▎ 摘　　要

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-Buttiker formalism show evidence for spin-degenerate ballistic valley-helical edge modes, which are key for the development of valleytronics. By interfacing graphene with other materials it is possible to break the intrinsic inversion symmetry and observe interesting quantum transport phenomena. Here, the authors conduct transport measurements of encapsulated graphene at different alignment angles and find evidence of nonlocal resistance above and below 60 K suggesting the existence of a quantum valley Hall state.