Observation of plasmon-phonons in a metamaterial superconductor using inelastic neutron scattering*

Recent experiments have demonstrated that the metamaterial approach is capable of drastically increasing the critical temperature, T_c, of composite metal-dielectric epsilon near zero (ENZ) metamaterial superconductors. For example, a tripling of the critical temperature was observed in bulk Al-Al₂O₃ ENZ core-shell metamaterials. A theoretical model based on the Maxwell-Garnett effective medium approximation provides a microscopic explanation of this effect in terms of electron-electron pairing mediated by a hybrid plasmon-phonon excitation in the composite metal-dielectric metamaterial. We report the first observation of a hybrid plasmon-phonon excitation in Al-Al₂O₃ ENZ core-shell metamaterials using inelastic neutron scattering. These results provide strong support for this novel mechanism of superconductivity in ENZ metamaterials and provide an explanation for the 50 year old mystery of enhanced T_c in granular aluminum thin films.