First-principles Studies And Designs On Some Low-dimensional Structures And Devices

Using the first-principles methods within the framework of Density Functional Theory, we studied the physical properties of several low-dimensional systems, and suggested some corresponding potential applications.First, we investigated the magnetism in III-V nanotubes (GaN and BN nanotubes) induced by two different ways: magnetic doping (Mn-doped GaN nanotubes) and structural defects (open BN nanotubes). The substitution of one Ga atom by Mn leads to 4μB magnetic moments which is mainly contributed by the hybridization of Mn 3d orbitals and its nearest N 2p orbitals. In the open BN nanotubes, the dangling bonds on the unpaired mouth atoms are responsible for the local magnetism, and the magnetic moment is determined by the chirality of the BN tube. Both of the two kinds of systems mentioned above can result in large spin-splitting effect (splitting of the highest occupied molecular orbital energies of different spin components), but the mechanisms are very different. We put forward some ideas of designing spintronic devises based on the III-V nanotubes, such as spin-polarized electron emitters and the tip of the spin-polarized scan tunneling microscope.Second, we explored the phase transition behaviors of GaN and BN nanotube bundles under transverse hydrostatic pressure and investigated the different physical mechanisms of the transitions. When the intertube distance is small enough, the GaN nanotubes of the bundles will aggregate spontaneously to form the new porous GaN phase via sp2-sp3 hybridization transition. We proposed microscopic descriptions of the structural phase transitions in the BN nanotube bundles, and found that curvature effect and the match of the tube chirality and the lattice symmetry play the important roles in the phase transition behavior of BN nanotube bundles. More importantly, we observed the BN crystal phase with the lightest density andthe one with the largest band gap among all the BN crystals.Finally, we simulated the polarization behaviors of the KNbO3/(KTaO3)m superlattice with variation of composition (different m). We concluded that in a B-site modulation ferro-paraelectric superlattic, due to the breaking of the translation symmetry of the BO6 oxygen octahedron, the polarization is very sensitive to the composition (i.e. with increasing m, the spontaneous polarization in KNbO3/(KTaO3)m superlattice will vanish quickly). We demonstrated the existence of antiferroelectric phase in 1×1 KNbO3/KTaO3 superlattice, consistent with a previous experimental result.