We report results of electrical conductivity and thermoelectric power measurements on ${\mathrm{Ni}}_{1-x}{\mathrm{Co}}_x{\mathrm{S}}_2$ ($0<~x<~0.12$) in the temperature range $77 \mathrm{K}<T<\mathrm{}600\mathrm{} \mathrm{K}$. When analyzed on the basis of conventional one-electron theories, these results are difficult to explain. However, the measurements can be quantitatively interpreted in terms of a model involving polaron transport in highly correlated, narrow energy bands associated primarily with the transition-metal cations.

• Received 19 May 1977

DOI:https://doi.org/10.1103/PhysRevLett.39.494