  Electronic correlations are crucial to the low energy physics of metallic
systems with localized $d$ and $f$ states; however, their effect on band
insulators and semiconductors is typically negligible. Here, we measure the
electronic structure of the half-Heusler compound FeVSb, a band insulator with
filled shell configuration of 18 valence electrons per formula unit ($s^2 p^6
d^{10}$). Angle-resolved photoemission spectroscopy (ARPES) reveals a mass
renormalization of $m^{*}/m_{bare}= 1.4$, where $m^{*}$ is the measured
effective mass and $m_{bare}$ is the mass from density functional theory (DFT)
calculations with no added on-site Coulomb repulsion. Our measurements are in
quantitative agreement with dynamical mean field theory (DMFT) calculations,
highlighting the many-body origin of the mass renormalization. This mass
renormalization lies in dramatic contrast to other filled shell intermetallics,
including the thermoelectric materials CoTiSb and NiTiSn; and has a similar
origin to that in FeSi, where Hund's coupling induced fluctuations across the
gap can explain a dynamical self-energy and correlations. Our work calls for a
re-thinking of the role of correlations and Hund's coupling in intermetallic
band insulators.
