Probing the limits of the phonon quasi-particle picture: The transition from underdamped to overdamped dynamics in CsPbBr3

The soft modes associated with continuous-order phase transitions are commonly associated with particularly strong anharmonicity. Here, we show that this can lead to overdamped behavior already far above the actual transition temperature using molecular dynamics simulations and a machine-learned potential. While in the overdamped limit the interpretation of lattice vibrations as phonon quasi-particles with a specified frequency and relaxation time becomes physically questionable the mathematical description in terms of damped oscillators still holds. A physically more intuitive picture can be obtained by considering the relaxation times of the mode coordinate and its conjugate momentum, which at the instability approach infinity and the inverse damping factor, respectively. We demonstrate this behavior quantitatively for the prototypical case of the cubic-to-tetragonal phase transition of the inorganic halide perovskite CsPbBr3, and show that the overdamped region extends almost 200 K (or about 60%) above the transition temperature.