Jisang Hong

Ferromagnetism induced by Zn vacancy defect and lattice distortion in ZnO

Through the full potential linearized augmented plane wave method, we have explored the vacancy defect induced magnetism in wurtize ZnO. It has been found that the Zn vacancy defect brings a spin polarized state in the nearest neighbor oxygen atoms, whereas the oxygen vacancy defect has no influence on the magnetism. However, it is found that the lattice distortion is a crucial factor for the Zn vacancy induced ferromagnetism because the ferromagnetic ground state cannot be achieved if there is no lattice distortion due to Zn vacancy defect. The magnetic moment of oxygen atom in the nearest neighbor from the Zn vacancy site is ranged from 0.10 to 0.19 μB and the spin polarized oxygen atoms have metallic feature in both spin states. These results are quite different from those found in other calculation [Q. Wang et al., Phys. Rev. B 77, 205411 (2008)]. In addition, we have found that the ferromagnetic exchange interaction among oxygen atoms is mediated by Zn 3d state. Along with these, the theoretically ca...

Metal free half metallicity in 2D system: structural and magnetic properties of g-C4N3 on BN

Synthesis of a half metallic material on a substrate is highly desirable for diverse applications. Herein, we have investigated structural, adsorptive, and magnetic properties of metal free graphitic carbon nitride (g-C4N3) layer on hexagonal BN layer (h-BN) using the optB88-vdW van der Waals density functional theory. It is found that g-C4N3 layer can be adsorbed on BN layer due to the change of lattice constant of the hybridized system. The newly found lattice constant of g-C4N3 was 9.89 Å, which is approximately 2% lower and larger than to those of free standing BN and g-C4N3, respectively. Also, 2 × 2 surface reconstruction geometry predicted in free standing g-C4N3 layer disappears on the BN layer. Interestingly, we have found that metal free half metallic behavior in g-C4N3 can be preserved even on BN layer and the characters of spin polarized planar orbitals suggest that our theoretical prediction can be verified using normal incidence of K-edge X-ray magnetic circular dichroism (XMCD) measurement.

Anisotropic bias dependent transport property of defective phosphorene layer

Phosphorene is receiving great research interests because of its peculiar physical properties. Nonetheless, no systematic studies on the transport properties modified due to defects have been performed. Here, we present the electronic band structure, defect formation energy and bias dependent transport property of various defective systems. We found that the defect formation energy is much less than that in graphene. The defect configuration strongly affects the electronic structure. The band gap vanishes in single vacancy layers, but the band gap reappears in divacancy layers. Interestingly, a single vacancy defect behaves like a p-type impurity for transport property. Unlike the common belief, we observe that the vacancy defect can contribute to greatly increasing the current. Along the zigzag direction, the current in the most stable single vacancy structure was significantly increased as compared with that found in the pristine layer. In addition, the current along the armchair direction was always greater than along the zigzag direction and we observed a strong anisotropic current ratio of armchair to zigzag direction.

Anisotropic Effective Mass, Optical Property, and Enhanced Band Gap in BN/Phosphorene/BN Heterostructures

Phosphorene is receiving great research interests because of its peculiar physical properties. Nonetheless, the phosphorus has a trouble of degradation due to oxidation. Hereby, we propose that the electrical and optical anisotropic properties can be preserved by encapsulating into hexagonal boron nitride (h-BN). We found that the h-BN contributed to enhancing the band gap of the phosphorene layer. Comparing the band gap of the pristine phosphorene layer, the band gap of the phosphorene/BN(1ML) system was enhanced by 0.15 eV. It was further enhanced by 0.31 eV in the BN(1ML)/phosphorene/BN(1ML) trilayer structure. However, the band gap was not further enhanced when we increased the thickness of the h-BN layers even up to 4 MLs. Interestingly, the anisotropic effective mass and optical property were still preserved in BN/phosphorene/BN heterostructures. Overall, we predict that the capping of phosphorene by the h-BN layers can be an excellent solution to protect the intrinsic properties of the phosphorene.

Geometry, electronic structures and optical properties of phosphorus nanotubes

Using a first principles approach, we investigated the geometry, electronic structures, and optical properties of phosphorus nanotubes (PNTs). Two possible 1D configurations, the so-called α-PNTs and β-PNTs, are proposed, which are structurally related to blue and black phosphorus monolayers, respectively. Hereby, we predict that both armchair and zigzag geometries can be synthesized in α-PNTs, but the zigzag form of β-PNT is highly unfavorable because of large strain and conformation energies. The band gap of α-PNTs is expected to be ∼2.67 eV, and this is insensitive to the chirality when the tubes inner diameter is larger than 1.3 nm, while the armchair β-PNTs have a much smaller band gap. Interestingly, we find nearly flat band structures in the zigzag α-PNT system. This may indicate that an excited particle-hole pair has a huge effective mass. We also find asymmetric optical properties with respect to the polarization direction. The armchair α-PNT for parallel polarization shows a large refractive index of 2.6 near the ultraviolet wavelength, and also we find that the refractive index can be even smaller than 1 in certain frequency ranges. The zigzag tubes show very weak reflectivity for parallel polarization, while the armchair tube displays high reflectivity.

Local magnetic moment induced by Ga vacancy defect in GaN

Through the full potential linearized augmented plane wave method, we have explored the possibility of defect-induced magnetism in wurtize GaN. The N vacancy defect structure has no sign of a magnetic state. Nonetheless, very interestingly it has been found that the GaN with a Ga vacancy defect can show induced local magnetic moment in N atoms. The four N atoms in the tetrahedron sites neighboring the Ga vacancy have magnetic moments of 0.23 and 0.29μB depending on their positions. The spin-polarized N atoms have a metallic state. It has been observed that the px,y state mainly contributes to the spin polarization of N atoms in the base layer, while the pz state is important for the other N atoms. In addition, the theoretically calculated x-ray absorption spectroscopy and x-ray magnetic circular dichroism of the K edge have been presented.

Magnetic ordering and x-ray magnetic circular dichroism of Co doped ZnO

Using the full potential linearized augmented plane wave method, we explored the magnetization and magnetic ordering of Co doped ZnO. Energetically Co prefers nonuniform distribution and antiferromagnetic ordering. Both Zn and O display measurable x-ray magnetic circular dichroism spectra.

Effect of ion irradiation on a Co-based amorphous ribbon

The effects of ion irradiation on a giant magnetoimpedance (GMI) have been investigated for a Co-based amorphous ribbon with various kinds of ions such as Xe, Ar, and N. The GMI ratio and M-H loop parameters were used to characterize the samples before and after the ion irradiation. The GMI ratio increased considerably in the ion irradiated samples and the GMI response showed a strong dependence on the irradiated ion species and driving frequencies. It was shown that the ion irradiations lead to a substantial modification of the magnetic properties including a large coercivity and shearing of the in-plane magnetization loops, thus suggesting the reduction of an exchange coupling.

Manipulation of Magnetic State in Armchair Black Phosphorene Nanoribbon by Charge Doping.

Using first-principles studies, we investigated the width-dependent magnetic properties of armchair black phosphorene nanoribbons (APNRs) by controlling the electron charge doping. In the unrelaxed APNRs the antiferromagnetic coupling between two phosphorus atoms in the same edge was found. However, the edge magnetic moment vanished after structure relaxation, and all of the APNRs showed a semiconducting feature. Interestingly, the charge doping substantially altered the band structures of the APNRs because the metallic states reappeared in the charge-doped APNRs. Besides this, the magnetic moment was found in the charge-doped systems. We found that the Stoner condition could nicely explain the magnetic moment at the edge atoms. Moreover, we propose that the edge-to-edge magnetic coupling can be manipulated by charge doping because the transition from the antiferromagnetic to ferromagnetic state was achieved. Our findings may bring interesting issues for spintronics applications.

Hot electron spin polarization and Schottky barrier in spin-valve transistor

We have explored phenomenological temperature dependence of hot electron magnetotransport in a spin valve transistor. We stress spin polarization of hot electrons and spatial inhomogeneity of Schottky barriers to explain the peculiar temperature and spin dependence of collector currents. Qualitative trends are established for collector current with changes in temperature, thickness of spin-valve base, along with height and width of Schottky barriers.