چكيده به لاتين
Carbon is one of the most crucial elements in nature; carbon-based materials have brought a great deal of concentration during the past decades. All organisms on Earth consist of carbon-based structures, and many materials relevant to human society are based on carbon. Carbon is the Group 14 element that has four valence electrons and forms different types of chemical bonds (covalent bonds) using several hybridizations (sp, sp2, and sp3). Physicists and chemists are quite familiar with this flexibility, as it allows the formation of a wide variety of fascinating materials. The electronic and magnetic properties of carbon-based nanostructures are currently one of the most critical research areas in science. Many researchers and engineers are working on the design and synthesis of new carbon-based materials, which play essential roles in human society, with the range of potential applications continually increasing. Intrinsic magnetism involves the elements belonging to either the d- or f- block of the periodic table. However, The main carbon allotropes are known to be non-magnetic. This seems to be correct not only about the well known solid phases diamond and graphite but also concerning the nanoscopic phases manufactured in nanotechnology. Hence, the emergence of magnetism in nanographenes offers unique opportunities for future technological applications such as spintronic devices and field-effect transistors. Controlled fabrication of magnetic materials based on carbon looks to be beneficial because of their lower production costs when compared with their metal counterparts. This thesis is based on graphene-derived systems (nanographene) in which electronic band structure, magnetic correlations that emerge as a result of reduced dimensions and disorders, and quantum spin transport are investigated. In particular, two-dimensional graphene nanoribbons with its two high-symmetry crystallographic directions ( armchair and zigzag), both as an infinite and finite-size (quantum dots and antidots) structure and, one-dimensional carbon nanotubes are covered.
In the first part of this thesis, we survey a method that has demonstrated to be efficient in modeling carbon nanomaterials' band structure behavior. Procedures based on the simple and usual tight-binding model, which plays an important role in explaining the electronic architecture of graphene and its derivatives. Tight-binding schemes have successfully addressed remarkable phenomena on these materials. This method predicts that N-aGNRs are metallic for every $N=3l+2$ (where $l$ is a positive integer), and semiconducting otherwise. In the case of carbon nanotubes , including the zone-folding approach, the tight-binding model predicts metallic behavior for 1/3 of nanotubes, while the other 2/3 are semiconducting. Moreover, the physical mechanisms of the emergence of magnetism in nanographenes are illustrated with the help of computational many-body codes based on nearest-neighbor tight-binding Hamiltonian in combination with the single-orbital Hubbard model at the mean-field approximation. High density of low-energy electronic states suggests a possibility of magnetic ordering in zGNRs. The self-consistent mean-field Hubbard model solution for zigzag graphene nanoribbons reveals magnetic moments localized at the edges. The localized magnetic moments show ferromagnetic and antiferromagnetic ordering along the zigzag edge. The antiferromagnetic ground-state has a band gap at the Fermi energy while, the ferromagnetic interedge orientation is a metal with two bands crossing the Fermi level. Furthermore, a comprehensive argument made for the existence of zero-energy states in bipartite and non-bipartite nanographene quantum dots. Finally, graphene appears highly suitable for spin quantum transport. In the first place, this feature is related to the low weight of carbon, which indicates weak spin-orbit as well as hyperfine coupling. Both of these interactions induce spin relaxation and decoherence, thus limiting the lifetime of propagating spins. In addition, practical strategies for explaining the quantum spin transport properties of zigzag graphene nanoribbons have been outlined by using Landauer formalism, and Greens functions approach. This includes the implementation of Rashba interactions, which provide the mechanism underlying the spin transistor concept. These and other outcomes specific to nanographenes make them appear notably useful as carrier materials for spin currents in spintronics circuits.
