The antiferromagnetic phase of a wurtzite nickel sulfide monolayer.

Two-dimensional intrinsic long-range magnetic monolayers with high transition temperatures have attracted great interest in both fundamental studies and practical applications. In this study, we use a combination of first-principles calculations based on density functional theory (DFT), and unitary transformation of the effective Heisenberg model to investigate the electronic structure and magnetic properties of a [NiS]2 monolayer. The phonon calculations reveal that the [NiS]2 monolayer is dynamically stable in the wurtzite phase. This material is an out-of-plane easy-axis antiferromagnetically ordered monolayer with the Néel temperature close to room temperature. The intrinsic AFM ground state arises from the presence of top and bottom FM sublattices coupled together via AFM coupling, in which the net magnetic moment of each Ni atom is evaluated as 0.5µB. The spectrum of the spin-wave of [NiS]2 is investigated within the spin-wave theory of antiferromagnets in terms of the first-order Holstein-Primakoff approximation of the anisotropic Heisenberg model combined with the Bogoliubov diagonalization transformation. For the long wavelength limit, the magnon dispersion shows linear behavior with the wave vector, which is expected for conventional antiferromagnetism. The magnon velocity of approximately ∼600 m s-1 is predicted for the [NiS]2 monolayer by calculating the slope of the magnon spectrum. Due to strong spin-orbit coupling, the [NiS]2 monolayer has relatively large magnetic anisotropy energy, causing the existence of the 12 meV gap at the Γ point in the magnon spectrum. The magnon energy gap limits the number of thermally excited states, which is essential for maintaining intrinsic long-range antiferromagnetic order in two dimensions.

Robust ferromagnetism in two-dimensional GeC/CrN heterobilayers.

We have investigated the electronic and finite temperature magnetic properties of germanium carbide (GeC) and ferromagnetic chromium nitride (CrN) heterobilayers by using first-principles calculations based on density functional theory with Hubbard U correction and an effective anisotropic Heisenberg spin model. The dynamical stability of different stacking formations of heterobilayers is ensured by considering the phonon spectra. All the stacking patterns show half-metallicity with an out-of-plane easy-axis ferromagnetic ground state. We find a high Curie temperature for GeC/CrN heterobilayers within the random phase approximation (RPA). In addition to the symmetric stackings, i.e., AA and AB, the electronic properties of non-symmetric stackings at three different twist angles are also analyzed. The electronic structure analysis of twisted structures demonstrates that the half-metallicity of the GeC/CrN heterobilayer is stack independent. Furthermore, we have investigated the electronic properties, magnetic anisotropy energy, Curie temperature, and spin wave spectrum in the presence of biaxial strain. It is shown that the compressive strain dramatically reduces the magnetic anisotropy energy of the GeC/CrN heterobilayer and Curie temperature, but the Curie temperature still remains well above room temperature for all strain values. The increasing values of tensile strain reduce the magnetic exchange while it increases the magnetic anisotropy energy of the heterobilayer system which enhances the Curie temperature of the structures. The monolayer CrN on the GeC with a wide band gap and commensurate lattice together with a high Tc value can be a feasible candidate for future spintronic applications.

Easy-axis rotation in ferromagnetic monolayer CrN induced by fluorine and chlorine functionalization.

On the basis of first-principles calculations, we investigate the absorption of fluorine and chlorine on ferromagnetic monolayer CrN focusing on the mechanism of spin reorientation. We use density functional theory in combination with the spin Hamiltonian approach to study the electronic and magnetic properties of monolayer CrN upon single-side adsorption of F and Cl atoms. While the electronic structure of ferromagnetic CrN remains half-metallic after functionalization, its preferred axis of magnetization is rotated toward the in-plane direction due to the orbital moment suppression. The half-coverage of CrN is found to be thermodynamically stable and ferromagnetically ordered at room temperature. Our findings demonstrate the possibility of altering the magnetic properties of a two-dimensional magnet after the adsorption of F and Cl, which opens a route to the detection of these gases using magnetic or optical measurements.

Electronic structure and optical properties of novel monolayer gallium nitride and boron phosphide heterobilayers.

Motivated by the increasing number of studies on optoelectronic applications of van der Waals (vdW) heterostructures, we have investigated the electronic and optical properties of monolayer gallium nitride (MGaN) and boron phosphide (MBP) heterobilayers by using first-principle calculations based on density functional theory. We have ensured the dynamical stability of the structures by considering their binding energies and phonon spectra. We show that the magnitude and status (direct or indirect) of the band gap are strongly dependent on the stacking pattern of the heterobilayers. Furthermore, we have investigated the band splittings in the presence of an external electric field which show the effect of the field on the band alignment of the structure. We have also shown the band gap, charge redistributions and work function of the structures are highly dependent on the magnitude and direction of the electric field such that its magnitude yields indirect to direct band gap transitions around |E⊥| ∼ 0.7 V Å-1 at the Γ point and the order of the band gap is varied according to the direction of the electric field. Moreover, we examine optical properties of MGaN/MBP heterobilayers as part of DFT calculations. The band gap and work function being tunable with changes in the external field together with the prominent absorption over the UV range make the MGaN/MBP heterobilayer a feasible candidate for optoelectronic applications.

The effect of strain and functionalization on the optical properties of borophene.

Following its synthesis, borophene has drawn noticeable attention due to its remarkable intrinsic properties. Understanding and modifying these properties are crucial for implementation of borophene in high-technological applications. In this study, we employed ab initio techniques to examine the variation of the optoelectronic properties of buckled borophene by strain and surface functionalization. We find that the optical response can be tuned by applying compressive/tensile strain and covering the surface with hydrogen and fluorine atoms. It is shown that the variations in optical properties can be correlated with structural deformations and modifications in the electronic band structure. Revealing the tunability of the optical response of borophene can pave the way for its potential uses in various optoelectronic devices.