Extreme electron-phonon coupling in boron-based layered superconductors

Abstract

The phonon-mode decomposition of the electron-phonon coupling in the ${\mathrm{MgB}}_{2}$-like system ${\mathrm{Li}}_{1\ensuremath{-}x}\mathrm{BC}$ is explored using first-principles calculations. It is found that the high-temperature superconductivity of such systems results from extremely strong coupling up to only $\ensuremath{\sim}2%$ of the phonon modes. Noteworthy characteristics of ${E}_{2g}$ branches include (1) ``mode $\ensuremath{\lambda}$'' values of 25 and greater compared to a mean of $\ensuremath{\sim}0.4$ for other modes, (2) a precipitous Kohn anomaly, and (3) ${E}_{2g}$ phonon linewidths within a factor of $\ensuremath{\sim}2$ of the frequency itself, indicating impending breakdown of conventional electron-phonon theory. This behavior in borne out by recent inelastic x-ray scattering studies of ${\mathrm{MgB}}_{2}$ by Shukla et al.