Multiple phonon modes in Feynman path-integral variational polaron mobility

The Feynman path-integral variational approach to the polaron problem\cite{Feynman1955}, along with the associated FHIP linear-response mobility theory\cite{Feynman1962}, provides a computationally amenable method to predict the frequency-resolved temperature-dependent charge-carrier mobility, and other experimental observables in polar semiconductors. We show that the FHIP mobility theory predicts non-Drude transport behaviour, and shows remarkably good agreement with the recent diagrammatic Monte-Carlo mobility simulations of Mishchenko et al.\cite{Mishchenko2019} for the abstract Fr\"ohlich Hamiltonian. We extend this method to multiple phonon modes in the Fr\"ohlich model action. This enables a slightly better variational solution, as inferred from the resulting energy. We carry forward this extra complexity into the mobility theory, where it shows richer structure in the frequency and temperature dependent mobility, due to the different phonon modes activating at different energies. The method provides a computationally efficient and fully quantitative method of predicting polaron mobility and response in real materials.