Nature Of Edge States Research Articles

Superconducting pairing symmetry and spin-orbit coupling in proximitized graphene

Graphene may exhibit different topological phases as a result of proximity to different substrates. We study the effect of superconductivity in such systems using the effective Bogolyubov-de Gennes Hamiltonian with different superconducting pairing order parameters. We analyze the topological phase transition and symmetry class of the system in different parameter regimes. A particularly interesting situation occurs when nearest-neighbor spin-singlet superconducting pairing is present in phases of proximitized graphene that exhibit either inverted band or quantum spin Hall behavior. Both superconducting phases show similar characteristics in the low-energy range, including the appearance of robust edge states, and are neighboring phases across a transition that closes the quasiparticle gap as the chemical potential changes. Detailed construction and analysis of the existence and nature of edge states are presented in different system regimes.

Sublattice engineering and voltage control of magnetism in triangular single and bi‐layer graphene quantum dots

magnified imageWhen a Dirac electron is confined to a triangular graphene quantum dot with zigzag edges, its low‐energy spectrum collapses to a shell of degenerate states at the Fermi level leading to a magnetized edge. The shell degeneracy and the total magnetization are proportional to the edge size and can be made macroscopic. In this review, we start with a general discussion of magnetic properties of graphene structures and its relation to broken sublattice symmetry. Then, we discuss single electronic properties of single and bilayer triangular graphene quantum dots, focusing on the nature of edge states. Finally, we investigate the role of electronic correlations in determining the nature of ground state and excitation spectra of triangular graphene quantum dots as a function of dot size and filling fraction of the shell of zero‐energy states. The interactions are treated by a combination of tight‐binding, Hartree–Fock and configuration interaction methods. We show that the spin polarization of the triangular graphene quantum dots can be controlled through gating, i.e., by adding or removing electrons. In bilayer graphene dots, the relative filling of edge states in each layer and the magnetization can be tuned down to single localized spin using an external vertical electrical field. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)

Zigzag graphene nanoribbon edge reconstruction with Stone-Wales defects

In this article, we study zigzag graphene nanoribbons with edges reconstructed with Stone-Wales defects, by means of an empirical (first-neighbor) tight-binding method, with parameters determined by ab-initio calculations of very narrow ribbons. We explore the characteristics of the electronic band structure with a focus on the nature of edge states. Edge reconstruction allows the appearance of a new type of edge states. They are dispersive, with non-zero amplitudes in both sub-lattices; furthermore, the amplitudes have two components that decrease with different decay lengths with the distance from the edge; at the Dirac points one of these lengths diverges, whereas the other remains finite, of the order of the lattice parameter. We trace this curious effect to the doubling of the unit cell along the edge, brought about by the edge reconstruction. In the presence of a magnetic field, the zero-energy Landau level is no longer degenerate with edge states as in the case of pristine zigzag ribbon.