In this paper we present the results on Raman-active modes calculation in achiral $(n,n)$ and $(n,0)$ carbon nanotubes (CNTs). vibrational spectra are derived as the eigenvalues of corresponding dynamical matrix at the $\Gamma$-point of Brillouin zone. Diagonal components and $\alpha_{ xy} (=\alpha_{ yx})$ are the only non-zero components of polarization tensor. Selection rules for Raman-active modes are determined by direct estimations of matrix elements responsible for the intensities of corresponding vibrational transitions. Few $E_{ 2g}$ modes can be non-Raman-active because of their ``fine'' vibrational structure. We claim that numbers of Raman-active modes for achiral CNTs are: 5 ($(n,n)$ and $(n,0)$, $n$-even); 6 ($(n,n)$, $n$-odd); 8 ($(n,0)$, $n$-odd).
Categories: cond-mat.mtrl-sci cond-mat.mes-hall