  The ability to switch between distinct states of polarization comprises the
defining property of ferroelectrics. However, the microscopic mechanism
responsible for switching is not well understood and theoretical estimates
based on coherent monodomain switching typically overestimate experimentally
determined coercive fields by orders of magnitude. In this work we present a
detailed first principles characterization of domain walls (DWs) in
two-dimensional ferroelectric GeS, GeSe, SnS and SnSe. In particular, we
calculate the formation energies and migration barriers for 180 and 90 DWs, and
then derive a general expression for the coercive field assuming that
polarization switching is mediated by DW migration. We apply our approach to
the materials studied and obtain good agreement with experimental coercive
fields. The calculated coercive fields are up to two orders of magnitude
smaller than those predicted from coherent monodomain switching in GeSe, SnS
and SnSe. Finally, we study the optical properties of the compounds and find
that the presence of 180 DWs leads to a significant red shift of the absorption
spectrum, implying that the density of DWs may be determined by means of simple
optical probes.
