  Electronic flat bands in momentum space, arising from strong localization of
electrons in real space, are an ideal stage to realize strong correlation
phenomena. In certain lattices with built-in geometrical frustration,
electronic confinement and flat bands can naturally arise from the destructive
interference of electronic hopping pathways. Such lattice-borne flat bands are
often endowed with nontrivial topology if combined with spin-orbit coupling,
while their experimental realization in condensed matter system has been
elusive so far. Here, we report the direct observation of topological flat
bands in the vicinity of the Fermi level in frustrated kagome system CoSn,
using angle-resolved photoemission spectroscopy and band structure
calculations. The flat band manifests itself as a dispersionless electronic
excitation along the G-M high symmetry direction, with an order of magnitude
lower bandwidth (below 150 meV) compared to the Dirac bands originating from
the same orbitals. The frustration-driven nature of the flat band is directly
confirmed by the real-space chiral d-orbital texture of the corresponding
effective Wannier wave functions. Spin-orbit coupling opens a large gap of 80
meV at the quadratic band touching point between the Dirac and flat bands,
endowing a nonzero Z2 topological invariant to the flat band in the
two-dimensional Brillouin zone. Our observation of lattice-driven topological
flat band opens a promising route to engineer novel emergent phases of matter
at the crossroad between strong correlation physics and electronic topology.
