Theoretical Investigations Of The Spin Polarization In Low Dimensional Carbon Materials

As new-fashioned carbon materials, fullerene, carbon nanotube andgraphene with many novel electronic properties have been drawing lots ofattentions. Moreover, some novel carbon structures have been predictedtheoretically, such as graphyne and carbon foam, which further stimulate therelevant experimental investigations. It is noteworthy that experiments haveconfirmed the theoretical prediction about the magnetic ordering in some ofthese new carbon materials, including rhombohedral C60, zigzag-edgedgraphene nanoribbon(ZGNR) and the graphene adsorbingorganic molecules.The very weak spin-orbital coupling of the carbon atom results in the trivial spindephasing effect in the carbon materials. Hence, the magnetism based on the porbitals in the carbon materials is very promising to be applied in the field ofspintronics. Therefore, it is very important to study the magnetism in carbonmaterials to satisfy some specific device functions in spintronics. In this thesis, Iwill report our theoretical investigations about the magnetisms of the3dtransition metal atom(TMA) doped graphene, some kinds of grapheyne and thesurface of (1,1)zigzag carbon foam.First of all, we study the influence of doping TMAs on the edge magnetismof graphene. After doping the Ti, V, Cr, Mn, Ni, Cu or Zn atom into the latticeposition very close to the zigzag edge of ZGNR, the edge magnetism will bestrongly suppressed. According to our analysis, there are two main reasonsresponsible for the phenomenon of magnetism suppression: The first one is thatthe edge states will hybridize strongly with the TMA orbitals, which results insimultaneous occupations of the spin-up and spin-down edge states.Consequently, the edge magnetism disappears; The second one is that afterdoping the TMA into the position close to the zigzag edge, the edge will sufferfrom a nontrivial lattice distortion, in addition, such an edge distortion willdestroy the edge states. As a result, the edge magnetism disappears. In the cases of the Ti and Mn doping, the first reason is the main one for the suppression ofedge magnetism. And the second reason is mainly responsible for the case of Cudoping. However, as for most cases, the two reasons always play roles together.Another notable point is that the ZGNR with Sc-doping will display verydifferent magnetism from the case of the two dimensional graphene withSc-doping. For the latter case, the system is not magnetic, but the Sc-dopedZGNR is always spin polarized.Secondly, we discuss the magnetism of graphene with558-typeperiodically extended line defect (ELD-G) after adsorbing the TMA. Comparedwith the ordinary lattice position of graphene, the position near the ELD canproduce a bigger adsorption energy, so that the ELD is prone to the TMAadsorption. Moreover, the systems with TMA adsorbed onto different positionin ELD-G present different magnetic moments. As for the structure with TMAadsorbed onto the center of the octagon (TMA@H8), we establish a simplehybridization picture between the TMA orbitals and the pzorbitals of the eightcarbon atoms closest to the TMA. By this hybridization picture, the magneticmoment and the adsorption energy in the TMA@H8systems can be wellexplained. Our electronic transport calculations indicate that the Fe@ELD-Gand Co@ELD-G structures display the great spin-polarized current. Especiallyfor the Fe@ELD-G, the spin filter efficiency always exceeds76%as long as thebias is in the0.2-0.4V range, regardless of the specific adsorption position.Thirdly, we study the edge magnetism of four kinds of graphyne, namelyα,β,γ and (6,6,12)-graphyne. Same as ZGNR, the dispersionless sub-bands andantiparallel edge magnetic ordering exist in these zigzag-edged graphynenanoribbons. It is noteworthy that for β-graphyne, the edge magnetism and thewell-defined flat sub-bands do not exist until the nanoribbon is sufficiently wide.In addition, a very simple tight-binding(TB) model is established based on thecarbon atoms’ pzorbitals, which can appropriately describe the sub-bands ofthese ribbons. By using the TB model, we calculate the sub-band structure ofthe β-graphyne nanoribbon. And we find that it has much more transportchannels than ZGNR and other graphyne nanoribbons, so it can be used as anoptimal metal wire to carry a relatively strong current flow. Finally, we study the surface magnetism of (1,1) zigzag carbon foam,which is an array made of the (6,0)zigzag carbon nanotubes with the nearesttubes sharing the tube walls. After cutting a (1,1) zigzag carbon foam, we canget two kinds of surfaces, namely the ones terminated with the sp2and sp3hybridized carbon atoms(labeled as sp2and sp3surface), and the saturation ofthe dangling bonds with hydrogen can increase the stability of them. The sp2surface with hydrogen saturation (labeled as sp2H surface) and the sp3surfacehave the ferromagnetic and antiferromagnetic ground states respectively. Boththeir Curie temperature and Neel temperature are over100K. Moreover, thestrain can control the sp2H surface magnetism, e.g., increasing lattice constantof the sp2H surface could strengthen the spin polarization therein. Moreimportantly, in the bulk band gap, the surface states of the sp2H surface existand at the K and K’ points, they display the spin-polarized Dirac-cone-like bandstructure.