Anisotropic gap formation in CeM₂Al₁₀ (M = Ru, Os)

We report an anomalous energy gap formation in CeM₂Al₁₀ (M = Ru, Os) by using a polarized optical conductivity [σ(ω)] measurement. Near 28 K (= T₀), both compounds undergo an antiferromagnetic transition order, the origin of which is not yet clear. We found that the σ(ω) spectra along the orthorhombic b-axis have characteristic peak structure at the energy gap edge, which implies the existence of the band nesting due to a charge instability. The peak structure does not originate from the conventional Kondo semiconducting property but can be explained by the charge-density wave (CDW) fluctuation based on the mean-field theory. This suggests that the opening of energy gap along the b-axis originates from the CDW or spin-density wave formation.