标题： 单极子和多极子等离子体在二维系统中 显示英文标题

标题： Monopole and multipole plasmons in a two-dimensional system

摘要： 使用单层石墨烯作为纯二维（2D）电子气体的模型系统，我们通过能量电子损失谱学，以高能量和动量分辨率显示，不仅一个而且两个色散损失的激发具有显著的概率。两种损失的出现与衬底无关（我们在6H-SiC(0001)的Si面和Ir(111)上测试了石墨烯，且在有和没有插入Na层的情况下），色散曲线中的斜率比在1.4（SiC）到2之间变化。虽然较低的色散曲线可以归因于单极子等离子体的激发，这与理论模型计算一致，但较高的色散分支之前在二维电子气体的等离子体激发中尚未被识别，我们将它归因于多极片状等离子体的激发。 显示英文摘要

摘要： Using monolayer graphene as a model system for a purely two-dimensional (2D) electron gas, we show by energy electron loss spectroscopy, highly resolved both in energy and momentum, that there is a significant probability for the excitation of not only one but two dispersing losses. The appearance of both losses is independent of the substrate (we tested graphene on the Si face of 6H-SiC(0001), and on Ir(111) without and with intercalated Na layer), and the ratio of the slope in the dispersion curves varies between 1.4 (SiC) and 2. While the lower dispersion curve can be attributed to the excitation of the monopole plasmon, in agreement with theoretical model calculations, the upper dispersion branch has not been identified before for plasmonic excitations in a 2D electron gas, and we assign it to the excitation of a multipole sheet plasmon.