The Electronic Transport Properties In Quantum Wires Modulated By Stubs And Defects

In condensed matter physics, study on the electronic transport properties of mesoscopic structure is essential to study quantum coherence and Fano effects. Various structures of the quantum wires provide a wealth of coherent path for the electron transport. When studying electronic transport properties of quantum wires, it must reveal the images of quantum coherence. In this paper we used Green function methods and KNIT software to systematically study the electronic transport properties of several typical quantum wire structures and got some meaningful results.Firstly, for the conductance spectra of ordinary T-shaped quantum wire, when the energies of incident electrons are fixed at some specific values, there will be a phenomenon that the conductance value decline sharply. According to quantum coherence mechanisms, the main reason for this phenomenon is that when electrons move in the T-shaped quantum wire there is always destructive interference between different partitions. When the stub is embedded on the quantum wire, the changing of stub’s scale will influence electronic transport. When two stubs are embedded on quantum waveguide and move along the quantum waveguide, their relative positions on the quantum waveguide have a huge impact on the electron transport. When two stubs are located in different sides of quantum waveguide, they have different effects on the electronic transport. If the stubs are added onsite energy, the electronic transport will become more complicated.Secondly,5-5-8type line defects, which are periodically and alternately stacked by two pentagons and one octagon, are embedded in armchair graphene nano-ribbons. When the electrons move in armchair graphene nano-ribbons with different widths, there will produce a wealth of physical phenomena and the most typical phenomena are the Fano effects and the continuous bound state. In the spectra of density of electronic states near the Dirac point, due to the presence of line defect, some localized quantum states come up, which have different degree of localization and lead to different transport results. When the heterostructure is composed by graphene and metal, where the grapheme is embedded the line defect, it will cause strong localized states in the vicinity of Dirac point. The coupling between these localized states and the electrode leads to resonant tunneling phenomenon, and the property of resonant tunneling depends on the size of armchair graphene nano-ribbon. In some specific cases, armchair graphene nano-ribbons with a line defect can be seen as quantum dots. When there are two line defects, the peak of conductance can be seen as molecular quantum dot. Changing the coupling strength between line defects and armchair graphene nano-ribbon has some impact on Fano effect.Finally, a new line defect, which is alternately and periodically stacked by a pentagon and a heptagon, is introduced. The line defect mainly destroys the electronic transport in the conduction-band region, and causes abundant Fano effects determined by the width of the armchair graphene nano-ribbon. Typically, the results of M≤17are completely different from those of M>17. The spectra of the density of electron states show that the line defect induces some localized quantum states, and that the different localizations of these states lead to the distinct transport results. When such line defect embedded in the armchair graphene nano-ribbon, whose width M=3n+2, it has little effect on electron transport on the valence band region, but Fano anti-resonance always exist on conduction band region. In addition, the coupling strength between such a line defect and armchair graphene nano-ribbon can also affect the conductance value of entire region.