Semiclassical Approximation of Electron-Phonon Scattering Beyond Fermi's Golden Rule

We derive a quantum mechanical correction to the semiclassical Fermi golden rule operator for scattering of electrons in a crystal. This correction takes into account the fact that electron-phonon interaction is not instantaneous in quantum mechanics. The corrective term is derived via an oscillatory, i.e., weak, limit in the Levinson equation for large timescales.