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

Abstract

CeCoSi with no local inversion symmetric crystal structure (P4/nmm) exhibits a phase transition of unknown origin (Hidden Order: HO) at about 12 K (T0) above the antiferromagnetic transition temperature (TN = 9.4 K). The electronic structure change across T0 was investigated with high-precision optical reflection spectroscopy. The optical spectrum changed from a typical metallic behavior above T0 to a gap-like structure at around 15 meV below T0. 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 T0 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.