ELECTRON-ELECTRON CORRELATIONS IN CARBON NANOTUBES

Single-wall carbon nanotubes(1,2) are ideally suited for electron-tran sport experiments on single molecules because they have a very robust atomic and electronic structure and are sufficiently long to allow ele ctrical connections to lithographically defined metallic electrodes. T he electrical transport properties of single nanotubes(3) and bundles of nanotubes(4) have so far been interpreted by assuming that individu al electrons within the nanotube do not interact, an approximation tha t is often well justified for artificial mesoscopic devices such as se miconductor quantum dots(5). Here we present transport spectroscopy da ta on an individual carbon nanotube that cannot be explained by using independent-particle models and simple shell-filling schemes. For exam ple, electrons entering the nanotube in a low magnetic field are obser ved to all have the same spin direction, indicating spin polarization of the nanotube. Furthermore, even when the number of electrons on the nanotube is fixed, we find that variation of an applied gate voltage can significantly change the electronic spectrum of the nanotube and c an induce spin flips. The experimental observations point to significa nt electron-electron correlations. We explain our results phenomenolog ically using a model that assumes that the capacitance of the nanotube depends on its many-body quantum state.