Ultrafast pump-probe measurements of short small-polaron lifetimes in the mixed-valence perovskite Cs2Au2I6 under high pressures

We study the ultrafast phonon response of mixed-valence perovskite Cs2Au2I6 using pump-probe spectroscopy under high pressure in a diamond-anvil cell. We observed a remarkable softening and broadening of the Au-I stretching phonon mode with both applied pressure and photoexcitation. Using a double-pump scheme we measured a lifetime of the charge-transfer excitation into single-valence Au2+ of less than 4 ps, which is an indication of the local character of the Au2+ excitation. Furthermore, the strong similarity between the pressure and fluence dependence of the phonon softening shows that the intervalence charge transfer plays an important role in the structural transition.