Observation of Electronic Structure Modification in the Hidden Order Phase of CeCoSi

CeCoSi with no local inversion symmetric crystal structure (P4/nmm) exhibits a phase transition of unknown origin (Hidden Order: HO) at about 12K (T-0) above the antiferromagnetic transition temperature (T-N = 9.4 K). The electronic structure change across T-0 was investigated with high-precision optical reflection spectroscopy. The optical spectrum changed from a typical metallic behavior above T-0 to a gap-like structure at around 15 meV below T-0. The gap-like structure was unchanged across TN except for the narrowing of the Drude component of carriers due to the suppression of magnetic fluctuations. This result suggests a slight change from the typical metallic electronic structure above T-0 to that with an energy gap near the Fermi level in the HO phase. The change in electronic structure in the HO phase was concluded to be due to electron=valence instability.