Universal interaction-driven gap in metallic carbon nanotubes*

Metallic carbon nanotubes (m-CNTs) exhibit a remarkably large energy gap for electronic excitations. The gap often exceeds 100 meV when the m-CNT is suspended in free space, but the gap disappears when the m-CNT lies on a metal surface. The theoretical description of this gap remains controversial and more experiments are needed. We have built ultra-clean suspended CNT devices from CNTs of known diameter and chiral angle. The CNT chirality is identified with spectrally resolved scanning photocurrent microscopy and the energy gap is determined by measuring thermally-activated electron transport. Our results show that the gap scales exactly as 1/D, with no dependence on chiral angle. We also demonstrate that the gap can be tuned by submerging the suspended m-CNT in dielectric liquids. Our results put new constraints on competing theoretical descriptions of a Mott insulator state versus an excitonic insulator state.