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 [?(?)] measurement. Near 28 K (= T0), both compounds undergo an antiferromagnetic transition order, the origin of which is not yet clear. We found that the ?(?) 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.