  Many crystalline solids possess strongly anharmonic soft phonon modes
characterized by diminishing frequency as temperature approaches a critical
point associated with a symmetry breaking phase transition. While electron-soft
phonon coupling can introduce unique scattering channels for charge carriers in
ferroelectrics, recent studies on the non-ferroelectric lead halide perovskites
have also suggested the central role of anharmonic phonons bearing resemblance
to soft modes in charge carrier screening. Here we apply coherent phonon
spectroscopy to directly study electron coupling to the soft transverse optical
(TO) phonon mode in a ferroelectric semiconductor SbSI. Photo-generated charge
carriers in SbSI are found to be exceptionally long lived and are associated
with a transient electro-optical effect that can be explained by interactions
between charge carriers and thermally stimulated soft-phonon excitations. These
results provide strong evidence for the role of electron-soft phonon coupling
in the efficient screening of charge carriers and in reducing charge
recombination rates, both desirable properties for optoelectronics.
