Fig. 1: Material characterisation and Faraday rotation measurements.

a Sketch of the 2D perovskite BA₂FAPbI₇ structure and of lattice deformation associated with polaronic states. b Absorption (yellow line), and photoluminescence spectra (black line) of BA₂FAPbI₇ at 77 K. Spectra of the pump pulses used in TA and FR measurements are shown as red and blue shaded lines, respectively. c Experimental setup for FR measurements using a balanced photodetector (BPD). Probe photon energy is set to 1.91 eV, below the material absorption onset. d Temperature-dependent FR dynamics (spin polarisation as a function of time) following excitation with 2.17 eV and 2.43 eV pump photon energies at 77 K (solid lines) and at room temperature (dashed lines). All measurements had an exciton density smaller than 5 × 10¹⁶ cm^–3 (see Supplementary Fig. S2). Lowering the temperature under 2.17 eV excitation decreases the spin lifetime, while under 2.43 eV excitation it leads to an increase.