Calculating thermopower due to fluctuation-assisted tunnelling with application to carbon nanotube ropes

We show how Sheng's theory of fluctuation-assistedtunnelling can be used to calculate thermopower due to the presence of quantum mechanical tunnelling barriers. For a hole-like situation, this calculated thermopower behaviour increases approximately linearly with increasing temperature (commencing from zero in the zero temperature limit) until a knee is reached, after which the thermopower continues to increase linearly but with a reduced slope. In Sheng's theory, the amplitude for the barrier fluctuations increases with increasing temperature and the knee occurs when the thermal fluctuations become comparable to the magnitude of the tunnelling barriers. Interestingly, this thermopower behaviour compares quite favourably with that measured for systems of metallic single-walled carbonnanotubes (SWCNs) and, in particular, the knee seen in the temperature dependence of the carbonnanotubethermopower data at T similar to 100 K arises naturally in our calculation. For a system of carbonnanotubes, tunnelling barriers could occur either between individual carbonnanotubes or else due to defects within the carbonnanotubes. (c) 2001 Elsevier Science B.V. All rights reserved.