Instability of vibrational modes in hexagonal latticeElena A. Korznikova, Dmitry V. Bachurin, Sergey Yu. Fomin, Alexander P. Chetverikov and Sergey V. DmitrievEur. Phys. J. B, 90 2 (2017) 23DOI: https://doi.org/10.1140/epjb/e2016-70595-2