  The notion of an electronic flat band refers to a collectively degenerate set
of quantum mechanical eigenstates in periodic solids. The vanishing kinetic
energy of flat bands relative to the electron-electron interaction is expected
to result in a variety of many-body quantum phases of matter. Despite intense
theoretical interest, systematic design and experimental realization of such
flat band-driven correlated states in natural crystals have remained a
challenge. Here we report the realization of a partially filled flat band in a
new single crystalline kagome metal Ni$_3$In. This flat band is found to arise
from the Ni $3d$-orbital wave functions localized at triangular motifs within
the kagome lattice plane, where an underlying destructive interference among
hopping paths flattens the dispersion. We observe unusual metallic and
thermodynamic responses suggestive of the presence of local fluctuating
magnetic moments originating from the flat band states, which together with
non-Fermi liquid behavior indicate proximity to quantum criticality. These
results demonstrate a lattice and orbital engineering approach to designing
flat band-based many-body phenomena that may be applied to integrate
correlation with topology and as a novel means to construct quantum
criticality.
