Peculiar anharmonicity of Ruddlesden Popper metal halides: Temperature-dependent phonon dephasing

The anharmonicity of the Ruddlesden Popper metal-halide lattice, and its consequences on their electronic and optical properties, is paramount in their basic semiconductor physics. It is thus critical to identify specific anharmonic optical phonons that govern their photophysics . Here, we address the nature of phonon-phono scattering probabilities of the resonantly excited optical phonons that dress the electronic transitions in these materials by means of variable-temperature resonant impulsive stimulated Raman measurements. Based on the temperature dependence of the coherent phonon lifetimes, we isolate the dominant anharmonic phonon and quantify its phonon-phonon interaction strength. Intriguingly, we also observe that the anharmonicity is distinct for different phonons, with a few select modes exhibiting temperature-independent coherence lifetimes, indicating their predominantly harmonic nature. However, the population and dephasing dynamics of excitons are dominated by the anharmonic phonon.