2D materials with multi-phase and multi-element crystals such as transition atoms (V-, Cr-, Mn-, Fe-, Cd-, Pt-, and Pd-) based chalcogenides (TMCs) and phosphorous chalcogenides (TMPCs), offer a unique platform to explore novel physical phenomena including 2D ferromagnetism, 2D superconductivity, high-Tc topological superconductivity, Majorana bound states, and many-body excitons 1-9 . However, synthesis of a singlephase/composition of these 2D crystals is still challenging since the growth kinetics is difficult to be controlled during chemical vapor deposition (CVD) 10 . Here, we unravel a competitive-chemical reaction growth mechanism via controlling the kinetic parameters to manipulate the nucleation and growth rate. Based on this mechanism, chemical reactions of 2D crystals with the defined phase, controllable structure, and tunable component can be realized. Specifically, we synthesized 67 types of 2D compounds including 27 binary metal chalcogenides with different chemical compositions, 12 ternary metal phosphorous chalcogenides, 24 alloys, and 4 heterostructures. The ferromagnetism and superconductivtity in FeXy can be tuned with y value, such as superconductivity observed in FeX and ferromagnetism in the FeS2 monolayers, demonstrating the high quality of as-
Quantum spin Hall (QSH) effect is promising for achieving dissipationless transport devices which can be achieved only at extremely low temperature presently. The research for new large-gap QSH insulators is critical for their realistic applications at room temperature. Based on first-principles calculations, we propose a QSH insulator with a sizable bulk gap as large as ∼0.22 eV in stanene film functionalized with the organic molecule ethynyl (SnC 2 H), whose topological electronic properties are highly tunable by the external strain. This large-gap is mainly due to the result of the strong spinorbit coupling related to the p xy orbitals at the Γ point of the honeycomb lattice, significantly different from that consisting of the p z orbital as in free-standing group IV ones. The topological characteristic of SnC 2 H film is confirmed by the Z 2 topological order and an explicit demonstration of the topological helical Dirac type edge states. The SnC 2 H film on BN substrate is observed to support a nontrivial large-gap QSH, which harbors a Dirac cone lying within the band gap. Owing to their high structural stability, this two-dimensional large-gap QSH insulator is promising platforms for topological phenomena and new quantum devices operating at room temperature in spintronics.
Manipulating valley-dependent Berry phase effects provides remarkable opportunities for both fundamental research and practical applications. Here, by referring to effective model analysis, we propose a general scheme for realizing topological magneto-valley phase transitions. More importantly, by using valley-half-semiconducting VSi2N4 as an outstanding example, we investigate sign change of valley-dependent Berry phase effects which drive the change-in-sign valley anomalous transport characteristics via external means such as biaxial strain, electric field, and correlation effects. As a result, this gives rise to quantized versions of valley anomalous transport phenomena. Our findings not only uncover a general framework to control valley degree of freedom, but also motivate further research in the direction of multifunctional quantum devices in valleytronics and spintronics.
Two-dimensional (2D) materials have attracted great attention and spurred rapid development in both fundamental research and device applications. The search for exotic physical properties, such as magnetic and topological order, in 2D materials could enable the realization of novel quantum devices and is therefore at the forefront of materials science. Here, we report the discovery of two-fold degenerate Weyl nodal lines in a 2D ferromagnetic material, a single-layer gadoliniumsilver compound, based on combined angle-resolved photoemission spectroscopy measurements and theoretical calculations. These Weyl nodal lines are symmetry protected and thus robust against external perturbations. The coexistence of magnetic and topological order in a 2D material is likely to inform ongoing efforts to devise and realize novel nanospintronic devices.Spintronics is an emerging technique that uses electron spin as a medium for data storage and transfer [1,2]. Ferromagnets are widely used as spintronic materials because they possess electronic band structures that are spin split by the exchange interaction. Recently, the desire to miniaturize future quantum devices has stimulated great research interest in low-dimensional materials. For example, various two-dimensional (2D) materials have been realized, such as graphene [6,7], phosphorene [8,9], and borophene [10,11], providing the possibility to realize novel quantum devices at the atomic scale. The search for 2D ferromagnetic materials is thus a promising route towards future nanospintronics. However, the long-range ferromagnetic order in 2D systems is vulnerable to low-energy spinwave excitations, making it difficult to realize 2D ferromagnetism; such low-energy excitations can be gapped out by magnetic anisotropy [12]. Intrinsic 2D ferromagnetism has been experimentally observed only recently in several van der Waals crystals, including Cr 2 Ge 2 Te 6 , CrI 3 , and VSe 2 [13-15].The transport properties of a material, which are crucial for its use in spintronic devices, largely depend on the band structure near the Fermi level. Therefore, exploration of ferromagnetic materials with exotic band structures provides great opportunities to realize novel spintronic devices. Recently, topological band structures including the Dirac cone, Weyl cone, and Dirac/Weyl nodal line have attracted great attention because of their potential device applications [16][17][18][19]. To date, most materials that host topological band structures are non-magnetic and three dimensional, strongly limiting their applicability to spintronic devices. Therefore, it remains important to search for novel 2D materials with both magnetic order and topological band structure.Here, we study a novel 2D ferromagnet, single-layer GdAg 2 , which can be synthesized using a bottom-up approach. Our ab initio molecular dynamic (AIMD) simulation results show that freestanding single-layer GdAg 2 is stable at room temperature.Moreover, all the atoms of GdAg 2 are coplanar; therefore, its thickness reaches the atomic li...
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