Instability of vibrational modes in hexagonal lattice Elena A. Korznikova, Dmitry V. Bachurin, Sergey Yu. Fomin, Alexander P. Chetverikov and Sergey V. Dmitriev Eur. Phys. J. B, 90 2 (2017) 23 Published online: 01 February 2017 DOI: 10.1140/epjb/e2016-70595-2