Thermoelectric Power of Single-Walled Carbon Nanotubes

We have measured the temperature-dependent thermoelectric power (TEP) of crystalline ropes of single-walled carbon nanotubes. The TEP is large and holelike at high temperatures and approaches zero as $T\to 0$. The results argue against the opening of a gap at low temperature in these materials. When derived from a simple band structure picture, the TEP of a single metallic nanotube is significantly lower than the measured TEP, strongly suggesting that the predicted electron-hole symmetry of metallic nanotubes is broken when the tubes self-assemble into ropes. Different models for the symmetry breaking are considered.