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

TL;DR

This study reveals a significant electronic structure change in CeCoSi during the Hidden Order phase, characterized by a gap formation near the Fermi level, indicating electron/valence instability.

Contribution

It provides direct optical evidence of a gap-like electronic structure modification associated with the Hidden Order phase in CeCoSi.

Findings

Optical spectrum shows a transition from metallic to gap-like behavior below T_0.

The gap structure remains unchanged across the antiferromagnetic transition.

Electronic structure change suggests electron/valence instability in the HO phase.

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 ($T_{\rm N}=9.4$ 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 $T_{\rm N}$ 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.