Pressure dependence of the thermoelectric power of single-walled carbon nanotubes

We have measured the thermoelectric power (S) of high-purity single-walled carbon nanotube mats as a function of temperature at various hydrostatic pressures up to 2.0 GPa. The thermoelectric power is positive, and it increases in a monotonic way with increasing temperature for all pressures. The low-temperature $(T<\mathrm{}40\mathrm{}\hspace{0.5em}\mathrm{K})$ linear thermoelectric power is pressure independent and is characteristic for metallic nanotubes. At higher temperatures it is enhanced and though $S\left(T\right)\mathrm{}$ is linear again above about 100 K it has a nonzero intercept. This enhancement is strongly pressure dependent and is related to the change of the phonon population with hydrostatic pressure.