Understanding the effect of beyond-Fröhlich interactions on large polarons*

The large polaron, an electron interacting with a continuum of lattice phonons, is one of the most fundamental and well-known problems of many-body physics. Large polarons are often described using the Fröhlich Hamiltonian, which assumes a linear electron-phonon interaction. However, in recent years significant interest has been raised in additional interaction terms, such as the 1-electron-2-phonon interaction. In our work, we extend Fröhlich theory to include this interaction and investigate the properties of the resulting polaron.

We derive an analytical expression for the interaction strength of an electron coupling to LO phonons. For cubic materials, the interaction strength only depends on a single scalar parameter, making it well-suited for analytical calculations. Since the resulting Hamiltonian is quadratic, we may investigate several properties using the path integral formalism: these include the energy and effective mass of the new polaron, and formation of bipolarons. It is shown that the additional term leads to significant additional trapping of the electron, broadens the bipolaron stability regime, and causes a secondary absorption peak in the optical conductivity.