Observation of Ultrahigh Mobility Surface States in a Topological Crystalline Insulator by Infrared Spectroscopy

TL;DR

This study uses infrared spectroscopy to reveal ultrahigh mobility surface states in a topological crystalline insulator, demonstrating surface-dominated conduction with potential for novel topological phenomena and applications.

Contribution

First direct infrared measurement showing surface states dominate conductivity in a TCI, with record-high surface mobility of 40000 cm²/(Vs).

Findings

Surface states dominate IR conductivity due to small penetration depth.

Surface mobility measured at 40000 cm²/(Vs), among the highest in topological materials.

Evidence supports surface-dominated conduction in thin TCI crystals.

Abstract

Topological crystalline insulators (TCIs) 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 (IR) reflectance measurements of a TCI, (001) oriented $Pb_{1-x}Sn_{x}Se$ in zero and high magnetic fields. We demonstrate that the far-IR conductivity is unexpectedly dominated by the surface states as a result of their unique band structure and the consequent small IR penetration depth. Moreover, our experiments yield a surface mobility of 40000 $cm^{2}/(Vs)$, which is one of the highest reported values in topological materials, suggesting the viability of surface-dominated conduction in thin TCI crystals. These findings pave the way for exploring many exotic transport and optical phenomena and applications predicted for TCIs.