An investigation of electron-phonon coupling via phonon dispersion measurements in graphite using angle-resolved photoelectron spectroscopy.

Electron-phonon coupling (EPC) plays an important role in solid state physics. Here, we demonstrate an experimental method that enables investigation of the elemental processes of the indirect transition, in which EPC participates in photoexcitation in solids, by resolving the energy and momentum of phonons and electrons simultaneously. For graphite, we used angle-resolved photoelectron spectroscopy to observe electron emission at the Γ-point being scattered from the K-point by a phonon. Energy conservation during phonon emission implies that the step-like structure in the spectrum is near the Fermi level, and angle-resolved measurements revealed phonon dispersions that contribute to EPC because of parallel momentum conservation. The observed phonon branch depends on the photon energy, i.e., the final photoexcitation state; this dependency is partly explained by the selection rule, which is determined by the electron state symmetry for the initial, intermediate, and final states and the phonon.