Topological valley currents via ballistic edge modes in graphene superlattices near the primary Dirac point

TL;DR

This paper demonstrates the existence of ballistic valley-helical edge modes in graphene/hBN superlattices near the primary Dirac point, revealing potential for valleytronics applications through nonlocal resistance measurements.

Contribution

It provides experimental evidence for spin-degenerate ballistic valley-helical edge modes in graphene/hBN superlattices, advancing understanding of topological valley currents.

Findings

Negligible nonlocal resistance at secondary Dirac point

Quantized nonlocal resistance approaching h/2e^2 at primary Dirac point

Evidence for ballistic valley-helical edge modes

Abstract

Graphene on hexagonal boron nitride (hBN) can exhibit a topological phase via mutual crystallographic alignment. Recent measurements of nonlocal resistance ($R_{nl}$) near the secondary Dirac point (SDP) in ballistic graphene/hBN superlattices have been interpreted as arising due to the quantum valley Hall state. We report hBN/graphene/hBN superlattices in which $R_{nl}$ at SDP is negligible, but below 60 K approaches the value of $h/2e^{2}$ in zero magnetic field at the primary Dirac point with a characteristic decay length of 2 ${\mu}$m. Furthermore, nonlocal transport transmission probabilities based on the Landauer-B\"uttiker formalism show evidence for spin-degenerate ballistic valley-helical edge modes, which are key for the development of valleytronics