Fig. 6: Schematic illustration of the relevant pathways of ultrafast charge and energy transfer.

Interband transitions between bulk and surface states due to electron–electron (e–e) and electron–phonon (e–ph) scatterings are highlighted, with distinct contributions from low-k and high-k transfers, as well as energy dissipation into the lattice. Blue and red colors emphasize different branches of the surface state with opposite spin (SS±). Bulk conduction and bulk valence band states are denoted as BCB and BVB, respectively.