Extreme electron-phonon coupling in boron-based layered superconductors

The phonon-mode decomposition of the electron-phonon coupling in the MgB2-like system Li1-xBC 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 similar to2% of the phonon modes. Noteworthy characteristics of E-2g branches include (1) "mode lambda" values of 25 and greater compared to a mean of similar to0.4 for other modes, (2) a precipitous Kohn anomaly, and (3) E-2g phonon linewidths within a factor of similar to2 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 MgB2 by Shukla et al.