Anisotropic optical spectra of BaCo_1−xNi_xS₂: Effect of Ni substitution on the electronic structure of the Co_1−xNi_xS

Polarized optical reflectivity is studied in single crystals of BaCo_1−xNi_xS₂ over a wide compositional range 0 ≤ x ≤ 0.28 from the antiferromagnetic insulating to the paramagnetic metallic phase. BaCoS₂ is a charge-transfer-type Mott insulator with an anisotropic optical gap of about (or less than) 1 eV for E⊥c (parallel to the CoS plane) and 3.5 eV for Ec (perpendicular to the CoS plane). Ni substitution for Co has different effects on the in-plane σ_ab(ω) and out-of-plane σ_c(ω) conductivities. As the Ni substitution proceeds, a Drude-like peak develops in σ_ab(ω) above x=0.18, while a mid-infrared absorption band develops and a Drude-like peak is not confirmed in σ_c(ω). This indicates that the two-dimensionality is stronger than predicted from band calculations. However, the mid-infrared spectral weight induced in σ_c(ω) is much larger than that of high-T_c cuprates. This suggests that the charge carriers are not so tightly confined within the plane as in high-T_c cuprates. The looseness of the charge confinement probably originates from a strong interlayer-coupling effect via the 3d_3z²−r² orbital.