Dirac points at high symmetry K and K' points in Brillouin zone. When considering the SOC effect, CdCl, CdBr, and CdI monolayers behave as topological insulators with global band gap, whereas CdF is converted from Dirac semimetal into topological metal with local band gap. The nontrivial topological properties are further proved by their nontrivial edge states. The fascinating properties of the CdX (X = F, Cl, Br, and I) monolayers show their potential for future quantum computing and next generation of high‐speed electronic devices. [ABSTRACT FROM AUTHOR]

Based on first-principles calculation, we found a new type of three-dimensional (3D) topological semimetal: β-PbO2. When spin-orbit coupling (SOC) is ignored, β-PbO2 behaves as a node-line semimetal, but in the presence of SOC, the system turns into 3D Dirac semimetal, and the Dirac points are protected by C4 crystal symmetry. Moreover, the surface states and Fermi arcs on the semi-infinite edge clearly reveal its topological nature. Therefore, β-PbO2 provides a flexible platform for exploring exotic physical phenomena of Dirac semimetals. [ABSTRACT FROM AUTHOR]