Competing Gap Opening Mechanisms of Monolayer Graphene and Graphene Nanoribbons on Strong Topological Insulators

Abstract

Graphene is a promising material for designing next-generation electronic and valleytronic devices, which often demand the opening of a bandgap in the otherwise gapless pristine graphene. To date, several conceptually different mechanisms have been extensively exploited to induce bandgaps in graphene, including spin-orbit coupling and inversion symmetry breaking for monolayer graphene, and quantum confinement for graphene nanoribbons (GNRs). Here, we present a multiscale study of the competing gap opening mechanisms in a graphene overlayer and GNRs proximity-coupled to topological insulators (TIs). We obtain sizable graphene bandgaps even without inversion symmetry breaking and identify the Kekulé lattice distortions caused by the TI substrates to be the dominant gap opening mechanism. Furthermore, Kekulé distorted armchair GNRs display intriguing nonmonotonous gap dependence on the nanoribbon width, resulting from the coexistence of quantum confinement, edge passivation, and Kekulé distortions. The present study offers viable new approaches for tunable bandgap engineering in graphene and GNRs.

Resonant transport in Kekulé-distorted graphene nanoribbons

“…where the first solutions are given by ε = ±3∆ 0 t. Therefore, the size of the gap is given by 6∆ 0 t, which coincides with the gap for an infinite Kek-O distorted graphene sheet 22,25 . This last result applies only to zigzag GNR, for the Kek-O armchair GNRs the gap turn to be considerable smaller 29,40 . Notice that there is an entirely flat band at zero energy, this band will not contribute to transport since it has vanishing group velocity 41 .…”

“…For instance it has been pointed out that a Kek-O distortion allows the realization of charge fractionalization with time reversal symmetry in 2D graphenelike systems 25,26 , leading to the formation of states with topological properties 27,28 . There is no experimental evidence yet of the Kek-O phase in graphene 20,29 , although it can be achieved in analogues systems like molecular graphene 21 and vibrational systems 30 . As for the Kek-Y phase, it became a subject of scrutiny after its recent experimental realization by C. Gutierrez, et al 31 , whom showed amazing STM images of standing-wave in the local density of states of graphene on Cu(111) surface forming a Kek-Y pattern.…”

Activating impurity effect in edge nitrogen-doped chevron graphene nanoribbons

“…The calculations are performed using first-principle theory imbedded in the QUNTUM ESPRESSO code [17]. The Perdew-Burke-Ernzerhof generalized gradient approximation (PBE-GGA) functional is employed and the pseudopotential is ultrasoft [18,19]. We have also verified the GGA with the Van der Waals forces correction and the result differences are negligible.…”

Unconventional thermal magnon Hall effect in a ferromagnetic topological insulator

“…In graphene, bond modulations can be induced by local changes in the position of the carbon atoms due to the absorption of adatoms on its surface [32] or by a proximity effect [33,34]. One such example is the KOC modulation, depicted in Fig.…”