Moiseenko Ilya M. RussiaSaratovSaratov Branch of Kotel’nikov Institute of Radio Engineering and Electronics of the Russian Academy of Sciences, 38, Zelenaya Str., Saratov 410019, RussiaMoiseenkoIM@yandex.ru Author's articles Linear Model of Surface Terahertz Plasmons Amplification in Two Parallel Graphene Sheets