Fig. 1. Spin lifetime τ_s of electrons of CsPbBr₃.

We compare electron and hole τ_s in Supplementary information (SI) Fig. S7 and they have the same order of magnitude at all conditions we investigated. a τ_s due to both the electron-phonon (e-ph) and electron-electron (e-e) scatterings calculated as a function of T at different electron densities n_e compared with experimental data. In Fig. S6, we show τ_s versus T using log-scale for both y- and x-axes to highlight low-T region. Exp. A are our experimental data of $T_2^{*}$ of free electrons in bulk CsPbBr₃ at a small external transverse magnetic field. For Exp. A, the density of photo-excited carriers is estimated to be about 10¹⁸ cm⁻³. Exp. B are experimental data of exciton τ_s of CsPbBr₃ films from Ref. ¹¹. Exp. C and Exp. D are experimental data of spin relaxation time T₁ of bulk CsPbBr₃ and CsPbBr₃ nanocrystals measured by the spin inertia method from Ref. ⁹ and³⁴ respectively. In Ref. ³⁴, it was declared that quantum confinement effects do not modify the spin relaxation/dephasing significantly (see its Table 1), so that their T₁ data can be compared with our theoretical results. For Exp. C and D, the measured lifetimes cannot be unambiguously ascribed to electrons or holes and can be considered as values between electron and hole T₁. The carrier densities are not reported for Exp. C and D. b τ_s due to both the e-ph and e-e scatterings as a function of n_e at different T. The vertical dashed line in b corresponding to n_e with chemical potential μ_F,c at the conduction band minimum (CBM).