Gain characteristics of THz surface plasmons in electrically pumped monolayer grapheneYu-Ping Zhang, Ya-Qing Liu, Yan-Yan Cao, Huan-Huan Lv, Tong-Tong Li, Xiaoyan Huang, Hui-Yun Zhang and Guang-Jun RenEur. Phys. J. Appl. Phys., 69 1 (2015) 10803DOI: https://doi.org/10.1051/epjap/2014140425