Large Enhancement of Electronic Thermal Conductivity and Lorenz Number in Topological Insulator Bi2Te2Se Thin Films

TL;DR

This study reveals significant enhancement of thermal and electrical conductivities in thin Bi2Te2Se topological insulator films below 20 nm, with an exceptionally high surface Lorenz number, highlighting unique surface state transport phenomena.

Contribution

It provides the first detailed investigation of thickness-dependent thermal transport in topological insulator thin films, emphasizing the role of surface states and revealing a large Lorenz number.

Findings

Thermal conductivity increases in films below 20 nm

Surface Lorenz number exceeds 10 times the Sommerfeld value

Surface states are near charge neutrality at small thicknesses

Abstract

Topological insulators (TI) have attracted extensive research effort due to their insulating bulk states but conducting surface states. However, investigation and understanding of thermal transport in topological insulators, particularly the effect of surface states are lacking. In this work, we studied thickness-dependent in-plane thermal conductivity of Bi2Te2Se TI thin films. A large enhancement of both thermal and electrical conductivity was observed for films with thicknesses below 20 nm, which is attributed to the surface states and bulk-insulating nature of these films. Surprisingly, a surface Lorenz number of over 10 times the Sommerfeld value was found. Transport measurements indicated that the surface is near the charge neutrality point when the film thickness is below 20 nm. Possible reasons for the large Lorenz number include the electrical and thermal current decoupling in the surface state Dirac fluid and the bipolar diffusion transport involving surface states.