Extreme Electron-Phonon Coupling in Boron-based Layered Superconductors

TL;DR

This paper investigates the electron-phonon coupling in boron-based layered superconductors, revealing extremely strong coupling to a small subset of phonon modes that drive high-temperature superconductivity.

Contribution

It uncovers the dominant role of a few phonon modes with unprecedented coupling strength in MgB2-like systems through first principles calculations.

Findings

High mode lambda values (~25) for specific phonon modes

Presence of a sharp Kohn anomaly in phonon dispersion

Phonon linewidths comparable to phonon frequencies indicating breakdown of linear theory

Abstract

The phonon-mode decomposition of the electron-phonon coupling in the MgB2-like system Li_{1-x}BC is explored using first principles calculations. It is found that the high temperature superconductivity of such systems results from extremely strong coupling to only ~2% of the phonon modes. Novel characteristics of E_2g branches include (1) ``mode lambda'' values of 25 and greater compared to a mean of $\sim 0.4$ for other modes, (2) a precipitous Kohn anomaly, and (3) E_2g phonon linewidths within a factor of ~2 of the frequency itself, indicating impending breakdown of linear electron-phonon theory. This behavior in borne out by recent inelastic x-ray scattering studies of MgB2 by Shukla et al.