Discovery of dispersive quasiparticles in the band gap of candidate correlated semiconductor FeGa3

The narrow-gap semiconductor FeGa3 has attracted considerable attention due to its colossal thermoelectric Seebeck effect, intriguing magnetic anomaly and ferromagnetic quantum criticality. Although the quasiparticle excitations in the band gap have been speculated to be directly associated with exotic characters of FeGa3, the physical origin of these in gap states and their direct connection to either of these puzzling properties are under debate. Here, we report the discovery of the dispersive in-gap states in semiconducting FeGa3 through high-resolution angle-resolved photoemission spectroscopy. Combined with first-principles calculations and comprehensive analysis, the observed coherent in-gap states are identified as a manifestation of slight off-stoichiometry induced ordered secondary phase. Notably, these dispersive in-gap states provide a consistent explanation for the exotic electronic and magnetic properties of FeGa3, establishing their origin in the previously overlooked secondary phase rather than in strong correlation effects or the trivial disorders.