▎ 摘　　要

A mean-field theory formalism is employed to analyze the nonlocal plasmon dispersion relation of monolayer graphene which is Coulomb coupled to a thick conductor. We calculate numerically the undamped plasmon excitation spectrum for arbitrary wave number. For gapped graphene, both the low-frequency (acoustic) and high-frequency (surface) plasmons may lie within an undamped opening in the particle-hole region. Furthermore, we find undamped plasmon excitations in a region of frequency-wave vector space which has no counterpart for free-standing gapped graphene.