Theory of electron energy-loss spectroscopy in atomically thin metallic films

We study strongly confined plasmons in ultrathin gold and silver films by simulating electron energy-loss spectroscopy. Plasmon dispersion relations are directly retrieved from the energy- and momentum-resolved loss probability under normal incidence conditions, whereas they can also be inferred for aloof parallel beam trajectories from the evolution of the plasmon features in the resulting loss spectra as we vary the impinging electron energy. We find good agreement between nonlocal quantum-mechanical simulations based on the random-phase approximation and a local classical dielectric description for silver films of different thicknesses down to a few atomic layersThis work has been supported in part by the ERC(Advanced Grant No. 789104-eNANO), the SpanishMINECO (MAT2017-88492-R and SEV2015-0522), theCatalan CERCA Program, the Fundació Privada Cellex, andthe Quscope center sponsored by the Villum Foundation.Postprint (published version