Transport approach to the superconducting proximity effect in carbon nanotubes

A microscopic approach is developed to account for the magnitudes of the supercurrents observed experimentally in carbon nanotubes placed between superconducting contacts. We build up a model for the nanotube ropes encompassing the electron repulsion from the Coulomb interaction and the effective attraction given by phonon exchange. We show that the available experimental data are consistent with the expected decay of the supercurrents along the length of the nanotube samples. Our results stress that the propagation of the Cooper pairs is favored in the thick ropes, as a consequence of the reduction in the strength of the Coulomb interaction from the electrostatic coupling between the metallic nanotubes. We also provide an explanation for the temperature dependence of the supercurrents observed in the experiments, remarking the existence of a crossover from a very flat behavior at low temperatures to a pronounced decay in the long nanotubes. Submitted to J. Phys.: Condens. Matter (special issue on Nanostructured Surfaces) [email protected]