Fig. 1. Crystal structure and Raman spectra of CrI₃.

a Top and side views of two layers of CrI₃ with monoclinic structure and a schematic defining angles θ and φ with respect to the a- and b-crystal axes and the polarization vectors of the incoming ε_i and scattered ε_s light. The two CrI₃ layers are color-coded differently. b Raman spectra for both xx (black) and xy (red) polarization configurations at T = 5 K and B = 0 T. Spectra in xx were divided by two for clarity. Peaks that appear below T_c are highlighted with blue asterisks. Inset shows intensity as a function of θ for P₂ and ${\mathrm{A}}_{\mathrm{g}}^6$ in a polar plot.