Gate-tunable Topological Valley Transport in Bilayer Graphene

TL;DR

This paper demonstrates that applying a perpendicular electric field to bilayer graphene induces tunable, topologically protected valley transport, with a giant nonlocal response observable even at room temperature, promising for valleytronic devices.

Contribution

It shows gate-tunable topological valley transport in bilayer graphene, revealing controllable, robust valley pseudospin transport with potential for technological applications.

Findings

Giant nonlocal response observed due to valley topological transport

Valley transport fully tunable by external gates

Nonlocal signal persists at room temperature and over long distances

Abstract

Valley pseudospin, the quantum degree of freedom characterizing the degenerate valleys in energy bands, is a distinct feature of two-dimensional Dirac materials. Similar to spin, the valley pseudospin is spanned by a time reversal pair of states, though the two valley pseudospin states transform to each other under spatial inversion. The breaking of inversion symmetry induces various valley-contrasted physical properties; for instance, valley-dependent topological transport is of both scientific and technological interests. Bilayer graphene (BLG) is a unique system whose intrinsic inversion symmetry can be controllably broken by a perpendicular electric field, offering a rare possibility for continuously tunable valley-topological transport. Here, we used a perpendicular gate electric field to break the inversion symmetry in BLG, and a giant nonlocal response was observed as a result of the topological transport of the valley pseudospin. We further showed that the valley transport is fully tunable by external gates, and that the nonlocal signal persists up to room temperature and over long distances. These observations challenge contemporary understanding of topological transport in a gapped system, and the robust topological transport may lead to future valleytronic applications.