  Many thermoelectrics like Bi exhibit Rashba spin-orbit surface bands for
which topological insulator behavior consisting of ultrahigh mobilities and
enhanced thermopower has been predicted. Bi nanowires realize surface-only
electronic transport since they become bulk insulators when they undergo the
bulk semimetal-semiconductor transition as a result of quantum confinement for
diameters close to 50 nm. We studied 20-, 30-, 50- and 200-nm trigonal Bi
wires. Shubnikov-de Haas magnetoresistance oscillations caused by surface
electrons and bulklike holes enable the determination of their densities and
mobilities. Surface electrons have high mobilities exceeding 2(m^2)/(Vsec) and
contribute strongly to the thermopower, dominating for temperatures T< 100 K.
The surface thermopower is - 1.2 T microvolt/(K^2), a value that is consistent
with theory, raising the prospect of developing nanoscale thermoelectrics based
on surface bands.
