Coherent band-edge oscillations and dynamic longitudinal-optical phonon mode splitting as evidence for polarons in perovskites

• Published in: Physical review. B 2020-03, Vol.101 (11)
• Main Authors: Liu, Z., Vaswani, C., Luo, L., Cheng, D., Yang, X., Zhao, X., Yao, Y., Song, Z., Brenes, R., Kim, R. J. H., Jean, J., Bulović, V., Yan, Y., Ho, K.-M., Wang, J.
• Format: Article
• Language: English
• Subjects: ATOMIC AND MOLECULAR PHYSICS; CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSICS; CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND SUPERFLUIDITY; MATERIALS SCIENCE
• ISSN: 2469-9950; 2469-9969

The coherence of collective modes, such as phonons and polarons, and their modulation of electronic states is long sought in complex systems, which is a crosscutting issue in photovoltaics and quantum electronics. In photovoltaic cells and lasers based on metal halide perovskites, the presence of polarons, i.e., photocarriers dressed by the macroscopic motion of charged lattice, assisted by terahertz (THz) longitudinal-optical (LO) phonons, has been intensely studied yet is still debated. This may be key for explaining the remarkable properties of the perovskite materials, e.g., defect tolerance, long charge lifetimes, and diffusion lengths. In this work we use the intense single-cycle THz pulse with peak electric field up to ETHz=1000 kV/cm to drive coherent polaronic band-edge oscillations at room temperature in CH3NH3PbI3 (MAPbI3). We reveal the oscillatory behavior is dominated by a specific quantized lattice vibration mode at $ω$LO ~ 4 THz, which is both dipole and momentum forbidden. THz-driven coherent polaron dynamics exhibits distinguishing features: room temperature coherent oscillations at ωLO longer than 1 ps in both single crystals and thin films, mode-selective modulation of different band-edge states assisted by electron-phonon interaction, and dynamic mode splitting at low temperature due to entropy and anharmonicity of organic cations. Our results demonstrate intense THz-driven coherent band-edge modulation is a powerful probe of electron-lattice coupling phenomena and polaronic quantum control in perovskites.