Terahertz Phonons and Electron–Phonon Interactions in Two-Dimensional Lead Halide Perovskites*

While Ruddlesden–Popper halide perovskites have attracted much attention in recent years due to their promising properties for applications in optoelectronics, they are also known to possess novel phononic properties due to the soft lattice as well as strong electron-phonon interactions. The lowest-frequency optical phonon modes in these materials lie in the terahertz (THz) frequency range. It has previously been demonstrated that in bulk perovskites both free carriers and optical phonons contribute to the THz optical conductivity with intriguing interterplay between them. Here, we used THz time-domain spectroscopy to study optical phonon modes in (BA)₂(MA)_n-1Pb_nI_3n-1 samples with different values of n (1 to 5). We observed pronounced resonances in the 0.3-2.5 THz range and systematically studied them as a function of temperature, including the temperature range where a structural phase transition occurs in these compounds. Further, we performed optical-pump/THz-probe spectroscopy measurements, which elucidated the nonequilibrium dynamics and interactions of free charge carriers and phonons.