  Boundary helical Luttinger liquid (HLL) with broken bulk time-reversal
symmetry belongs to a unique topological class which may occur in
antiferromagnets (AFM). Here, we search for signatures of HLL on the edge of a
recently discovered topological AFM, MnBi2Te4 even-layer. Using scanning
superconducting quantum interference device, we directly image helical edge
current in the AFM ground state appearing at its charge neutral point. Such
helical edge state accompanies an insulating bulk which is topologically
distinct from the ferromagnetic Chern insulator phase as revealed in a magnetic
field driven quantum phase transition. The edge conductance of the AFM order
follows a power-law as a function of temperature and source-drain bias which
serves as strong evidence for HLL. Such HLL scaling is robust at finite fields
below the quantum critical point. The observed HLL in a layered AFM
semiconductor represents a highly tunable topological matter compatible with
future spintronics and quantum computation.
