Abstract
Graphene on hexagonal boron nitride (hBN) can exhibit a topological phase via mutual crystallographic alignment. Recent measurements of nonlocal resistance (Rnl) 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 Rnl 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.
Highlights
Graphene on hexagonal boron nitride can exhibit a topological phase via mutual crystallographic alignment
In atomically aligned graphene/hexagonal boron nitride (hBN), a band gap at the primary Dirac point (DP, VD) is formed[6,7,8] and secondary Dirac points (SDPs) are stabilized at energy relating to the moiré wavelength[4,9]
Nonlocal resistances (Rnl) in aligned graphene/hBN Hall bars at the DP and in zero magnetic field have been interpreted as being related to a finite Berry curvature, which leads to the valley Hall effect (VHE) due to a coupling between the valley and the electron orbital motion[3,8,10,11]
Summary
Graphene on hexagonal boron nitride (hBN) can exhibit a topological phase via mutual crystallographic alignment. Recent measurements of nonlocal resistance (Rnl) near the secondary Dirac point (SDP) in ballistic graphene/hBN superlattices have been interpreted as arising due to the quantum valley Hall state. Nonlocal resistances (Rnl) in aligned graphene/hBN Hall bars at the DP (or SDP) and in zero magnetic field have been interpreted as being related to a finite Berry curvature, which leads to the valley Hall effect (VHE) due to a coupling between the valley and the electron orbital motion[3,8,10,11]. We report Rnl measurements in encapsulated graphene Hall bars (hBN/graphene/hBN) with different alignment angles and focus on nonlocal transport near the DP where Rnl approaches h/2e2 in zero magnetic field below 60 K, in contrast to refs. We report Rnl measurements in encapsulated graphene Hall bars (hBN/graphene/hBN) with different alignment angles and focus on nonlocal transport near the DP where Rnl approaches h/2e2 in zero magnetic field below 60 K, in contrast to refs. 8,10
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