Electron-phonon interaction and thermopower nonlinearities in Chevrel-phase compounds

We show that previously reported measurements for the Chevrel compounds ${\mathrm{Cu}}_{1.8}$ ${\mathrm{Mo}}_6$ ${\mathrm{S}}_{8\mathrm{-}\mathrm{y}}$ ${\mathrm{Se}}_{\mathrm{y}}$ and ${\mathrm{Cu}}_{1.8}$ ${\mathrm{Mo}}_6$ ${\mathrm{S}}_{8\mathrm{-}\mathrm{y}}$ ${\mathrm{Te}}_{\mathrm{y}}$ provide evidence for electron-phonon enhancement of thermopower, when scattering is sufficiently large to suppress phonon drag. This enhancement effect is similar to that hitherto seen in glassy metals, and is in agreement with theoretical calculations. The apparent sizes of the enhancement (λ≲1) are consistent with estimates from the McMillan formula using usual parameter values, in contrast to the unusually large values of λ and Coulomb repulsion ${\mu }^{\mathrm{*}}$ recently suggested for similar systems.