  The interplay between electronic correlations and topological protection may
offer a rich avenue for discovering emergent quantum phenomena in condensed
matter. However, electronic correlations have so far been little investigated
in Weyl semimetals (WSMs) by experiments. Here, we report a combined optical
spectroscopy and theoretical calculation study on the strength of electronic
correlations in a kagome magnet Co3Sn2S2 and the influence of electronic
correlations on its WSM state expected within a single-particle picture. The
electronic kinetic energy estimated from our optical data is about half of that
obtained from single-particle ab initio calculations, which indicates
intermediate-strength electronic correlations in this system. Furthermore,
comparing the energy ratios between the interband-transition peaks at high
energies in the experimental and single-particle-ab-initio-calculation derived
optical conductivity spectra with the electronic bandwidth renormalization
factors obtained by many-body calculations enables us to estimate the
Coulomb-interaction strength (U ~ 4 eV) of electronic correlations in Co3Sn2S2.
Our many-body calculations with U ~ 4 eV show that a WSM state, which is
characterized by bulk Weyl cones and surface Fermi arcs, survives in this
correlated electron system. Besides, a sharp experimental optical conductivity
peak at low energy, which is absent in the
single-particle-ab-initio-calculation-derived optical conductivity spectrum but
is consistent with the optical conductivity peaks obtained by many-body
calculations, indicates that an electronic band connecting the two Weyl cones
is flattened by electronic correlations and emerges near the Fermi energy in
Co3Sn2S2. Our work paves the way for exploring flat-band-generated quantum
phenomena in WSMs.
