Figure 2: Feynman diagram of phonon self-energy.
From: Giant phonon anomaly associated with superconducting fluctuations in the pseudogap phase of cuprates
Dashed lines are phonon propagators (D) and thick grey lines are Leggett mode propagators (L). B is the effective interaction vertex between phonons and Leggett modes, which consists of a particle–particle electronic bubble (G is the electron Green’s function). The incoming phonon is scattered forwardly to a nearby wavevector by the absorption and emission of the Leggett mode. Due to the separation of energy scales between the Leggett mode and the electron, all quantities in green are neglected.
