Electronic and mechanical coupling between guest and host in carbon peapods

Pristine single-wall carbon nanotubes (SWCNT’s) filled with ${\mathrm{C}}_{60}$ are investigated with resonance Raman spectroscopy. Both totally symmetric modes of ${\mathrm{C}}_{60}$ exhibit a surprising splitting into two components with a slightly different resonance behavior and an apparent loss of polarization. The latter can be understood from a symmetry reduction of the fullerene molecules in the center of the tubes and/or by an anisotropy of the electric field seen by the ${\mathrm{C}}_{60}$ peas inside the SWCNT pods due to depolarization effects. The splitting is explained from molecular-dynamics simulations which show that the two bands emerge from a coupling of the ${\mathrm{C}}_{60}$ totally symmetric modes to the fullerene translational mobility inside the tube. The doublet is thus the spectroscopic fingerprint of different mobilities of ${\mathrm{C}}_{60}$ fullerenes in the tubes.