Topological valley currents in bilayer graphene/hexagonal boron nitride superlattices

Abstract

Graphene superlattices have recently been attracting growing interest as an emergent class of quantum metamaterials. In this paper, we report the observation of nonlocal transport in bilayer graphene (BLG) superlattices encapsulated between two hexagonal boron nitride (hBN) layers, which formed hBN/BLG/hBN moiré superlattices. We then employed these superlattices to detect a long-range charge-neutral valley current using an all-electrical method. The moiré superlattice with broken inversion symmetry leads to a "hot spot" at the charge-neutral point (CNP), and it harbors satellites of the CNP. We observed nonlocal resistance on the order of 1 kΩ, which obeys a scaling relation. This nonlocal resistance evolves from an analog of the quantum Hall effect but without magnetic field/time-reversal symmetry breaking, which is associated with a hot-spot-induced topological valley current. This study should pave the way to developing a Berry-phase-sensitive probe to detect hot spots in gapped Dirac materials with inversion-symmetry breaking. _____________________________ Graphene superlattices are an emergent class of quantum metamaterials with considerable promise. In this letter, we explore the transport properties of bilayer graphene (BLG) superlattices, focusing on the topological current associated with a valley degree of freedom. A valley in an energy band implies a degenerate local minimum (maximum) in the conduction (valence) band and is referred to as K and K' in the case of graphene, which correspond to valley pseudospin states [1]. Graphene is associated with two valley pseudospin states, which transform into each other under spatial inversion. The broken inversion symmetry induces valleycontrasted physics through the emergence of a "hot spot" for each valley, which leads to the generation of valley Hall currents owing to the finite Berry curvature in the energy band [1,2]. The nonlocal transport is a Berry-phase-sensitive probe to detect such a hot spot in graphene superlattices [3,4,5]. More recently, quantum valley current was detected through the nonlocal resistance in the quantum limit [5]. This is in analogy with quantum Hall effect but without magnetic-field/time-reversal symmetry breaking, which is associated with hot-spot-induced

Enhanced electron-phonon coupling in doubly aligned hexagonal boron nitride bilayer graphene heterostructure

“…One well established example is graphene on hexagonal boron nitride (hBN). The weak periodic potential due to underlying hBN gives rise to modulation of graphene electronic band structure [2][3][4] , with the emergence of clone Dirac cones 5 , wherein, the Fermi velocity could be controlled by the relative twist angle between graphene and hBN, showing interesting physics like the Hofstader butterfly 6 , resonant tunneling 7 , change of topological winding number 8 , topological valley current 9 , Brown-Zak oscillations 10 etc. Moreover, twisting individual layers leads to flat bands 11 , where emergent phenomenon like correlated insulating state 12 , superconductivity 13 , quantum anomalous Hall effect 14 , fractional Chern insulating states 15,16 , moiré excitons 17 , and ferromagnetism 14 has been observed.…”

Topology-Bounded Superfluid Weight in Twisted Bilayer Graphene

“…Recently, superconductivity has been observed in other Moiré systems, including twisted double bilayer graphene, and multilayer graphene/boron nitride heterostructure [68][69][70][71][72][73][74][75][76]. In these systems, a displacement electric field is necessary for superconductivity.…”

Valley Hall effect and nonlocal resistance in locally gapped graphene

“…Certain defects or strain deformation fields have been theoretically proposed for achieving valley filtering, but the corresponding experimental implementation remains challenging . Possible signatures of valley transport phenomena have been discussed in relation to nonlocal resistance (R NL ) measurements in commensurately stacked graphene/hexagonal boron nitride (hBN) systems [34][35][36]. Large R NL signals have been interpreted in terms of an intrinsic valley Hall effect (VHE), driven by bulk Berry curvature [37][38][39][40], and which would be related to a uniform mass term induced by the interaction between graphene and hBN.…”