Giant electron-phonon interactions in molecular crystals and the importance of non-quadratic coupling

TL;DR

This paper demonstrates that electron-phonon interactions in certain molecular crystals are exceptionally strong, significantly affecting electronic properties, and emphasizes the importance of considering non-quadratic coupling beyond traditional theoretical models.

Contribution

It provides first-principles calculations revealing large vibrational corrections to band gaps and critiques the limitations of the Allen-Heine-Cardona theory in modeling these effects.

Findings

Vibrational corrections to band gaps are comparable to electron-electron correlation effects.

Strong electron-phonon coupling involves both high-frequency molecular modes and lower frequency lattice modes.

Traditional Allen-Heine-Cardona theory shows significant discrepancies with more accurate methods.

Abstract

We investigate electron-phonon coupling in the molecular crystals CH$_4$, NH$_3$, H$_2$O, and HF, using first-principles quantum mechanical calculations. We find vibrational corrections to the electronic band gaps at zero temperature of -1.97 eV, -1.01 eV, -1.52 eV, and -1.62 eV, respectively, which are comparable in magnitude to those from electron-electron correlation effects. Microscopically, the strong electron-phonon coupling arises in roughly equal measure from the almost dispersionless high-frequency molecular modes and from the lower frequency lattice modes. We also highlight the limitations of the widely used Allen-Heine-Cardona theory, which gives significant discrepancies compared to our more accurate treatment.