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/2e² in zero magnetic field at the primary Dirac point with a characteristic decay length of 2 μm. Furthermore, nonlocal transport transmission probabilities based on the Landauer-Büttiker formalism show evidence for spin-degenerate ballistic valley-helical edge modes, which are key for the development of valleytronics.

六方氮化硼（hBN）上的石墨烯可以通过相互的晶体学排列显示出拓扑相。非局部阻抗（的最近的测量- [R _NL）在弹道石墨烯二次狄拉克点（SDP）接近/六方氮化硼超晶格被解释成由于量子霍尔谷状态引起的。我们报告了hBN /石墨烯/ hBN超晶格，其中SDP处的R _nl可忽略不计，但低于60 K时接近h / 2 e ²的值在零狄拉克点的零磁场中具有2μm的特征衰减长度。此外，基于Landauer-Büttiker形式主义的非本地运输传输概率显示出自旋简并的弹道山谷螺旋边缘模式的证据，这是Valleytronics发展的关键。