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Changing character of electronic transitions in graphene : From single-particle excitations to plasmons

In this paper we clarify the nature of π and π + σ electron excitations in pristine graphene. We clearly demonstrate the continuous transition from single particle to collective character of such excitations and how screening modifies their dispersion relations. We prove that π and π + σ plasmons do exist in graphene, though occurring only for a particular range of wave vectors and with finite damping rate. Particular attention is paid to comparing the theoretical results with available EELS measurements in optical (Q ≈ 0) and other (Q = 0) limits. The conclusions, based on microscopic numerical results, are confirmed in an approximate analytical approach.

Pi plasmon ; graphene ; EELS

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