Topological crystalline insulators possess metallic surface states protected by crystalline symmetry, which are a versatile platform for exploring topological phenomena and potential applications. However, progress in this field has been hindered by the challenge to probe optical and transport properties of the surface states owing to the presence of bulk carriers. Here, we report infrared reflectance measurements of a topological crystalline insulator, (001)-oriented Pb1−xSnxSe in zero and high magnetic fields. We demonstrate that the far-infrared conductivity is unexpectedly dominated by the surface states as a result of their unique band structure and the consequent small infrared penetration depth. Moreover, our experiments yield a surface mobility of 40,000 cm2 V−1 s−1, which is one of the highest reported values in topological materials, suggesting the viability of surface-dominated conduction in thin topological crystalline insulator crystals. These findings pave the way for exploring many exotic transport and optical phenomena and applications predicted for topological crystalline insulators.

We report resistivity, specific heat and optical spectroscopy measurements on single crystal samples of [Formula: see text] [Formula: see text] [Formula: see text] and [Formula: see text] [Formula: see text] [Formula: see text]. We observed clear temperature-induced spectral weight suppression below 4000 [Formula: see text] for both compounds in the conductivity spectra [Formula: see text], indicating the progressive formation of gap-like features with decreasing temperature. The suppressed spectral weight transfers mostly to the higher energy region. This observation reflects the presence of the correlation effect in the compounds. We attribute the correlation effect to the Co 3d electrons.