What is the material with the highest mass enhancement factor?

Question

The electron phonon coupling mass enhancement factor $\lambda$ is a measure of the strength of this coupling. This quantity can be measured experimentally. For instance, Pb has a factor of 1.55 whereas Nb is ~1.0 - 1.2 [source]. Because of this, Pb is typically considered a material with strong electron-phonon coupling.

What is the material with the strongest $\lambda$ known so far? Is Pb the highest ever measured?

Edit: I found this paper which compiled experimentally measured $\lambda$ at the time (1972) where we see some Pb alloys having values up to 3.30. I wonder if there is a general table for all measured values of $\lambda$.

Answer 1

For mass enhancement from electron-phonon (e-ph) coupling, you need to look for strongly polar (ionic) compounds such as LiF and other alkali halides. These systems form small (localized) polarons with nearly flat bands and thus very large mass enhancement. In metals such as Pb the e-ph coupling is short ranged and cannot be as strong as in ionic systems, which are governed by long-range (Frohlich) e-ph coupling.

Another interaction causing mass enhancement is the electron-electron interaction. In strongly correlated materials with d- or f-valence electrons, one can heavy large mass enhancement (and correspondingly small quasiparticle weight Z) caused by the strong Coulomb repulsion at d or f orbitals. Examples include ruthenates and other transition metal oxides, f-electron oxides, as well as heavy fermion compounds.