Thermoelectric Power of Single Bi Microwires at Helium Temperatures

The thermopower S has two contributions. Diffusion thermopower arises from a diffusion of charge carriers opposite to the temperature gradient; phonon drag thermopower results from a quasi-momentum transfer from the phonons to the charge carriers. The latter term is dominant at low temperatures, yields important information about phonon-wall scattering in nanostructures, and has been studied in confined systems such as two-dimensional electron gases and carbon nanotubes. The thermopower of monocrystalline Bi (pure and doped with Te or Sn) microwires with diameters ranging from 0.1 to 3 mum were measured in the temperature range 4-300 K. Samples of Bi nanowires that are monocrystalline were spun as a fiber by the high frequency liquid-phase casting method. The low-temperature diffusion thermopower of Bi is linear with temperature. Instead, the dominant feature of the thermopower at temperatures below 12 K is a peak, which is due to phonon drag. We observe that the phonon-drag thermopower depends on the wire diameter and increases with increasing diameter of the sample, which is qualitatively explained by the suppression of two-step phonon processes in the finer wires due to the shortening of the phonon mean free path for normal (momentum conserving) processes due to diffusive wall scattering [Gitsu, D, et. al., 2005]. We have also studied the dependence of the phonon drag peak with wire length. Thus we have observed considerable decreasing of the phonon drag in the short samples when the length of the samples is smaller than 1 mm. In this case only thick samples d = 2.5 and 1 mum have the appreciable positive peak at around ap5 K. A possible explanation of these experimental results is presented