Direct probe of linearly dispersing 2D interband plasmons in a free-standing graphene monolayerM. K. Kinyanjui, C. Kramberger, T. Pichler, J. C. Meyer, P. Wachsmuth, G. Benner and U. KaiserEPL, 97 5 (2012) 57005DOI: https://doi.org/10.1209/0295-5075/97/57005