Coherent Exciton Spin Relaxation Dynamics and Exciton Polaron Character in Layered Two-Dimensional Lead-Halide Perovskites.

The quantum-well-like two-dimensional lead-halide perovskites exhibit strongly confined excitons due to the quantum confinement and reduced dielectric screening effect, which feature intriguing excitonic effects. The ionic nature of the perovskite crystal and the "softness" of the lattice induce the complex lattice dynamics. There are still open questions about how the soft lattices decorate the nature of excitons in these hybrid materials. Herein, we reveal the polaronic character of excitons and coherent exciton spin relaxation dynamics in layered hybrid perovskites by using chirality-dependent impulsive vibrational spectroscopy. We identify an intrinsic exciton spin dynamics property, giving rise to a short spin relaxation lifetime in the sub-picosecond time scale. The exciton polaron formation is confirmed by the blue-shift of the phonon frequency under resonant conditions compared to that in below-resonance excitation cases. The phonon vibrational wavepackets show a cosine- and sine-like oscillation as a function of time via on- and below-resonance excitation scenarios due to the displacive and impulsive mechanisms, respectively. Our findings provide profound insights concerning the polaronic character of excitons in two-dimensional perovskites, underpinning the prospective developments in optical and optoelectronic applications.