The far-infrared conductivity and dielectric function of ${\mathrm{BaCo}}_{0.9}{\mathrm{Ni}}_{0.1}{\mathrm{S}}_{1.9}$ in the insulating and metallic phases were obtained from temperature-dependent reflectivity measurements. We found a Drude-like response for the carriers in the metallic phase with remarkably narrow width $(<10\mathrm{}{\mathrm{cm}}^{-1\mathrm{}})$ and a very small ratio of carrier density to effective mass $\left({\mathrm{nm}}_e{/m}^{*}\sim {10}^{18}{\mathrm{cm}}^{-3\mathrm{}}\right)$. Our result suggests that the metallic state is anomalous and associated with some type of coherent or collective response of the carriers.

• Received 31 October 1996

DOI:https://doi.org/10.1103/PhysRevLett.78.4498