M. Machón

Origin of the high‐energy Raman modes in single‐wall carbon nanotubes

We present a new interpretation of the origin of the first‐order Raman modes in single‐wall carbon nanotubes. Similar to the disorder‐induced D mode, the high‐energy modes are determined by double‐resonant scattering. Our model predicts an excitation‐energy dependence of the Raman frequencies, which we observe experimentally as well. We present preliminary results on the Raman spectra of metallic tubes.

Phonons and symmetry properties of (4,4) picotube crystals

The recently grown picotube crystals are the closest to a monochiral nanotube sample achieved up to now. We present an experimental and theoretical study of the vibrational properties of these crystals, including polarization dependent Raman spectra and ab initio calculations. We assign symmetries to the most intense peaks, A1 in most cases. From ab initio calculations we obtain the underlying atomic displacements. We find, among others, modes related to the high‐energy mode and the radial‐breathing mode of carbon nanotubes.

Ab initio studies of electron-phonon coupling in single-walled nanotubes

We present ab initio calculations of electron‐phonon coupling in single‐walled nanotubes and graphene. The perturbation of the electronic energies due to the atomic distortion caused by totally symmetric phonons was calculated, yielding the matrix elements of the electron‐phonon interaction. For the radial breathing mode (RBM) we obtained a decrease of the electron‐phonon interaction with increasing diameter. This is in good agreement with the fact that the equivalent mode for graphene is an out‐of‐plane translation which cannot affect the electronic system. The matrix elements for the RBM and the optical A1g mode show different behaviours for armchair and zig‐zag nanotubes.

Optical properties of 4 Å‐diameter single‐wall nanotubes

We studied the electronic band structure and optical absorption of carbon nanotubes with small diameters (d ≈ 4 A), which grow in the channels of a zeolite crystal. The size of the channels determines the diameter of the nanotubes and restricts their possible chiralities to three [(3,3), (5,0), and (4,2)]. We performed ab initio calculations of the electronic band structure and optical absorption of the mentioned nanotubes. We discuss our results and compare them with optical and Raman data.