Coherent phonons in excited-state carbon nanotubes: A simulation by tight-binding molecular dynamics

We have made tight-binding molecular dynamics simulations on the coherent phonons induced by electron–phonon coupling in the low energy optically excited states of single-walled carbon nanotubes (SWCNTs) with different diameters and chiralities. It is found that: (1) SWCNT’s radial breathing mode (RBM) and longitudinal optical (LO) mode are easily excited due to their direct coupling with band electrons, which then causes band gap oscillations with the same frequencies as those of RBM and LO modes. (2) Excitation of coherent phonons exhibits an interesting family-behavior. For example, for the family-II tubes, such as (11,0), (17,0) and (10,5) tubes with their mod(n−m,3)=2, the amplitude of their coherent RBM phonons in the first optical E11 transition is smaller than that in the second optical E22 one, while the opposite situation appears for a family-I tube with its mod(n−m,3)=1, e.g., (10,0) and (16,0) tubes. (3) Transverse optical (TO) modes cannot be excited in zigzag (10,0) and (11,0) tubes, but can be in a chiral (10,5) tube. The above obtained results are in good agreement with experimental observations.