  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.
