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 a symmetry breaking phase transition. While electron-soft phonon coupling can introduce unique scattering channels for charge carriers in ferroelectrics, recent studies on the nonferroelectric 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 phonon mode in a ferroelectric semiconductor SbSI. Photogenerated 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.