“…For the (C) 44 structure, the left val ues correspond to the 〈11〉 direction, and the right val ues correspond to the 〈10〉 direction. The values of the elastic rigidities c 11 , c 12 , and c 13 are taken from [12]. We call attention that both the Young's modulus and elas tic rigidities of a two dimensional structure have the dimension of a force divided by a length.…”