  Electronic flat band systems are a fertile platform to host
correlation-induced quantum phenomena such as unconventional superconductivity,
magnetism and topological orders. While flat band has been established in
geometrically frustrated structures, such as the kagome lattice, flat
band-induced correlation effects especially in those multi-orbital bulk systems
are rarely seen. Here we report negative magnetoresistance and signature of
ferromagnetic fluctuations in a prototypical kagome metal CoSn, which features
a flat band in proximity to the Fermi level. We find that the magnetoresistance
is dictated by electronic correlations via Fermi level tuning. Combining with
first principles and model calculations, we establish flat band-induced
correlation effects in a multi-orbital electronic system, which opens new
routes to realize unconventional superconducting and topological states in
geometrically frustrated metals.
