Anharmonic organic cation vibrations in the hybrid lead halide perovskite CH3NH3PbI3

Ultrafast vibrational dynamics of inorganic-organic hybrid lead halide perovskite $\mathrm{C}{\mathrm{H}}_{3}\mathrm{N}{\mathrm{H}}_{3}\mathrm{Pb}{\mathrm{I}}_{3}$ in the room-temperature tetragonal phase is investigated under weak photoexcitation at the band edge. Time-resolved optical Kerr effect signals exhibit periodic modulations at 4 and 8 THz due to the libration and twisting of the methylammonium molecule, in addition to the inorganic sublattice deformation at 1.2 THz. These oscillations are heavily damped and resemble the short-lived ground-state oscillation reported in a recent THz-pump study [Z. Liu et al., Phys. Rev. B 101, 115125 (2020)]. The vibrational frequencies exhibit blueshifts with increasing pump fluence, contrary to the expectation from conventional laser heating. Our ab initio lattice dynamics simulations reveal strongly anharmonic vibrational potentials for the zone-center optical phonon modes, and thereby confirm the anharmonicity to be the origin of the experimentally observed frequency blueshifts.