Measurement of the topological surface state optical conductance in bulk-insulating Sn-doped Bi$_{1.1}$Sb$_{0.9}$Te$_2$S single crystals

TL;DR

This study measures the optical conductance of topological surface states in bulk-insulating Sn-doped Bi$_{1.1}$Sb$_{0.9}$Te$_2$S crystals using terahertz spectroscopy, revealing magnetic field suppression effects and enabling detailed surface state characterization.

Contribution

The paper demonstrates a method to isolate and measure topological surface state conductance in thick bulk-insulating crystals, overcoming previous limitations of optical detection.

Findings

Magnetic field enhances transmission by 5% at 7 T due to surface state suppression.

Surface state suppression is independent of sample thickness, confirming 2D transport.

Extracted surface conductance and mobility provide insights into topological surface state properties.

Abstract

Topological surface states have been extensively observed via optics in thin films of topological insulators. However, in typical thick single crystals of these materials, bulk states are dominant and it is difficult for optics to verify the existence of topological surface states definitively. In this work, we studied the charge dynamics of the newly formulated bulk-insulating Sn-doped Bi$_{1.1}$Sb$_{0.9}$Te$_2$S crystal by using time-domain terahertz spectroscopy. This compound shows much better insulating behavior than any other bulk-insulating topological insulators reported previously. The transmission can be enhanced an amount which is 5$\%$ of the zero-field transmission by applying magnetic field to 7 T, an effect which we believe is due to the suppression of topological surface states. This suppression is essentially independent of the thicknesses of the samples, showing the two-dimensional nature of the transport. The suppression of surface states in field allows us to use the crystal slab itself as a reference sample to extract the surface conductance, mobility, charge density and scattering rate. Our measurements set the stage for the investigation of phenomena out of the semi-classical regime, such as the topological magneto-electric effect.