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The van der Waals (vdW) magnetic heterostructures provide flexible ways to realize particular magnetic properties that possess both scientific and practical significance. Here, by firstprinciples calculation, we predict strong Dzyaloshinskii-Moriya interactions (DMIs) by constructing CrI3/Metal vdW heterostructures. The underlaying mechanisms are ascribed the large spin-orbital coupling (SOC) of the I atom and the structural distortion in CrI3 layer caused by interlayer interaction. This is different from the traditional way that deposit magnetic films on substrate to generate DMI, wherein DMI is dominated by interlayer hybridization and large SOC of substrates. In addition, both Heisenberg exchange and magnetic anisotropy are modulated dramatically, such as Heisenberg exchange is nearly doubled on Au(111), and the out-of-plane magnetism is enhanced by 88% on Ir(111). Our work may provide a experimentally accessible strategy to induce DMI in vdW magnetic materials, which will be helpful to the design of spintronics devices.

Ferrovalley materials with spontaneous valley polarization are crucial to valleytronic application. Based on first-principles calculations, we demonstrate that two-dimensional (2D) YX$_2$(X= I, Br,and Cl) in 2H structure constitute a series of promising ferrovalley semiconductors with large spontaneous valley polarization and high Curie temperature. Our calculations reveal that YX$_2$ are dynamically and thermally stable 2D ferromagnetic semiconductors with a Curie temperature above 200 K. Due to the natural noncentrosymmetric structure, intrinsic ferromagnetic order and strong spin orbital coupling, the large spontaneous valley polarizations of 108.98, 57.70 and 22.35 meV are also predicted in single-layer YX$_2$(X = I, Br and Cl),respectively. The anomalous valley Hall effect is also proposed based on the valley contrasting Berry curvature. Moreover, the ferromagnetism and valley polarization are found to be effectively tuning by applying a biaxial strain. Interestingly, the suppressed valley physics of YBr$_2$ and YCl$_2$ can be switched on via applying a moderate compression strain. The present findings promise YX$_2$ as competitive candidates for the further experimental studies and practical applications in valleytronics.

The newly discovered 2D magnetic materials provide new opportunities for basic physics and device applications. However, their low Curie temperature (TC) is a common weakness. In this paper, by combining magnetic Hamiltonian, Wannier functions and first-principle calculations, we systematically study the magnetic properties of monolayer CrI3 functionalized by halogen. The magnetic exchange coupling (EX) and magnetic anisotropy (MA) are found to increase significantly by X (X=F, Cl and Br) atom adsorption, and increase along with the coverage of X atom. In the frame work of superexchange theory, the enhanced EX can be ascribed to the reduced energy difference and increased hopping strength between Cr d and I p orbitals, due to the states of I ligand are engineered by X adatom. Besides, the X adatom may provide additional ferromagnetic superexchange channel. Finally, the CrI3 that one side is fully adsorbed by F atoms is found to be a room temperature ferromagnetic semiconductor with TC=650 K. Our results not only give an insightful understanding for the enhancement of ferromagnetism of CrI3 by atom adsorption, but also propose a promising way to improve the ferromagnetism of 2D magnetic materials.

Aiming to clarify the mechanisms governing the interlayer magnetic coupling, we have investigated the stacking energy and interlayer magnetism of bilayer CrBr$_3$ systemically. The magnetic ground states of bilayer CrBr$_3$ with different R-type and H-type stacking orders are established, which is found to be in good agreement with recent experiment (Science $\mathbf{366}$,983(2019)).Further analyses indicate that the stacking energy is mainly determined by the Coulomb interaction between the interlayer nearest-neighbor Br-Br atoms. While interlayer magnetism can be understood by a competition between super-super-exchange interactions involving $t_{2g}$-$t_{2g}$ and $t_{2g}$-$e_g$ orbitals and semi-covalent exchange interactions of $e_g$-$e_g$ orbitals. Our studies give an insightful understanding for stacking order and interlayer magnetism of bilayer CrBr$_3$, which should be useful to understand quantum confinement effect of other layered magnets in two-dimensional limit.

The 2D ferromagnets, such as CrX3 (X=Cl, Br and I), have been attracting extensive attentions since they provide novel platforms to fundamental physics and device applications. Integrating CrX3 with other electrodes and substrates is an essential step to their device realization. Therefore, it is important to understand the interfacial properties between CrX3 and other 2D materials. As an illustrative example, we have investigated the heterostructures between CrX3 and graphene (CrX3/Gr) from firstprinciples. We find unique Schottky contacts type with strongly spin-dependent barriers in CrX3/Gr. This can be understood by synergistic effects between the exchange splitting of semiconductor band of CrX3 and interlayer charge transfer. The spinasymmetry of Schottky barriers may result in different tunneling rates of spin-up and down electrons, and then lead to spin-polarized current, namely spin-filter (SF) effect. Moreover, by introducing X vacancy into CrX3/Gr, an Ohmic contact forms in spin-up direction. It may enhance the transport of spin-up electrons, and improve SF effect. Our systematic study reveals the unique interfacial properties of CrX3/Gr, and provides a theoretical view to the understanding and designing of spintronics device based on magnetic vdW heterostructures.

The recent experimental discovery of intrinsic ferromagnetism in single-layer CrI3 opens a new avenue to low-dimensional spintronics. However, the low Curie temperature Tc=45 K is still a large obstacle to its realistic device application. In this work, we demonstrate that the Tc and magnetic moment of CrX3(X=Br, I) can be enhanced simultaneously by coupling them to buckled two-dimensional Mene (M=Si, Ge) to form magnetic van der Waals (vdW) heterostructures. Our first-principles calculations reveal that n-doping of CrX3, induced by a significant spin-dependent interlayer charge transfer from Mene, is responsible for its drastic enhancement of Tc and magnetic moment. Furthermore, the diversified electronic properties including halfmetallicity and semi-conductivity with configuration dependent energy gap are also predicted in this novel vdW heterostructure, implying their broad potential applications in spintronics. Our study suggests that the vdW engineering may be an efficient way to tune the magnetic properties of 2D magnets, and the Mene_CrX3 magnetic vdW heterostructures are wonderful candidates in spintronics and nanoelectronics device.

Searching the novel 2D semiconductor is crucial to develop the next-generation low-dimensional electronic device. Using first-principles calculations, we propose a class of unexplored binary V-V compound semiconductor (PN, AsN, SbN, AsP, SbP and SbAs) with monolayer black phosphorene ($\alpha$) and blue phosphorene ($\beta$) structure. Our phonon spectra and room-temperature molecular dynamics (MD) calculations indicate that all compounds are very stable. Moreover, most of compounds are found to present a moderate energy gap in the visible frequency range, which can be tuned gradually by in-plane strain. Especially, $\alpha$-phase V-V compounds have a direct gap while $\beta$-SbN, AsN, SbP, and SbAs may be promising candidates of 2D solar cell materials due to a wide gap separating acoustic and optical phonon modes. Furthermore, vertical heterostructures can be also built using lattice matched $\alpha$($\beta$)-SbN and phosphorene, and both vdW heterostructures are found to have intriguing direct band gap. The present investigation not only broads the scope of layered group V semiconductors but also provides an unprecedented route for the potential applications of 2D V-V families in optoelectronic and nanoelectronic semiconductor devices., Comment: 7 pages, 7 figures

Two-dimensional (2D) intrinsic ferromagnetic (FM) semiconductors are crucial to develop low-dimensional spintronic devices. Using density functional theory, we show that single-layer chromium trihalides (SLCTs) (CrX$_3$,X=F, Cl, Br and I) constitute a series of stable 2D intrinsic FM semiconductors. A free-standing SLCT can be easily exfoliated from the bulk crystal, due to a low cleavage energy and a high in-plane stiffness. Electronic structure calculations using the HSE06 functional indicate that both bulk and single-layer CrX$_3$ are half semiconductors with indirect gaps and their valence bands and conduction bands are fully spin-polarized in the same spin direction. The energy gaps and absorption edges of CrBr$_3$ and CrI$_3$ are found to be in the visible frequency range, which implies possible opt-electronic applications. Furthermore, SLCTs are found to possess a large magnetic moment of 3$\mu_B$ per formula unit and a sizable magnetic anisotropy energy. The magnetic exchange constants of SLCTs are then extracted using the Heisenberg spin Hamiltonian and the microscopic origins of the various exchange interactions are analyzed. A competition between a near 90$^\circ$ FM superexchange and a direct antiferromagnetic (AFM) exchange results in a FM nearest-neighbour exchange interaction. The next and third nearest-neighbour exchange interactions are found to be FM and AFM respectively and this can be understood by the angle-dependent extended Cr-X-X-Cr superexchange interaction. Moreover, the Curie temperatures of SLCTs are also predicted using Monte Carlo simulations and the values can further increase by applying a biaxial tensile strain. The unique combination of robust intrinsic ferromagnetism, half semiconductivity and large magnetic anisotropy energies renders the SLCTs as promising candidates for next-generation semiconductor spintronic applications., Comment: 12 pages, 14 figures. published in J. Mater. Chem. C

First-principle calculations with different exchange-correlation functionals, including LDA, PBE and vdW-DF functional in form of optB88-vdW, have been performed to investigate the electronic and elastic properties of two dimensional transition metal dichalcogenides(TMDCs) with the formula of MX$_2$(M=Mo,W; X=O,S,Se,Te) in both monolayer and bilayer structures. The calculated band structures show a direct band gap for monolayer TMDCs at the K point except for MoO$_2$ and WO$_2$. When the monolayers are stacked into bilayer, the reduced indirect band gaps are found except for bilayer WTe$_2$, in which direct gap is still present at the K point. The calculated in-plane Young moduli are comparable to graphene, which promises the possible application of TMDCs in future flexible and stretchable electronic devices. We also evaluated the performance of different functionals including LDA, PBE, and optB88-vdW in describing elastic moduli of TMDCs and found that LDA seems to be the most qualified method. Moreover, our calculations suggest that the Young moduli for bilayers are insensitive to stacking orders and the mechanical coupling between monolayers seems to be negligible., Comment: 15 pages, 3 figures; Accepted by Chinese Physics B

Based on First-principles calculation, we have investigated electronic structure of a ZrCuSiAs structured superconductor LaNiPO. The density of states, band structures and Fermi surfaces have been given in detail. Our results indicate that the bonding of the La-O and Ni-P is strongly covalent whereas binding property between the LaO and NiP blocks is mostly ionic. It's also found that four bands are across the Fermi level and the corresponding Fermi surfaces all have a two-dimensional character. In addition, we also give the band decomposed charge density, which suggests that orbital components of Fermi surfaces are more complicated than cuprate superconductors., Comment: Submitted to Phys.Rev.B