Photoinduced dynamics of flat bands in the kagome metal CoSn

CoSn is a prototypical kagome compound showing lattice-born flat bands with suppressed bandwidth over large parts of the Brillouin zone. Here, by means of time- and angle-resolved photoelectron spectroscopy, we provide direct evidence of the response to photoexcitation of the flat bands, which underlie information about localization in real space. In particular, we detect a sudden shift and broadening of the flat bands, while after one picosecond only a broadening survives. We ascribe both these effects to an ultrafast disruption of electron localization, which renormalizes the effect of electron-electron interaction and affects the flat band dispersion. Since both variations are in the order of a few meV for an excitation fluence of a few hundreds of μJ/cm², our measurements suggest that the flat bands are resilient to near-infrared photoexcitation in a moderate fluence re\'egime.