Fig. 1. MAPbI₃ crystal, X-ray diffraction patterns and experimental set-up.

a MAPbI₃ single crystal where the (110) surface is in front and (100) surface is on the left (marked by a red line). b X-ray diffraction patterns of (100) and (110) crystal facets. c Schematic of the experimental apparatus for pump–probe transient photoinduced circular-polarized reflection, c-PPR(t). PEM is a photoelastic modulator for modulating the pump-beam polarization between left and right circular polarizations; λ/4 is a quarter-wavelength plate; LP is a linear polarizer; and BS is a beam-splitter. The MAPbI₃ crystal was mounted in a cryostat and cooled down to 4 K. An electromagnet generates a magnetic field B up to 700 mT parallel to the crystal surface (Voigt geometry).