Fig. 4: Physical picture of photo-induced scanning ultrafast electron microscopy (SUEM) contrast in α-RuCl₃.

a Schematic showing the competing mechanisms affecting the secondary electron (SE) yield as a function of photocarrier concentration. At low optical fluences, increased average electron energy due to photoexcitation enhances the SE yield. At higher optical fluences, increased local conductivity due to photo-induced insulator-to-metal transition suppresses the SE yield. b The electric field profile used in the time-dependent density functional theory (TDDFT) simulation as the optical excitation. c The ground state electronic structure calculated using the density functional theory (DFT) with the ACBN0 functional, showing an optical gap of 1 eV. The inset shows the first Brillouin zone and the high-symmetry points. d The electronic structure at 80 fs after photoexcitation simulated using TDDFT with an excitation level of 0.1 electron per unit cell, showing a complete melting of the Mott gap.