Electronic correlations in the semiconducting half-Heusler compound FeVSb

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 a filled shell configuration of 18 valence electrons per formula unit (${s}²{p}⁶{d}¹⁰$). Angle-resolved photoemission spectroscopy reveals a mass renormalization of ${m}^*/{m}_{\text{bare}}=1.4$, where ${m}^*$ is the measured effective mass and ${m}_{\text{bare}}$ is the mass from density functional theory calculations with no added on-site Coulomb repulsion. Our measurements are in quantitative agreement with dynamical mean-field theory 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 rethinking of the role of correlations and Hund's coupling in intermetallic band insulators.