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

TL;DR

This study investigates thermoelectric properties of topological surface states in 3D topological insulators, revealing distinct behaviors of metallic and gapped states and emphasizing the importance of energy-dependent relaxation times.

Contribution

First direct observation of thermoelectric properties of isolated topological surface states, highlighting the need for energy-dependent relaxation time analysis in 3D topological insulators.

Findings

Pure metallic topological surface states have high thermopower of -44 μV/K.

Gapped topological states exhibit enhanced thermopower of -212 μV/K.

Thermoelectric conductivity is affected by surface disorder, unlike electrical conductivity.

Abstract

We report thermoelectric (TE) properties of topological surface Dirac states (TSDS) in three-dimensional topological insulators (3D-TIs) purely isolated from 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 nontrivial topological surface states, a metallic TSDS (m-TSDS) and a gap-opened semiconducting topological state (g-TSDS), are successfully observed by electrical transport, and important TE parameters (electrical conductivity (${\sigma}$), thermal conductivity (${\kappa}$), and thermopower ($S$)) are accurately determined. Pure m-TSDS gives $S$=-44 {\mu}VK$^{-1}$, which is an order of magnitude higher than those of the conventional metals and the value is enhanced to -212 {\mu}VK$^{-1}$ for g-TSDS. It is clearly shown that the semi-classical Boltzmann transport equation (SBTE) in the framework of constant relaxation time (${\tau}$) most frequently used for conventional analysis cannot be valid in 3D-TIs and strong energy dependent relaxation time ${\tau}(E)$ beyond the Born approximation is essential for making intrinsic interpretations. Although ${\sigma}$ is protected on the m-TSDS, ${\kappa}$ is greatly influenced by the disorder on the topological surface, giving a dissimilar effect between topologically protected electronic conduction and phonon transport.