No 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 (T 0 ) above the antiferromagnetic transition temperature (T N = 9.4 K). We investigated the electronic structure at the HO phase with high-precision reflection spectroscopy. Below the photon energy ω of 300 meV, except for optical phonon peaks at ω ∼ 20 and 40 meV, a typical metallic behavior above T 0 changes to an energy-gap-like structure at around 15 meV overlaid on the Drude component below T 0 , implying the slight change of the electronic structure near the Fermi level. The electronic structure modification at the HO phase is considered to originate from the electronic/valence instability.