High spin polarization in formamidinium transition metal iodides: first principles prediction of novel half-metals and spin gapless semiconductors.

The electronic structure and magnetic properties of ten formamidinium transition metal iodides in the ground state and under strain have been studied. These formamidinium transition metal iodides have a stable cubic perovskite structure. In the ground state, FAVI3 is a spin gapless semiconductor, and FAScI3, FATiI3, FACrI3, FAFeI3, FACoI3 and FANiI3 are ferromagnetic half-metals. They all have 100% spin polarization and integer total magnetic moment. Under the action of strain, the high spin polarization of some formamidinium transition metal iodides can still be well maintained, and several novel spin gapless semiconductors such as FATiI3, FAFeI3 and FACoI3 have been discovered. Magnetic studies show that these formamidinium transition metal iodides with half-metal, semiconductor and spin-gapless semiconductor properties have integral total magnetic moments under strain ranging from -10.0% to 10.0%. These newly discovered half-metallic ferromagnetic materials and spin gapless semiconductors have broad application prospects in the field of spintronics due to their high spin polarization.

Bidirectional spin filter in a triple orbital molecule junction by tuning the magnetic field along a single direction.

Metal-molecule-metal junction is considered the basing block and key element of molecular spintronic devices, within which to generate spin polarized currents is one of the most fundamental issues for quantum computation and quantum information. In this paper, by employing a parallel triple orbital molecule junction with large inter-orbital tunneling couplings, we propose theoretically a bidirectional spin filter where both spin-up and spin-down currents could be obtained by simply adjusting the external magnetic field to different regimes along a single direction, and the filtered efficiencies could reach almost 100%. The Zeeman effect and the occupancy switching for the bonding and anti-bonding states are found to be responsible for the spin selective transport. We demonstrate that our scheme is robust for large parameter spaces of the orbital energy level, except the particle-hole symmetric point, and is widely suitable for the strong-, weak-, and non-interacting cases. To implement these problems, we use the Wilson's numerical renormalization group technique to treat such systems.

Stable antiferromagnetic property and tunable electronic structure of two-dimensional MnPX3(X = S and Se) from pristine structure to Janus phase.

Transition-metal phosphorus trichalcogenides have been considered as very promising two-dimensional (2D) magnetic candidates up-to-date. We performed a systematical first-principles study on the electronic structures and magnetic properties of pristine MnPX3(X = S and Se) and Janus Mn2P2S3Se3monolayers. All monolayers behave as a direct-band-gap semiconductor in antiferromagnetic ground state which is caused by strong direct and indirect exchange interactions. It is found that the electronic structures and magnetic properties can be manipulated by Janus phase. The calculated band gap is 2.44 eV, 1.80 eV and 1.86 eV for MnPS3, MnPSe3and Mn2P2S3Se3with a valley polarization with consideration of spin-orbital coupling (SOC), respectively. In particular, significant energy-splittings emerge in the SOC-band structures of Janus Mn2P2S3Se3due to its broken-inversion-symmetry. Estimated by Monte Carlo simulations, the Néel temperature is 96 K, 71 K and 79 K based on Ising model while halved down to 41 K, 33 K and 36 K on the basis ofXYmodel for MnPS3, MnPSe3and Mn2P2S3Se3, respectively, indicating theXYmodel should be more reliable to describe the spin dynamics. Our research offers an insight into the magnetic mechanism and paves a feasible path to modulate the magnetism for 2D magnets in realistic applications on spintronics.

Investigations on structural, electronic and optical properties of ZnO in two-dimensional configurations by first-principles calculations.

The electronic structures and optical properties of two-dimensional (2D) ZnO monolayers in a series of configurations were systematically investigated by first-principles calculations with HubbardUevaluated by the linear response approach. Three types of 2D ZnO monolayers, as planer hexagonal-honeycomb (Plan), double-layer honeycomb (Dlhc), and corrugated tetragonal (Tile) structures, show a mechanical and dynamical stability, while the Dlhc-ZnO is the most energetically stable configuration and Plan-ZnO is the second one. Each 2D ZnO monolayer behaves as a semiconductor with that Plan-, Dlhc-ZnO have a direct band gap of 1.81 eV and 1.85 eV at theΓpoint, respectively, while Tile-ZnO has an indirect band gap of 2.03 eV. Interestingly, the 2D ZnO monolayers all show a typical near-free-electron character for the bottom conduction band with a small effective mass, leading to a tremendous optical absorption in the whole visible and ultraviolet window, and this origination was further confirmed by the transition dipole moment. Our investigations suggest a potential candidate in the photoelectric field and provide a theoretical guidance for the exploration of wide-band-gap 2D semiconductors.

Mechanism of linear and nonlinear optical properties of the urea crystal family.

First-principles calculations of the second-order optical response functions and the dielectric functions of urea [CO(NH(2))(2)] and some of its derivatives such as monomethylurea (H(2)NCONHCH(3), MMU), and N,N'-dimethylurea (H(3)CHNCONHCH(3), DMU) crystals are performed. On the basis of the density functional theory (DFT) in the local-density approximation (LDA), the highly accurate full-potential projected augmented wave (FP-PAW) method was used to obtain the electronic structure. Over a wide frequency range (0.0-10.0 eV), the dielectric constants and second-harmonic generation (SHG) susceptibilities of the urea crystal family have been obtained, and the results are in good agreement with the experimental values. The origin of the linear and nonlinear optical (NLO) properties of the urea crystal family has been analyzed by coupling the calculated electronic structure and optical spectrum. The prominent spectra of χ((2)) are successfully correlated with the dielectric function ε(ω) in terms of single-photon and double-photon resonances. The virtual electron (VE) and virtual hole (VH) processes have also been performed for the urea crystal family. From the research into the electron deformation density, crystal configuration, substitutional group, and so forth, it is found that the origin of the SHG of the urea crystal family is the charge transfer due to the strong "(̀)push-pull" effect along the hydrogen bond, which favors a head-to-tail arrangement of the molecules and enhances the SHG response. The electron-donating substitutional group supplies more electrons to the electron-accepting group, and helps to form large dipoles in molecules. The influence on the NLO properties of the local symmetry of the substitutional group is also discussed in detail.

Ab-Initio Investigations of Magnetic Properties and Induced Half-Metallicity in Ga1-xMnxP (x = 0.03, 0.25, 0.5, and 0.75) Alloys.

Ab-initio calculations are performed to examine the electronic structures and magnetic properties of spin-polarized Ga1-xMnxP (x = 0.03, 0.25, 0.5, and 0.75) ternary alloys. In order to perceive viable half-metallic (HM) states and unprecedented diluted magnetic semiconductors (DMSs) such as spintronic materials, the full potential linearized augmented plane wave method is utilized within the generalized gradient approximation (GGA). In order to tackle the correlation effects on 3d states of Mn atoms, we also employ the Hubbard U (GGA + U) technique to compute the magnetic properties of an Mn-doped GaP compound. We discuss the emerged global magnetic moments and the robustness of half-metallicity by varying the Mn composition in the GaP compound. Using GGA + U, the results of the density of states demonstrate that the incorporation of Mn develops a half-metallic state in the GaP compound with an engendered band gap at the Fermi level (EF) in the spin-down state. Accordingly, the half-metallic feature is produced through the hybridization of Mn-d and P-p orbitals. However, the half-metallic character is present at a low x composition with the GGA procedure. The produced magnetic state occurs in these materials, which is a consequence of the exchange interactions between the Mn-element and the host GaP system. For the considered alloys, we estimated the X-ray absorption spectra at the K edge of Mn. A thorough clarification of the pre-edge peaks is provided via the results of the theoretical absorption spectra. It is inferred that the valence state of Mn in Ga1-xMnxP alloys is +3. The predicted theoretical determinations surmise that the Mn-incorporated GaP semiconductor could inevitably be employed in spintronic devices.