The Andreev Reflection Of Zero Line Mode In Graphene-superconductor Hybrid Junction

Graphene has caused the great attention in many research fields since been successfully fabricated in 2004 by two physicists:A. Geim and K. Novoselov, from the University of Manchester in England. Graphene has unique physical and chemical properties. As a result, graphene becomes one of the most popular topics in modern materials science and condensed matter physics. In resent years, not only the synthesis of graphene emerges in an endless stream, but also the study of its characteristics and applications have achieved great success. The Andreev reflection occurring between graphene and the superconducting interface is also different from the phenomenon between the ordinary one.In this thesis, we investigate the Andreev reflection effect of quasi one-dimensional state in graphene at the interface of graphene and superconductor. In two dimensional hexagonal materials, if there is a line defect exist that the electron effective mass changes its sign when across the line defect, the line defect carries one-dimensional electron wave, called zero line mode (ZLM). We report the theoretical investigation of the Andreev reflection of ZLM. For a two-terminal zigzag edged sample, the Andreev reflection coefficient can be either large or strongly suppressed depending on the incident wave' valley index. However, the Andreev reflection coefficient is large due to the absence of wave function symmetry. When ZLM changes its direction in a vertical path, a perfect Andreev reflection could happen when the incident electrons form a zigzag edged graphene ribbon. It stems from the fact that in a zigzag edged four-terminal hybrid model, the strengthening of interference from reflected holes leads to the annihilation of the crossed Andreev reflection and perfect Andreev reflection. For the armchair edged model, the interference effect disappears because the Andreev reflection from one of the paths is prohibited. The interference and Andreev reflection in four-terminal models can be explained by local density of states in the central scattering region as well. Our finding can provide useful guiding for the design of superconducting quantum interference device in valleytronics.