  Two-dimensional (2D) lateral heterojunctions of transition metal
dichalcogenides (TMDCs) have become a reality in recent years. Semiconducting
TMDC layers in their common H -structure have a nonzero in-plane electric
polarization, which is a topological invariant. We show by means of
first-principles calculations that lateral 2D heterojunctions between TMDCs
with a different polarization generate one-dimensional (1D) metallic states at
the junction, even in cases where the different materials are joined
epitaxially. The metallicity does not depend upon structural details, and is
explained from the change in topological invariant at the junction.
Nevertheless, these 1D metals are susceptible to 1D instabilities, such as
charge- and spin-density waves, making 2D TMDC heterojunctions ideal systems
for studying 1D physics.
