NMR Evidence for Gapped Spin Excitations in Metallic Carbon Nanotubes

TL;DR

This study uses 13C NMR to investigate spin dynamics in inner walls of double-wall carbon nanotubes, revealing a temperature-independent spin gap of 3.7 meV that influences their magnetic properties.

Contribution

It provides the first direct NMR evidence of a spin gap in metallic carbon nanotubes, challenging previous expectations of gapless behavior.

Findings

Spin-lattice relaxation time T1 shows consistent T and H dependence across most nanotubes.

At T > 150 K, behavior aligns with a 1D metallic chain model.

A spin gap of 3.7 meV is observed below 20 K, independent of magnetic field.

Abstract

We report on the spin dynamics of 13C isotope enriched inner-walls in double-wall carbon nanotubes (DWCNT) using 13C nuclear magnetic resonance (NMR). Contrary to expectations, we find that our data set implies that the spin-lattice relaxation time (T1) has the same temperature (T) and magnetic field (H) dependence for most of the innerwall nanotubes detected by NMR. In the high temperature regime (T > 150 K), we find that the T and H dependence of 1/T1T is consistent with a 1D metallic chain. For T < 150 K, we find a significant increase in 1/T1T with decreasing T, followed by a sharp drop below 20 K. The data clearly indicates the formation of a gap in the spin excitation spectrum, where the gap value 2 Delta = 40 K (= 3.7 meV) is H independent.