  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.
