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This is the first study of lead halide perovskites (LHPs) that compares the coupling strengths between longitudinal optical (LO) phonons and free or localized excitons. In this research, we measured the excitation density and temperature dependence of the photoluminescence (PL) spectra in the range of 4-300 K in a CsPbBr 3 single crystal. The PL peak energy of the free excitons monotonically blueshifts when the temperature increases, and so does the localized excitons until the temperature reaches 80 K. Above 80 K, however, the PL peak energy of the localized excitons redshifts when the temperature increases. We analyzed this unique temperature dependence using the configuration coordinate model, focusing on the exciton -LO phonon interaction. We showed that this unique temperature dependence can be exhibited as the localized excitons couple with the LO phonons with enhanced strength and have larger relaxation energy than the free excitons.

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