  We theoretically investigate the dynamics of atomic domain walls (DWs) in
antiferromagnets driven by a spin-orbit field. For a DW with the width of a few
lattice constants, we identify a Peierls-like pinning effect, with the
depinning field exponentially decaying with the DW width, so that a spin-orbit
field moderately larger than the threshold can drive the propagation of an
atomic DW in a step-wise manner. For a broad DW, the Peierls pinning is
negligibly small. However, the external spin-orbit field can induce a fast DW
propagation, accompanied by a significant shrinking of its width down to atomic
scales. Before stepping into the pinning region, noticeable spin waves are
emitted at the tail of the DW. The spin-wave emission event not only broadens
the effective width of the DW, but also pushes the DW velocity over the
magnonic barrier, which is generally believed to be the relativistic limit of
the DW speed. While the existing dynamic theory based on the continuum
approximation fails in the atomic scale, we develop an energy conversion theory
to interpret the DW dynamics beyond the relativistic limit.
