Fig. 5

CLAP of (HA)PbI₄, and the amplitude and coherence time of CLAPs in 2D RPs and MAPbI₃. a ΔR/R spectral map for (HA)PbI₄ single-crystal flake, excited with 400-nm pump at a fluence of 0.2 mJ cm⁻². b Left: crystal structure of (HA)PbI₄ (upper figure) and schematic of the corresponding bead-spring model (lower figure). The ionic interaction between the Pb–I octahedron (blue bead) and the HA²⁺ ion (purple bead) is represented by the purple spring with a spring constant of k_c. Right: crystal structure of N = 1 (upper figure) and schematic of the corresponding bead-spring model (lower figure). The van der Waals interaction between the two BA⁺ ions (black beads) is represented by the black spring with a spring constant of k_d. c Dependence of the acoustic phonon velocity of N = 1 on k_d and L, with L defined in b. d ΔR/R kinetic traces at 955 nm for N = 1–6 and MAPbI₃ (curves are offset for clarify). e Blue: amplitude of the sinusoidal oscillations at the delay times associated with the first ΔR/R peak (marked by cyan dotted lines in d); the plotted amplitudes are normalized by the fluence described in caption of Fig. 2. Red: calculated time-averaged mean displacement of the beads. f Coherence time of the CLAP oscillations. Error bars (i.e., standard deviations) in e and f are based on the fitting of ΔR/R kinetics at ten different wavelengths spanning the probed spectral window