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We report thermoelectric (TE) properties of topological surface Dirac states(TSDS) in three-dimensional topological insulators (3D-TIs) purely isolatedfrom the bulk by employing single crystal Bi$_{2-x}$Sb$_x$Te$_{3-y}$Se$_y$films epitaxially grown in the ultrathin limit. Two intrinsic nontrivialtopological surface states, a metallic TSDS (m-TSDS) and a gap-openedsemiconducting topological state (g-TSDS), are successfully observed byelectrical transport, and important TE parameters (electrical conductivity(${\sigma}$), thermal conductivity (${\kappa}$), and thermopower ($S$)) areaccurately determined. Pure m-TSDS gives $S$=-44 {\mu}VK$^{-1}$, which is anorder of magnitude higher than those of the conventional metals and the valueis enhanced to -212 {\mu}VK$^{-1}$ for g-TSDS. It is clearly shown that thesemi-classical Boltzmann transport equation (SBTE) in the framework of constantrelaxation time (${\tau}$) most frequently used for conventional analysiscannot be valid in 3D-TIs and strong energy dependent relaxation time${\tau}(E)$ beyond the Born approximation is essential for making intrinsicinterpretations. Although ${\sigma}$ is protected on the m-TSDS, ${\kappa}$ isgreatly influenced by the disorder on the topological surface, giving adissimilar effect between topologically protected electronic conduction andphonon transport.

Thermoelectric properties of 3D topological insulator: Direct observation of topological surface and its gap opened states