Protecting spin polarization in layered metal halide perovskites through dynamic polaron formation

Metal halide perovskites attracted intense research interest as promising materials for photovoltaic and spintronic applications. Due to the polar nature of their soft crystal lattice and quantum-well-like structures, they exhibit strong coupling between exciton and phonon modes. We investigated the impact of exciton-phonon coupling on the spin dynamics of BA₂FAPbI₇ (BA, Butylammonium; FA, Formamidinium) using femtosecond Faraday rotation and transient absorption spectroscopy. We observe that by coherently exciting far above the exciton resonance, the spin lifetime at 77K is increased by more than two orders of magnitude. Transient absorption measurements recover coherent phonon modes which are strongly coupled to excitons and enable the formation of a polaron state. The ultrafast formation of the polaron reduces the electron-hole exchange which, in turn, slows down the rate of spin precession and protects spin polarization. Our results demonstrate that strong exciton-phonon coupling in layered perovskites leads to a novel regime of exciton-polaron spin dynamics which can be optically selected. The ability to encode photoexcitation energy into spin lifetimes could be a promising strategy for future spintronics-photonics applications.