  High surface-mobility, which is attributable to topological protection, is a
trademark of three-dimensional topological insulators (3DTIs). Exploiting
surface-mobility indicates successful application of topological properties for
practical purposes. However, the detection of the surface-mobility has been
hindered by the inevitable bulk conduction. Even in the case of high-quality
crystals, the bulk state forms the dominant channel of the electrical current.
Therefore, with electrical transport measurement, the surface-mobility can be
resolved only below-micrometer-thick crystals. The evaluation of the
surface-mobility becomes more challenging at higher temperatures, where phonons
can play a role. Here, using spectroscopic techniques, we successfully
evaluated the surface-mobility of Bi2Te3 (BT) at room temperature (RT). We
acquired the effective masses and mean scattering times for both the surface
and bulk states using angle-resolved photoemission and terahertz time-domain
spectroscopy. We revealed a record-high surface-mobility for BT, exceeding
33,000 cm^2/(Vs) per surface sheet, despite intrinsic limitations by the
coexisting bulk state as well as phonons at RT. Our findings partially support
the interesting conclusion that the topological protection persists at RT. Our
approach could be applicable to other topological materials possessing
multiband structures near the Fermi level.
