Polarized optical reflectivity is studied in single crystals of ${\mathrm{BaCo}}_{1-x}{\mathrm{Ni}}_x{\mathrm{S}}_2$ over a wide compositional range $0<~x<~0.28$ from the antiferromagnetic insulating to the paramagnetic metallic phase. ${\mathrm{BaCoS}}_2$ is a charge-transfer-type Mott insulator with an anisotropic optical gap of about (or less than) 1 eV for $\mathbf{E}\perp c$ (parallel to the CoS plane) and 3.5 eV for $\mathbf{E}\Vert c$ (perpendicular to the CoS plane). Ni substitution for Co has different effects on the in-plane ${\sigma }_{\mathrm{ab}}\left(\omega \right)$ and out-of-plane ${\sigma }_c\left(\omega \right)$ conductivities. As the Ni substitution proceeds, a Drude-like peak develops in ${\sigma }_{\mathrm{ab}}\left(\omega \right)$ above $x=0.18,$ while a mid-infrared absorption band develops and a Drude-like peak is not confirmed in ${\sigma }_c\left(\omega \right).\mathrm{}$ This indicates that the two-dimensionality is stronger than predicted from band calculations. However, the mid-infrared spectral weight induced in ${\sigma }_c\left(\omega \right)$ is much larger than that of high-$T_{\mathrm{c}}$ cuprates. This suggests that the charge carriers are not so tightly confined within the plane as in high-$T_{\mathrm{c}}$ cuprates. The looseness of the charge confinement probably originates from a strong interlayer-coupling effect via the ${3d}_{{3z}^2-r^2}$ orbital.

• Received 8 October 2000

DOI:https://doi.org/10.1103/PhysRevB.63.115113