NMR evidence for gapped spin excitations in metallic carbon nanotubes

We report on the spin dynamics of C-13 isotope enriched inner walls in double-wall carbon nanotubes using C-13 nuclear magnetic resonance. Contrary to expectations, we find that our data set implies that the spin-lattice relaxation time (T-1) has the same temperature (T) and magnetic field (H) dependence for most of the inner-wall nanotubes detected by NMR. In the high-temperature regime (T greater than or similar to 150 K), we find that the T and H dependence of 1/T1T is consistent with a 1D metallic chain. For T less than or similar to 150 K we find a significant increase in 1/T1T with decreasing T, followed by a sharp drop below similar or equal to 20 K. The data clearly indicate the formation of a gap in the spin excitation spectrum, where the gap value 2 Delta similar or equal to 40 K (equivalent to 3.7 meV) is H independent.