Single polaron properties for double-well electron-phonon coupling

TL;DR

This paper introduces a double-well electron-phonon coupling model to accurately describe polaron properties in certain crystals, revealing significant differences from traditional Holstein models.

Contribution

The study develops a new double-well electron-phonon coupling model and analyzes its properties, highlighting differences from the Holstein model in the single carrier limit.

Findings

Small polarons form at strong coupling with distinct properties.

Double-well model cannot be replicated by linear Holstein models.

Key differences in polaron behavior compared to Holstein polarons.

Abstract

We show that in crystals where light ions are symmetrically intercalated between heavy ions, the electron-phonon coupling for carriers located at the light sites cannot be described by a Holstein model. We introduce the double-well electron-phonon coupling model to describe the most interesting parameter regime in such systems, and study it in the single carrier limit using the momentum average approximation. For sufficiently strong coupling, a small polaron with a robust phonon cloud appears at low energies. While some of its properties are similar to those of a Holstein polaron, we highlight some crucial differences. These prove that the physics of the double-well electron-phonon coupling model cannot be reproduced with a linear Holstein model.