Charge carrier coupling to the soft phonon mode in a ferroelectric semiconductor

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