  A charge order has been discovered to emerge deep into the antiferromagnetic
phase of the kagome metal FeGe. To study its origin, the evolution of the
low-lying electronic structure across the charge order phase transition is
investigated with angle-resolved photoemission spectroscopy. We do not find
signatures of nesting between Fermi surface sections or van-Hove singularities
in zero-frequency joint density of states, and there are no obvious energy gaps
at the Fermi level, which exclude the nesting mechanism for the charge order
formation in FeGe. However, two obvious changes in the band structure have been
detected, i.e., one electron-like band around the K point and another one
around the A point move upward in energy position when the charge order forms.
These features can be well reproduced by our density-functional theory
calculations, where the charge order is primarily driven by magnetic energy
saving via large dimerizations of a quarter of Ge1-sites (in the kagome plane)
along the c-axis. Our results provide strong support for this novel charge
order formation mechanism in FeGe, in contrast to the conventional nesting
mechanism.
