  A mirror twin boundary (MTB) in a transition metal dichalcogenide (TMD)
monolayer can host one-dimensional electron liquid of a topological nature with
tunable interactions. Unfortunately, the electrical characterization of such
boundaries has been challenging due to the paucity of samples with large enough
size and high quality. Here, we report an epitaxial growth of monolayer
molybdenum disulfide (MoS$_2$) bicrystals with well-isolated MTBs that are tens
of micrometers long. Conductance measurements of these MTBs exhibit power-law
behaviors as a function of temperature and bias voltage up to room temperature,
consistent with electrons tunneling into a Luttinger liquid. Transport
measurements of two distinct types of MTBs reveal the critical role of the
atomic-scale defects. This study demonstrates that MTBs in TMD monolayers
provide an exciting new platform for studying the interplay between electronic
interactions and topology.
