High throughput search for polaronic materials

Over the past 10 years, immense progress has been made in the ab initio calculation of electron-phonon coupling and its consequences, in particular through improvements on the Allen Heine Cardona theory and exploiting the Frohlich model with its coupling constant "alpha". The simple Frohlich model shows qualitative agreement with many polar materials. However, with just one longitudinal optical phonon mode and a single isotropic parabolic electron it should have strong limits in real materials. We perform a high throughput scan of existing phonon and electron band structures (Materials Project), to identify trends and outliers. We sketch the limits of validity of the Frohlich model, through comparison of the strong-coupling localisation length with interatomic distances, signaling small polaron formation, and by tracking the breakdown of the lowest-order perturbation theory treatment. Our approach accounts for anisotropic and degenerate electronic bands, and multiple phonon modes. A large variety of behaviors is found, and more accurate fully ab initio calculations are performed to analyze extreme cases, with Frohlich alpha beyond 6 (when perturbation theory breaks down), and very large zero point motion renormalizations of the band gap.