Title: Electron-phonon coupling mechanisms for hydrogen-rich metals at high pressure,
Abstract: The mechanisms for strong electron-phonon coupling predicted for
hydrogen-rich alloys with high superconducting critical temperature ($T_c$) are
examined within the Migdal-Eliashberg theory. Analysis of the functional
derivative of $T_c$ with respect to the electron-phonon spectral function shows
that at low pressures, when the alloys often adopt layered structures, bending
vibrations have the most dominant effect. At very high pressures, the H-H
interactions in two-dimensional (2D) and three-dimensional (3D) extended
structures are weakened, resulting in mixed bent (libration) and stretch
vibrations, and the electron-phonon coupling process is distributed over a
broad frequency range leading to very high $T_c$.