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Spin current generation from charge current in nonmagnetic materials promises an energy-efficient scheme for manipulating magnetization in spintronic devices. In some asymmetric two-dimensional (2D) materials, the Rashba and valley effects coexist owing to strong spin-orbit coupling (SOC), which induces the spin Hall effect due to spin-momentum locking of both effects. Herein, we propose a new Janus structure MoSiAs 2 Se with both valley physics and the Rashba effect and reveal an effective way to modulate the properties of this structure. The results demonstrated that applying an external electric field is an effective means to modulating the electronic properties of MoSiAs 2 Se, leading to both type I-II phase transitions and semiconductor-metal phase transitions. Furthermore, the coexistence of the Rashba and valley effects in monolayer MoSiAs 2 Se contributes to the spin Hall effect (SHE). The magnitude and direction of spin Hall conductivity can also be manipulated with an out-of-plane electric field. Our results enrich the physics and materials of the Rashba and valley systems, opening new opportunities for the applications of 2D Janus materials in spintronic devices.

The valley polarization, induced by the magnetic proximity effect, in monolayer transition metal dichalcogenides (TMDCs), has attracted significant attention due to the intriguing fundamental physics. However, the enhancement and modulation of valley polarization for real device applications is still a challenge. Here, using first-principles calculations we investigate the valley polarization properties of monolayer TMDCs CrS 2 and CrSe 2 and how to enhance the valley polarization by constructing Janus CrSSe (with an internal electric field) and modulate the polarization in CrSSe by applying external electric fields. Janus CrSSe exhibits inversion symmetry breaking, internal electric field, spin-orbit coupling, and compelling spin-valley coupling. A magnetic substrate of the MnO 2 monolayer can induce a modest magnetic moment in CrSe 2 , CrSe 2 , and CrSSe. Notably, the Janus structure with an internal electric field has a much larger valley p compared with its non-Janus counterparts. Moreover, the strength of valley polarization can be further modulated by applying external electric fields. These findings suggest that Janus materials hold promise for designing and developing advanced valleytronic devices.

Critical topological phases, possessing flat bands, provide a platform to study unique topological properties and transport phenomena under a many-body effect. Here, we propose that critical nodal points and nodal lines or rings can be found in Kagome lattices. After the C3 rotation symmetry of a single-layer Kagome lattice is eliminated, a quadratic nodal point splits into two critical nodal points. When the layered Kagome lattices are stacked into a three-dimensional (3D) structure, critical nodal lines or rings can be generated by tuning the interlayer coupling. Furthermore, we use Kagome graphene as an example to identify that these critical phases could be obtained in real materials. We also discuss flat-band-induced ferromagnetism. It is found that the flat band splits into two spin-polarized bands by hole-doping, and as a result the Dirac-type critical phases evolve into Weyl-type phases.

Topological metals/semimetals (TMs) have emerged as a new frontier in the field of quantum materials. A few two-dimensional (2D) boron sheets have been suggested as Dirac materials, however, to date TMs made of three-dimensional (3D) boron structures have not been found. Herein, by means of systematic first principles computations, we discovered that a rather stable 3D boron allotrope, namely 3D-α' boron, is a nodal-chain semimetal. In momentum space, six nodal lines and rings contact each other and form a novel spindle nodal chain. This 3D-α' boron can be formed by stacking 2D wiggle α' boron sheets, which are also nodal-ring semimetals. In addition, our chemical bond analysis revealed that the topological properties of the 3D and 2D boron structures are related to the π bonds between boron atoms, however, the bonding characteristics are different from those in the 2D and 3D carbon structures.

Coexistence of topological elements in topological metals/semimetals (TMs) has gradually attracted attention. However, non-topological factors always interfere with the Fermi surface and cover interesting topological properties. Here, we find that Ba3Si4 is a "clean" TM which contains coexisting nodal-chain networks, intersecting nodal rings (INRs) and triple points, in the absence of spin-orbit coupling (SOC). Moreover, the nodal rings in the topological phase exhibit diverse types: from type-I and type-II to type-III rings according to band dispersions. All of the topological elements are generated by crossings of three energy bands, and thus they are correlated rather than mutually independent. When some structural symmetries are eliminated by an external strain, the topological phase evolves into another phase including a Hopf link, a one-dimensional nodal chain and new INRs.

A semi-Dirac semimetal is a material that exhibits linear band dispersion in one direction and quadratic band dispersion in the orthogonal direction and, therefore, hosts massless and massive fermions at the same point in the momentum space. While a number of interesting physical properties have been predicted in semi-Dirac semimetals, it has been rare to realize such materials in condensed matter. Based on the fact that some honeycomb materials are easily oxidized or chemically absorb other atoms, here, we theoretically propose an approach of modifying their band structures by covalent addition of group-VI elements and strain engineering. We predict a silicene oxide with the chemical formula of Si 2 O to be a candidate semi-Dirac semimetal. Our approach is backed by the analysis and understanding of the effect of p-orbital frustration on the band structure of graphene-like materials.

Three single-layer tetragonal silicon carbides (SiCs), termed as T1, T2 and T3, are proposed by density functional theory (DFT) computations. Although the three structures have the same topological geometry, they show versatile electronic properties from a semiconductor (T1), a semimetal (T2) to a metal (T3). The versatile properties originate from the rich bonds between Si and C atoms. The nanoribbons of the three SiCs also show interesting electronic properties. Especially, T1 nanoribbons possess exotic edge states, where electrons only distribute on one edge's silicon or carbon atoms. The band gaps of the T1 nanoribbons are constant because of no interaction between the edge states.