Unconventional thermoelectric behaviors in topological insulator nanostructures

Te-Hsien Wang^1*, Horng-Tay Jeng^1,2,3

¹Physics, National Tsing Hua University, Hsinchu, Taiwan
²Physics, National Center for Theoretical Sciences, Hsinchu, Taiwan
³Physics, Academia Sinica, Taipei, Taiwan

* Presenter:Te-Hsien Wang, email:pilgrim0613@gmail.com

Thermoelectric (TE) devices, with the ability to convert heat to electricity, have attracted increasing attention. However, the TE devices remain suffering from low conversion efficiency due to the trade-off between the TE parameters for conventional materials. The novel quantum materials such as topological insulators (TIs) provide an opportunity to overcome this problem. Here, as an example, we calculate the TE properties of the Bi₂Se₃ ultra-thin film through density-functional theory as well as the Boltzmann transport equations. We demonstrate for epitaxial Bi₂Se₃ thin films with thickness slightly larger than six quintuple layers, the relaxation time of the in-gap topological surface states can reach hundreds of femtoseconds, which is 2 orders of magnitude larger than that of the bulk states. Such a strong relaxation time enhancement will violate the Wiedemann−Franz law causing approximately 3 times enhancement of the electrical- to thermal-conductance ratio. In addition, it also leads to a large Seebeck coefficient with an anomalous sign, and consequently the excellent TE figure of merit zT≈2.1 at room temperature with high TE efficiency over a wide temperature range. When defects are introduced in the bulk-like middle layers of the thin films, both the electronic and lattice thermal conductivities can be significantly reduced while the electrical conductivity will not be noticeably affected leading to a further enhancement of the TE performance.

Keywords: thermoelectric, topological insulator, Bi₂Se₃ thin film, Wiedemann−Franz law violation, anomalous Seebeck effect